Transactions Vol IV PDF

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Flow & Level Measurement
A Technical Reference Series Brought to You by Omega
VOLUME
4
TABLE OF CONTENTS
VOLUME 4—FLOW & LEVEL MEASUREMENT

Section Topics Covered Page
• The Flow Pioneers

D
• Flow Sensor Selection 08

• Accuracy vs. Repeatability
• Primary Element Options

• Pitot Tubes 16
• Variable Area Flowmeters
• Positive Displacement Flowmeters

• Turbine Flowmeters 34
• Other Rotary Flowmeters
• Magnetic Flowmeters
• Vortex Flowmeters 46
• Ultrasonic Flowmeters
• Coriolis Mass Flowmeters

• Thermal Mass Flowmeters 58
• Hot-Wire Anemometers
04 Volume 4 TRANSACTIONS
REFERENCE SECTIONS
Editorial 06 106 Information Resources
About OMEGA 07 110 Glossary

VOLUME 4—FLOW & LEVEL MEASUREMENT

Section Topics Covered Page
• Level Sensor Selection

• Boiling & Cryogenic Fluids 72
• Sludge, Foam, & Molten Metals
• Dry & Wet Leg Designs

• Bubbler Tubes 76
• Floats & Displacers
• Theory of Operation - --
-
--
--
- -- -- + ++
87
-
• Probe Designs - --
--
--
--
+ ++
++
++
++
+
+ ++
- + ++
+ ++
• Installation Considerations
+
++
++
+
• Radar & Microwave

• Ultrasonic Level Gages 93
• Nuclear Level Gages
• Thermal Switches
• Vibrating Switches 102
• Optical Switches
TRANSACTIONS Volume 4 05
A Flow Measurement Orientation
O ur interest in the measure-
ment of air and water flow
is timeless. Knowledge of
the direction and veloc-
ity of air flow was essential infor-
mation for all ancient navigators,
hydrodynamics, pneumatics, aerody-
namics) is based on the works of
the ancient Greek scientists Aristotle
and Archimedes. In the Aristotelian
view, motion involves a medium that
rushes in behind a body to prevent a
that Newton’s third law of motion
applies not only to stationary bodies,
but also to objects in motion.
The Flow Pioneers

A major milestone in the understand-
and the ability to measure water vacuum. In the sixth century A.D., John ing of flow was reached in 1783 when
flow was necessary for the fair Philoponos suggested that a body in the Swiss physicist Daniel Bernoulli
distribution of water through the motion acquired a property called published his Hydrodynamica. In it,
aqueducts of such early commu- impetus, and that the body came to he introduced the concept of the
conservation of energy for fluid
flows. Bernoulli determined that an
increase in the velocity of a flow-
ing fluid increases its kinetic energy
while decreasing its static energy. It
is for this reason that a flow restric-
tion causes an increase in the flowing
velocity and also causes a drop in
the static pressure of the flowing
fluid.
The permanent pressure loss
through a flowmeter is expressed
either as a percentage of the total
pressure drop or in units of velocity
heads, calculated as V2/2g, where V
is the flowing velocity and g is the
gravitational acceleration (32.2 feet/
second2 or 9.8 meters/second2 at
60° latitude). For example, if the
velocity of a flowing fluid is 10 ft/s,
the velocity head is 100/64.4 = 1.55
ft. If the fluid is water, the velocity
head corresponds to 1.55 ft of water
(or 0.67 psi). If the fluid is air, then
the velocity head corresponds to the
weight of a 1.55-ft column of air.
nities as the Sumerian cities of rest when its impetus died out. The permanent pressure loss
Ur, Kish, and Mari near the Tigris In 1687, the English mathematician through various flow elements can
and Euphrates Rivers around 5,000 Sir Isaac Newton discovered the law be expressed as a percentage of the
B.C. Even today, the distribution of of universal gravitation. The opera- total pressure drop (Figure 1-1), or it
water among the rice patties of Bali tion of angular momentum-type can be expressed in terms of veloc-
is the sacred duty of authorities mass flowmeters is based directly ity heads. The permanent pressure
designated the “Water Priests.” on Newton’s second law of angular loss through an orifice is four veloc-
Our understanding of the behav- motion. In 1742, the French math- ity heads; through a vortex shedding
ior of liquids and gases (including ematician Rond d’Alembert proved sensor, it is two; through positive
displacement and turbine meters, changed.
about one; and, through flow venturis, where C is the constant for units In 1883, the British mechanical engi-
less than 0.5 heads. Therefore, if an conversion. neer Osborne Reynolds proposed a
orifice plate (Figure 1-2) with a beta Over the past several years, the single, dimensionless ratio to describe
the velocity profile of flowing fluids:
Re = DVρ/μ
Where D is the pipe diameter, V is

the fluid velocity, ρ is the fluid den-
sity, and μ is the fluid viscosity.
He noted that, at low Reynolds
numbers (below 2,000) (Figure 1-5),
flow is dominated by viscous forces
and the velocity profile is (elongated)
parabolic. At high Reynolds numbers
(above 20,000), the flow is domi-
nated by inertial forces, resulting in
a more uniform axial velocity across
the flowing stream and a flat velocity
profile.
Until 1970 or so, it was believed
that the transition between laminar
and turbulent flows is gradual, but
increased understanding of turbu-
lence through supercomputer mod-
ratio of 0.3 (diameter of the orifice performance of magnetic flowmeters eling has shown that the onset of
to that of the pipe) has an unrecov- has improved significantly. Among turbulence is abrupt.
ered pressure loss of 100 in H2O, a the advances are probe and ceramic When flow is turbulent, the pres-
venturi flow tube could reduce that insert designs and the use of pulsed sure drop through a restriction is
pressure loss to about 12 in H2O for magnetic fields (Figure 1-4), but the proportional to the square of the
the same measurement. basic operating principle of Faraday’s flowrate. Therefore, flow can be
In 1831, the English scientist Michael law of electric induction has not measured by taking the square root
Faraday discovered the dynamo when
he noted that, if a copper disk is
rotated between the poles of a per-
manent magnet, electric current is
generated. Faraday’s law of electro-
magnetic induction is the basis for the
D
operation of the magnetic flowmeter.
As shown in Figure 1-3, when a liquid
conductor moves in a pipe having a
diameter (D) and travels with an aver-
age velocity (V) through a magnetic
field of B intensity, it will induce a
voltage (E) according to the relation-
ship:
E = BVDC
of a differential pressure cell output. velocity of the reflectors. This speed, dent of wind velocity and depends
When the flow is laminar, a linear in turn, could be used to calculate a solely on the diameter of the flag
relationship exists between flow and flowrate. pole. This is the theory behind the
pressure drop. Laminar flowmeters The history of the Coriolis flow- vortex flowmeter, which determines
are used at very low flowrates (capil- meter is similar. The French civil flow velocity by counting the num-
lary flowmeters) or when the viscos- engineer Gaspard Coriolis discovered ber of vortices passing a sensor.
in 1843 that the wind, the ocean Von Karman published his findings
currents, and even airborne artillery in 1954, and because by that time
shells will all drift sideways because the sensors and electronics required
of the earth’s rotation. In the northern to count vortices were already in
hemisphere, the deflection is to the existence, the first edition of the
right of the motion; in the southern Instrument Engineers’ Handbook in
hemisphere, it is to the left. Similarly, 1968 was able to report the avail-
a body traveling toward either pole ability of the first swirlmeter.
will veer eastward, because it retains The computer has opened new
the greater eastward rotational frontiers in all fields of engineering,
speed of the lower altitudes as it and flow measurement is no excep-
passes over the more slowly rotating tion. It was only as long ago as 1954
earth surface near the poles. Again, that another Hungarian-American
it was the slow evolution of sensors mathematician, John Von Neumann,
and electronics that delayed creation built Uniac—and even more recent-
of the first commercial Coriolis mass ly that yet another Hungarian-
flowmeter until the 1970’s. American, Andy Grove of Intel,
It was the Hungarian-American developed the integrated circuit. Yet
ity of the process fluid is high. aeronautical engineer Theodore these events are already changing
In the case of some flowmeter von Karman who, as a child growing the field of flowmetering. Intelligent
technologies, more than a century up in Transylvania (now Romania), differential pressure cells, for exam-
elapsed between the discovery of noticed that stationary rocks caused ple, can automatically switch their
a scientific principle and its use in vortices in flowing water, and that range between two calibrated spans
building a flowmeter. This is the case the distances between these travel- (one for 1-10%, the other for 10-100%
with both the Doppler ultrasonic and ing vortices are constant, no matter of D/P), extending orifice accuracy
the Coriolis meter. how fast or slow the water runs. to within 1% over a 10:1 flow range.
In 1842, the Austrian physicist
Christian Doppler discovered that, if a
sound source is approaching a receiv-
er (such as a train moving toward a
stationary listener), the frequency of
the sound will appear higher. If the
source and the recipient are moving
away from each other, the pitch will
drop (the wavelength of the sound
will appear to decrease). Yet it was
more than a century later that the first
ultrasonic Doppler flowmeter came Flow measurement options run the gamut from simple, economical paddle wheels (shown) to
on the market. It projected a 0.5-MHz sophisticated high-accuracy devices.
beam into a flowing stream containing
reflectors such as bubbles or particles. Later in life, he also observed that, Furthermore, it is possible to include
The shift in the reflected frequency when a flag flutters in the wind, the in this accuracy statement not only
was a function of the average traveling wavelength of the flutter is indepen- hysteresis, rangeability, and linearity
effects, but also drift, temperature, a device because it is less expensive. requiring that the following types of
humidity, vibration, over-range, and Those “inexpensive” purchases can be data be filled in for each application:
power supply variation effects. the most costly installations. • Fluid and flow characteristics:
With the development of super- The basis of good flowmeter In this section of the table, the
chips, the design of the universal selection is a clear understanding of name of the fluid is given and its
flowmeter also has become feasible. the requirements of the particular pressure, temperature, allowable
It is now possible to replace dye- application. Therefore, time should pressure drop, density (or specific
tagging or chemical-tracing meters be invested in fully evaluating the gravity), conductivity, viscosity
(which measured flow velocity by nature of the process fluid and of (Newtonian or not?) and vapor
dividing the distance between two the overall installation. The develop- pressure at maximum operating
points by the transit time of the
trace), with traceless cross-corre-
lation flowmeters (Figure 1-6). This
is an elegant flowmeter because it
requires no physical change in the
process—not even penetration of
the pipe. The measurement is based
on memorizing the noise pattern in
any externally detectable process
variable, and, as the fluid travels
from point A to point B, noting its
transit time.
Flow Sensor Selection

The purpose of this section is to
provide information to assist the
reader in making an informed selec-
tion of flowmeter for a particular
application. Selection and orien-
tation tables are used to quickly ment of specifications that state the temperature are listed, together
focus on the most likely candidates application requirements should be a with an indication of how these
for measurement. Tables 1-I and 1-II systematic, step-by-step process. properties might vary or interact.
have been prepared to make avail- The first step in the flow sensor In addition, all safety or toxicity
able a large amount of information selection process is to determine if information should be provided,
for this selection process. the flowrate information should be together with detailed data on the
At this point, one should consider continuous or totalized, and whether fluid’s composition, presence of
such intangible factors as familiarity of this information is needed locally bubbles, solids (abrasive or soft,
plant personnel, their experience with or remotely. If remotely, should the size of particles, fibers), tenden-
calibration and maintenance, spare transmission be analog, digital, or cy to coat, and light transmission
parts availability, mean time between shared? And, if shared, what is the qualities (opaque, translucent or
failure history, etc., at the particular required (minimum) data-update fre- transparent?).
plant site. It is also recommended that quency? Once these questions are • Expected minimum and maximum
the cost of the installation be com- answered, an evaluation of the prop- pressure and temperature values
puted only after taking these steps. erties and flow characteristics of the should be given in addition to the
One of the most common flow mea- process fluid, and of the piping that normal operating values. Whether
surement mistakes is the reversal of will accommodate the flowmeter, flow can reverse, whether it does
this sequence: instead of selecting a should take place (Table 1-I). In order not always fill the pipe, whether
sensor which will perform properly, an to approach this task in a systematic slug flow can develop (air-solids-
attempt is made to justify the use of manner, forms have been developed, liquid), whether aeration or pul-
sation is likely, whether sudden ards, or if there are other special Accuracy vs. Repeatability
temperature changes can occur, requirements such as compliance In applications where products are
or whether special precautions are with sanitary or clean-in-place sold or purchased on the basis of a
needed during cleaning and main- (CIP) regulations. meter reading, absolute accuracy is
tenance, these facts, too, should be The next step is to determine the critical. In other applications, repeat-
stated. required meter range by identify- ability may be more important than
absolute accuracy. Therefore, it is
advisable to establish separately the
accuracy and repeatability require-
ments of each application and to
state both in the specifications.
When a flowmeter’s accuracy is
stated in % CS or % FS units, its abso-
lute error will rise as the measured
flow rate drops. If meter error is
stated in % AR, the error in absolute
terms stays the same at high or low
flows. Because full scale (FS) is always
a larger quantity than the calibrated
span (CS), a sensor with a % FS per-
formance will always have a larger
error than one with the same % CS
specification. Therefore, in order to
compare all bids fairly, it is advisable
to convert all quoted error state-
ments into the same % AR units.
It is also recommended that the
user compare installations on the
basis of the total error of the loop.
For example, the inaccuracy of an ori-
fice plate is stated in % AR, while the
error of the associated d/p cell is in
% CS or % FS. Similarly, the inaccuracy
• Concerning the piping and the ing minimum and maximum flows of a Coriolis meter is the sum of two
area where the flowmeter is to be (mass or volumetric) that will be errors, one given in % AR, the other
located, the following information measured. After that, the required as a % FS value. Total inaccuracy is
should be specified: For the pip- flow measurement accuracy is deter- calculated by taking the root of the
ing, its direction (avoid downward mined. Typically accuracy is speci- sum of the squares of the component
flow in liquid applications), size, fied in percentage of actual read- inaccuracies at the desired flow rates.
material, schedule, flange-pressure ing (AR), in percentage of calibrated In well-prepared flowmeter speci-
rating, accessibility, up or down- span (CS), or in percentage of full fications, all accuracy statements are
stream turns, valves, regulators, and scale (FS) units. The accuracy require- converted into uniform % AR units and
available straight-pipe run lengths. ments should be separately stated these % AR requirements are specified
• In connection with the area, the at minimum, normal, and maximum separately for minimum, normal, and
specifying engineer must know if flowrates. Unless you know these maximum flows. All flowmeter speci-
vibration or magnetic fields are requirements, your meter’s perfor- fications and bids should clearly state
present or possible, if electric or mance may not be acceptable over both the accuracy and the repeatabil-
pneumatic power is available, if the its full range. ity of the meter at minimum, normal,
area is classified for explosion haz- and maximum flows.
Table 1 provides data on the range by making the calculation when flow flowmeter categories and one has
of Reynolds numbers (Re or RD) within and density are at their maximum and no moving parts, select the one
which the various flowmeter designs viscosity at its minimum. Conversely, without moving parts. Moving parts
can operate. In selecting the right the minimum RD is obtained by using are a potential source of problems,
flowmeter, one of the first steps is to minimum flow and density and maxi- not only for the obvious reasons
determine both the minimum and the mum viscosity. of wear, lubrication, and sensitivity
maximum Reynolds numbers for the If acceptable metering performance to coating, but also because mov-
application. Maximum RD is obtained can be obtained from two different ing parts require clearance spaces
that sometimes introduce “slippage” temperature. Changes in temperature full flowmeter and a point sensor,
into the flow being measured. Even also change the internal dimensions of it is generally advisable to use the
with well maintained and calibrated the meter and require compensation. flowmeter. Because point sensors do
meters, this unmeasured flow varies Furthermore, if one can obtain not look at the full flow, they read
with changes in fluid viscosity and the same performance from both a accurately only if they are inserted
to a depth where the flow veloc- References & Further Reading
ity is the average of the velocity • OMEGA Complete Flow and Level Measurement Handbook and
profile across the pipe. Even if this Encyclopedia®, Omega Press, 1995.
point is carefully determined at the • Omega Volume 29 Handbook & Encyclopedia, Purchasing Agents
time of calibration, it is not likely to Edition, Omega Press, 1995.
remain unaltered, since velocity pro- • “Advanced Process Control for Two-Phase Mixtures,” David Day,
files change with flowrate, viscosity, Christopher Reiner and Michael Pepe, Measurements & Control, June,
temperature, and other factors. 1997.
If all other considerations are the • Applied Fluid Flow Measurement, N.P. Cheremisinoff, Marcel Decker, 1979.
same, but one design offers less pres- • “Characteristics and Applications of Industrial Thermal Mass Flow
sure loss, it is advisable to select that Transmitters,” Jerome L. Kurz, Proceedings 47th Annual Symposium on
design. Part of the reason is that the Instrumentation for the Process Industries, ISA, 1992.
pressure loss will have to be paid • Developments in Thermal Flow Sensors, Jerome L. Kurz, Ph.D., Kurz
for in higher pump or compressor Instruments Inc., 1987.
operating costs over the life of the • “Differential Flow Measurement of Meter-Conditioned Flow,” Stephen A.
plant. Another reason is that a pres- Ifft and Andrew J. Zacharias, Measurements & Control, September, 1993.
sure drop is caused by any restriction • Dry Solids Flow Update, Auburn International Inc.
in the flow path, and wherever a • Flow Measurement Engineering Handbook, R.W. Miller, McGraw-Hill, 1983.
pipe is restricted becomes a potential • Flow Measurement for Engineers and Scientists, N.P. Cheremisinoff,
site for material build-up, plugging, or Marcel Dekker, 1988.
cavitation. • Flow Measurement, Bela Liptak, CRC Press, 1993.
Before specifying a flowmeter, it is • “Flowmeter Geometry Improves Measurement Accuracy,” Stephen A.
also advisable to determine whether Ifft, Measurements & Control, October, 1995.
the flow information will be more • Flowmeters, F. Cascetta, P. Vigo, ISA, 1990.
useful if presented in mass or vol- • Fluidic Flowmeter, Bulletin 1400 MX, Moore Products Co., June, 1988.
umetric units. When measuring the • Fundamentals of Flow Metering, Technical Data Sheet 3031, Rosemount
flow of compressible materials, volu- Inc., 1982.
metric flow is not very meaningful • Guide to Variable Area Flowmeters, Application No.: T-022 Issue I,
unless density (and sometimes also Brooks Instrument Co., 1986.
viscosity) is constant. When the veloc- • Incompressible Flow, Donald Panton, Wiley, 1996.
ity (volumetric flow) of incompress- • Industrial Flow Measurement, D.W. Spitzer, ISA, 1984.
ible liquids is measured, the pres- • “Installation Effects on Venturi Tube Flowmeters”, G. Kochen, D.J.M.
ence of suspended bubbles will cause Smith, and H. Umbach, Intech, October, 1989.
error; therefore, air and gas must be • Instrument Engineers’ Handbook, Bela Liptak, ed., CRC Press, 1995.
removed before the fluid reaches the • “Is a Turbine Flowmeter Right for Your Application?” Michael Hammond,
meter. In other velocity sensors, pipe Flow Control, April, 1998.
liners can cause problems (ultrasonic), • “Mass Flowmeters,” Measurements & Control, September, 1991.
or the meter may stop functioning if • Microprocessor-Based 2-Wire Swirlmeter, Bailey-Fischer & Porter Co., 1995.
the Reynolds number is too low (in • “Process Gas Mass Flow Controllers: An Overview,” J. G. Olin, Solid State
vortex shedding meters, RD > 20,000 is Technology, April, 1988.
required). • “Target Flowmeters,” George W. Anderson, Measurements & Control,
In view of these considerations, June, 1982.
mass flowmeters, which are insensi- • Thermal Approach to Flow Measurement, Joseph W. Harpster and
tive to density, pressure and viscos- Robert Curry, Intek, Inc. 1991.
ity variations and are not affected • “Ultrasonic Flowmeter Basics,” Gabor Vass, Sensors, October, 1997.
by changes in the Reynolds number, • “Ultrasonic Flowmeters Pick Up Speed,” Murry Magness, Control, April, 1996.
should be kept in mind. Also under- • “User Tips for Mass, Volume Flowmeters,” Donald Ginesi and Carl
utilized in the chemical industry are Annarummo, Intech, April, 1994.
the various flumes that can measure
Differential Pressure Flowmeters
T he calculation of fluid flow
rate by reading the pressure
loss across a pipe restriction
is perhaps the most common-
ly used flow measurement technique
in industrial applications (Figure 2-1).
the unrestricted pipe. The pressure
differential (h) developed by the flow
element is measured, and the velocity
(V), the volumetric flow (Q) and the
mass flow (W) can all be calculated
using the following generalized for-
(API), the American Society of
Mechanical Engineers (ASME), and the
American Gas Association (AGA), and
are included in many of the works
listed as references at the end of this
chapter.
The pressure drops generated by a mulas: The discharge coefficients of prima-
wide variety of geometrical restric- ry elements are determined by labora-
tions have been well characterized V = k (h/D)0.5 tory tests that reproduce the geome-
over the years, and, as compared or Q =kA(h/D)0.5 try of the installation. Published values
in Table 2, these primary or “head” or W= kA(hD)0.5 generally represent the average value
flow elements come in a wide variety for that geometry over a minimum of
of configurations, each with specific k is the discharge coefficient of 30 calibration runs. The uncertainties
application strengths and weaknesses. the element (which also reflects of these published values vary from
Variations on the theme of differen- the units of measurement), A is the 0.5% to 3%. By using such published
tial pressure (d/p) flow measurement cross-sectional area of the pipe’s discharge coefficients, it is possible to
obtain reasonably accurate flow mea-
surements without in-place calibra-
tion. In-place calibration is required
if testing laboratories are not avail-
able or if better accuracy is desired
than that provided by the uncertainty
range noted above. The relationship
between flow and pressure drop var-
ies with the velocity profile, which can
be laminar or turbulent (Figure 2-1) as a
function of the Reynolds number (Re),
which for liquid flows can be calcu-
lated using the relationship:
include the use of pitot tubes and opening, and D is the density of the
variable-area meters (rotameters), and flowing fluid. The discharge coeffi- Re = 3160(SG)(Q)/(ID)μ
are discussed later in this chapter. cient k is influenced by the Reynolds
number (see Figure 1-5) and by the where ID is the inside diameter of
Primary Element Options “beta ratio,” the ratio between the the pipe in inches, Q is the volumet-
In the 18th century, Bernoulli first bore diameter of the flow restriction ric liquid flow in gallons/minute, SG
established the relationship between and the inside diameter of the pipe. is the fluid specific gravity at 60°F,
static and kinetic energy in a flowing Additional parameters or cor- and μ is the viscosity in centipoises.
stream. As a fluid passes through rection factors can be used in the At low Reynolds numbers (gener-
a restriction, it accelerates, and derivation of k, depending on the ally under Re = 2,000), the flow is
the energy for this acceleration is type of flow element used. These laminar and the velocity profile is
obtained from the fluid’s static pres- parameters can be computed from parabolic. At high Reynolds num-
sure. Consequently, the line pressure equations or read from graphs and bers (well over Re = 3,000), the flow
drops at the point of constriction tables available from the American becomes fully turbulent, and the
(Figure 2-1). Part of the pressure drop National Standards Institute (ANSI), resulting mixing action produces a
is recovered as the flow returns to the American Petroleum Institute uniform axial velocity across the
pipe. As shown in Figure 1-5, the enter the application data and auto- that, at the low end of a 10:1 flow
transition between laminar and tur- matically find the recommended range (at 10% flow), corresponding
bulent flows can cover a wide range size, although these results should to a differential pressure range of
of Reynolds numbers; the relation- be checked for reasonableness by 100:1, the flowmeter would have an
ship with the discharge coefficient is hand calculation. error of ±20% AR. For this reason,
a function of the particular primary differential producing flowmeters
element. • Accuracy & Rangeability have historically been limited to use
Today, many engineering societies The performance of a head-type within a 3:1 or 4:1 range.
and organizations and most primary flowmeter installation is a function Flowmeter rangeability can be fur-
element manufacturers offer soft- of the precision of the flow ele- ther increased without adverse effect
ware packages for sizing d/p flow ment and of the accuracy of the on accuracy by operating several d/p
elements. These programs include d/p cell. Flow element precision is flowmeters in parallel runs. Only as
the required data from graphs, charts, typically reported in percentage of many runs are opened at a time as
and tables as well as empirical equa- actual reading (AR) terms, whereas are needed to keep the flow in the
tions for flow coefficients and cor- d/p cell accuracy is a percentage of active ones at around 75-90% of
rection factors. Some include data calibrated span (CS). A d/p cell usu- range. Another option is to stack
on the physical properties of many ally provides accuracy of ±0.2% of two or more transmitters in parallel
common fluids. The user can simply the calibrated span (CS). This means onto the same element, one for
1-10%, the other for 10-100% of full flow. pressure taps, or dampening soft-
scale (FS) d/p produced. Both of To minimize error (and the need ware can applied to the d/p cell
these techniques are cumbersome for density correction) when dealing output signal. One such filter is the
and expensive. Intelligent transmit- with compressible fluids, the ratio inverse derivative algorithm, which
ters offer a better option. of differential pressure (h) divided blocks any rate of change occurring
The accuracy of intelligent trans- by upstream pressure (P) should not more quickly than the rate at which
mitters is usually stated as ±0.1% exceed 0.25 (measured in the same the process flow can change.
CS, which includes only errors due engineering units).
to hysteresis, rangeability and lin- Metering errors due to incor- • Piping, Installation, & Maintenance
earity. Potential errors due to drift, rect installation of the primary Installation guidelines are published
temperature, humidity, vibration, element can be substantial (up to by various professional organiza-
overrange, radio frequency interfer- 10%). Causes of such errors can be tions (ISA, ANSI, API, ASME, AGA)
ence and power supply variation the condition of the mating pipe and by manufacturers of pro-
are all excluded. If one includes sections, insufficient straight pipe prietary designs. These guidelines
them, inaccuracy is about 0.2% CS. runs, and pressure tap and lead line include such recommendations as:
Because intelligent d/p transmitters design errors. • When, in addition to measuring
can—based on their own measure- Under turbulent flow conditions, the flow, the process temperature
ments—automatically switch ranges as much as 10% of the d/p signal or pressure is also to be measured,
between two calibrated spans (one can be noise caused by disturbances the pressure transmitter should
for 1-10%, the other for 10-100% of FS from valves and fittings, both up- not be installed in the process
d/p), it should be possible to obtain and downstream of the element, and pipe, but should be connected to
orifice installations with 1% AR inac- by the element itself. In the majority the appropriate lead line of the
curacy over a 10:1 flow range. of applications, the damping provid- flow element via a tee.
In most flowmetering applications, ed in d/p cells is sufficient to filter • Similarly, the thermowell used for
density is not measured directly. out the noise. Severe noise can be temperature measurement should
Rather, it is assumed to have some reduced by the use of two or more be installed at least 10 diameters
normal value. If density deviates from pressure taps connected in parallel downstream of the flow element,
this assumed value, error results. on both sides of the d/p cell. to prevent velocity profile distor-
Density error can be corrected if it Pulsating flow can be caused by tions.
is measured directly or indirectly by reciprocating pumps or compressors. • Welds should be ground smooth
measuring pressure in gases or tem- This pulsation can be reduced by and gaskets trimmed so that no
perature in liquids. Flow computing moving the flowmeter away from protrusion can be detected by
packages are also available that accept the source of the pulse, or down- physical inspection.
the inputs of the d/p transmitter and stream of filters or other dampening In order for the velocity profile
the other sensors and can simultane- devices. Pulsation dampening hard- to fully develop (and the pressure
ously calculate mass and volumetric ware can also be installed at the drop to be predictable), straight
pipe runs are required both up- and routed to the d/p cell, experience height condensate legs on both sides
downstream of the d/p element. the same temperature and are kept of the d/p cell. If for some reason
The amount of straight run required shielded from sunlight. the two legs are not of equal height,
depends on both the beta ratio of The d/p transmitter should be the d/p cell can be biased to zero
the installation and on the nature located as close to the primary ele- out the difference, as long as that
of the upstream components in the ment as possible. Lead lines should difference does not change.
pipeline. For example, when a single be as short as possible and of the If the process temperature exceeds
90° elbow precedes an orifice plate, same diameter. In clean liquid ser- the maximum temperature limitation
the straight-pipe requirement ranges vice, the minimum diameter is G", of the d/p cell, either chemical seals
from 6 to 20 pipe diameters as the while in condensable vapor service, have to be used or the lead lines need
diameter ratio is increased from 0.2 to the minimum diameter is 0.4". In to be long enough to cool the fluid. If
0.8. steam service, the horizontal lead a large temperature drop is required,
In order to reduce the straight lines should be kept as short as pos- a coiled section of tubing (pigtail) can
run requirement, flow straighteners sible and be tilted (with a minimum be installed in the lead lines to cool
(Figure 2-2) such as tube bundles, gradient of 1 in/ft with respect to the process fluids.
perforated plates, or internal tabs the piping) towards the tap, so that The frequency of inspection or
can be installed upstream of the condensate can drain back into the replacement of a primary element
primary element. pipe. Again, both lead lines should be depends on the erosive and cor-
The size and orientation of the
pressure taps are a function of both
the pipe size and the type of pro-
cess fluid. The recommended maxi-
mum diameter of pressure tap holes
through the pipe or flange is G" for
pipes under 2" in diameter, K" for 2"
and 3" pipes, H" for 4 to 8" and I" for
larger pipes. Both taps should be
of the same diameter, and, where
the hole breaks through the inside
pipe surface, it should be square
with no roughness, burrs, or wire
edges. Connections to pressure
holes should be made by nipples,
couplings, or adaptors welded to the
outside surface of the pipe. exposed to the same ambient condi- rosive nature of the process and
On services where the process tions and be shielded from sunlight. on the overall accuracy required.
fluid can plug the pressure taps In clean liquid or gas service, the lead If there is no previous experience,
or might gel or freeze in the lead lines can be purged through the d/p the orifice plate can be removed for
lines, chemical seal protectors cell vent or drain connections, and inspection during the first three, six,
can be used. Connection sizes are they should be flushed for several and 12 months of its operation. Based
usually larger (seal elements can minutes to remove all air from the on visual inspection of the plate, a
also be provided with diaphragm lines. Entrapped air can offset the reasonable maintenance cycle can
extensions), and, because of the zero calibration. be extrapolated from the findings.
space requirement, they are usu- Seal pots are on the wet leg in d/p Orifices used for material balance
ally installed at “radius tap” or cell installations with small ranges calculations should be on the same
“pipe tap” locations, as shown in (under 10 in H2O) in order to mini- maintenance cycle.
Figure 2-3. When chemical seals are mize the level variation in the legs.
used, it is important that the two In steam applications, filling tees • Sizing the Orifice Plate
connecting capillaries, as they are are recommended to ensure equal The orifice plate is commonly used
in clean liquid, gas, and steam ser- ever the plate is removed for main- under 2 inches in diameter. Corner
vice. It is available for all pipe sizes, tenance or inspection. In contrast, an taps are predominant in Europe for
and if the pressure drop it requires orifice fitting allows the orifice to be all sizes of pipe, and are used in the
is free, it is very cost-effective for removed from the process without United States for pipes under 2 inches
measuring flows in larger pipes (over depressurizing the line and shutting (Figure 2-3). With corner taps, the
6" diameter). The orifice plate is also down flow. In such fittings, the uni- relatively small clearances repre-
approved by many standards orga- versal orifice plate, a circular plate sent a potential maintenance prob-
nizations for the custody transfer of with no tab, is used. lem. Vena contracta taps (which
liquids and gases. The concentric orifice plate are close to the radius taps, Figure
The orifice flow equations used (Figure 2-4A) has a sharp (square- 2-4) are located one pipe diam-
today still differ from one another, edged) concentric bore that pro- eter upstream from the plate, and
although the various standards orga- vides an almost pure line contact downstream at the point of vena
nizations are working to adopt a sin- between the plate and the fluid, with contracta. This location varies (with
gle, universally accepted orifice flow negligible friction drag at the bound- beta ratio and Reynolds number)
equation. Orifice sizing programs ary. The beta (or diameter) ratios of from 0.35D to 0.8D.
usually allow the user to select the concentric orifice plates range from The vena contracta taps provide
flow equation desired from among 0.25 to 0.75. The maximum velocity the maximum pressure differential,
several. and minimum static pressure occurs but also the most noise. Additionally,
The orifice plate can be made of at some 0.35 to 0.85 pipe diam- if the plate is changed, it may require
any material, although stainless steel eters downstream from the orifice a change in the tap location. Also,
is the most common. The thickness plate. That point is called the vena in small pipes, the vena contracta
of the plate used (J-H") is a function contracta. Measuring the differential might lie under a flange. Therefore,
of the line size, the process tempera- pressure at a location close to the vena contracta taps normally are
ture, the pressure, and the differen- orifice plate minimizes the effect of used only in pipe sizes exceeding six
tial pressure. The traditional orifice pipe roughness, since friction has an inches.
is a thin circular plate (with a tab for effect on the fluid and the pipe wall. Radius taps are similar to vena
handling and for data), inserted into Flange taps are predominantly contracta taps, except the down-
the pipeline between the two flang- used in the United States and are stream tap is fixed at 0.5D from the
es of an orifice union. This method located 1 inch from the orifice plate’s orifice plate (Figure 2-3). Pipe taps are
of installation is cost-effective, but it surfaces (Figure 2-3). They are not located 2.5 pipe diameters upstream
calls for a process shutdown when- recommended for use on pipelines and 8 diameters downstream from
the orifice (Figure 2-3). They detect Concentric orifice plates are orifice is greater than that of the
the smallest pressure difference not recommended for multi-phase eccentric orifice, and, therefore, it is
and, because of the tap distance fluids in horizontal lines because preferred in applications with high
from the orifice, the effects of pipe the secondary phase can build up proportions of the secondary phase.
roughness, dimensional inconsisten- around the upstream edge of the These plates are usually used in pipe
cies, and, therefore, measurement plate. In extreme cases, this can sizes exceeding four inches in diam-
errors are the greatest. clog the opening, or it can change eter, and must be carefully installed
the flow pattern, creating measure- to make sure that no portion of the
• Orifice Types & Selection ment error. Eccentric and segmen- flange or gasket interferes with the
The concentric orifice plate is rec- tal orifice plates are better suited opening. Flange taps are used with
ommended for clean liquids, gases, for such applications. Concentric both types of plates, and are located
and steam flows when Reynolds orifices are still preferred for in the quadrant opposite the open-
numbers range from 20,000 to 107 in multi-phase flows in vertical lines ing for the eccentric orifice, in line
pipes under six inches. Because the
basic orifice flow equations assume
that flow velocities are well below
sonic, a different theoretical and
computational approach is required
if sonic velocities are expected. The
minimum recommended Reynolds
number for flow through an orifice
(Figure 1-5) varies with the beta ratio
of the orifice and with the pipe size.
In larger size pipes, the minimum
Reynolds number also rises.
Because of this minimum Reynolds
number consideration, square-edged
orifices are seldom used on viscous
fluids. Quadrant-edged and conical
orifice plates (Figure 2-5) are recom-
mended when the Reynolds number
is under 10,000. Flange taps, corner,
and radius taps can all be used with
quadrant-edged orifices, but only because accumulation of material with the maximum dam height for
corner taps should be used with a is less likely and the sizing data for the segmental orifice.
conical orifice. these plates is more reliable. For the measurement of low flow
Concentric orifice plates can be The eccentric orifice (Figure 2-4B) rates, a d/p cell with an integral
provided with drain holes to pre- is similar to the concentric except orifice may be the best choice. In
vent buildup of entrained liquids in that the opening is offset from the this design, the total process flow
gas streams, or with vent holes for pipe’s centerline. The opening of the passes through the d/p cell, elimi-
venting entrained gases from liquids segmental orifice (Figure 2-4C) is a nating the need for lead lines. These
(Figure 2-4A). The unmeasured flow segment of a circle. If the second- are proprietary devices with little
passing through the vent or drain ary phase is a gas, the opening of published data on their performance;
hole is usually less than 1% of the an eccentric orifice will be located their flow coefficients are based on
total flow if the hole diameter is towards the top of the pipe. If the actual laboratory calibrations. They
less than 10% of the orifice bore. secondary phase is a liquid in a gas or are recommended for clean, single-
The effectiveness of vent/drain a slurry in a liquid stream, the opening phase fluids only because even small
holes is limited, however, because should be at the bottom of the pipe. amounts of build-up will create sig-
they often plug up. The drainage area of the segmental nificant measurement errors or will
clog the unit. calculations, the quality of the instal- overall inaccuracy in the range of 2
Restriction orifices are installed to lation, and the condition of the plate to 5% AR.
remove excess pressure and usu- itself determine total performance. The typical custody-transfer
ally operate at sonic velocities with Installation factors include tap loca- grade orifice meter is more accu-
very small beta ratios. The pressure tion and condition, condition of the rate because it can be calibrated in
drop across a single restriction orifice process pipe, adequacy of straight a testing laboratory and is pro-
should not exceed 500 psid because pipe runs, gasket interference, mis- vided with honed pipe sections, flow
of plugging or galling. In multi-ele- alignment of pipe and orifice bores, straighteners, senior orifice fittings,
ment restriction orifice installations, and lead line design. Other adverse and temperature controlled enclo-
the plates are placed approximately conditions include the dulling of the sures.
one pipe diameter from one another sharp edge or nicks caused by corro-
in order to prevent pressure recovery sion or erosion, warpage of the plate • Venturi & Flowtubes
between the plates. due to waterhammer and dirt, and Venturi tubes are available in sizes
grease or secondary phase deposits up to 72", and can pass 25 to 50%
• Orifice Performance on either orifice surface. Any of the more flow than an orifice with the
Although it is a simple device, the above conditions can change the ori- same pressure drop. Furthermore,
the total unrecovered head loss
rarely exceeds 10% of measured
d/p (Figure 2-6). The initial cost of
venturi tubes is high, so they are
primarily used on larger flows or on
more difficult or demanding flow
applications. Venturis are insensi-
tive to velocity profile effects and
therefore require less straight pipe
run than an orifice. Their contoured
nature, combined with the self-
scouring action of the flow through
the tube, makes the device immune
to corrosion, erosion, and internal
scale build up. In spite of its high
initial cost, the total cost of owner-
ship can still be favorable because
of savings in installation and operat-
ing and maintenance costs.
The classical Herschel venturi has a
very long flow element characterized
by a tapered inlet and a diverging
outlet. Inlet pressure is measured at
the entrance, and static pressure in
the throat section. The pressure taps
feed into a common annular chamber,
orifice plate is, in principle, a preci- fice discharge coefficient by as much providing an average pressure read-
sion instrument. Under ideal condi- as 10%. In combination, these prob- ing over the entire circumference of
tions, the inaccuracy of an orifice lems can be even more worrisome the element. The classical venturi is
plate can be in the range of 0.75-1.5% and the net effect unpredictable. limited in its application to clean,
AR. Orifice plates are, however, quite Therefore, under average operating non-corrosive liquids and gases.
sensitive to a variety of error-induc- conditions, a typical orifice instal- In the short form venturi, the
ing conditions. Precision in the bore lation can be expected to have an entrance angle is increased and the
annular chambers are replaced by affected by calibration. The inaccura- lines. Plunger-like devices (vent clean-
pipe taps (Figure 2-7A). The short- cy of the discharge coefficient in a ers) can be installed to periodically
form venturi maintains many of the universal venturi, at Reynolds num- remove buildup from interior open-
advantages of the classical venturi, bers exceeding 75,000, is 0.5%. The ings, even while the meter is online.
but at a reduced initial cost, shorter inaccuracy of a classical venturi at Lead lines can also be replaced with
length and reduced weight. Pressure Re > 200,000 is between 0.7 and 1.5%. button-type seal elements hydrauli-
taps are located G to H pipe diam- Flowtubes are often supplied with cally coupled to the d/p transmitter
eter upstream of the inlet cone, and discharge coefficient graphs because using filled capillaries. Overall mea-
in the middle of the throat section. the discharge coefficient changes surement accuracy can drop if the
Piezometer rings can be used with as the Reynolds number drops. The chemical seal is small, its diaphragm
large venturi tubes to compensate variation in the discharge coefficient is stiff, or if the capillary system
for velocity profile distortions. In of a venturi caused by pipe rough- is not temperature-compensated or
slurry service, the pipe taps can be ness is less than 1% because there not shielded from direct sunlight.
purged or replaced with chemical is continuous contact between the
seals, which can eliminate all dead- fluid and the internal pipe surface. • Flow Nozzles
ended cavities. The high turbulence and the lack The flow nozzle is dimensionally
There are several proprietary of cavities in which material can more stable than the orifice plate,
flowtube designs which provide accumulate make flow tubes well particularly in high temperature and
even better pressure recovery than
the classical venturi. The best known
of these proprietary designs is the
universal venturi (Figure 2-7B). The
various flowtube designs vary in their
contours, tap locations, generated
d/p and in their unrecovered head
loss. They all have short lay lengths,
typically varying between 2 and 4
pipe diameters. These proprietary
flowtubes usually cost less than the
classical and short-form venturis
because of their short lay length.
However, they may also require suited for slurry and sludge services. high velocity services. It has often
more straight pipe run to condition However, maintenance costs can be been used to measure high flow-
their flow velocity profiles. high if air purging cannot prevent rates of superheated steam. The
Flowtube performance is much plugging of the pressure taps and lead flow nozzle, like the venturi, has
a greater flow capacity than the ficient data is available for Reynolds can justify the initial cost. The unique
orifice plate and requires a lower numbers as low as 5,000, it is advis- flow restriction is designed to last
initial investment than a venturi able to use flow nozzles only when the life of the installation without
tube, but also provides less pres- the Reynolds number exceeds 50,000. deterioration.
sure recovery (Figure 2-6). A major Flow nozzles maintain their accu- Wedge elements are used with
disadvantage of the nozzle is that it racy for long periods, even in dif- 3-in diameter chemical seals, elimi-
is more difficult to replace than the ficult service. Flow nozzles can be nating both the lead lines and any
orifice unless it can be removed as a highly accurate way to measure dead-ended cavities. The seals
part of a spool section. gas flows. When the gas velocity attach to the meter body immedi-
The ASME pipe tap flow nozzle reaches the speed of sound in the ately upstream and downstream of
is predominant in the United States throat, the velocity cannot increase the restriction. They rarely require
(Figure 2-7C). The downstream end any more (even if downstream pres- cleaning, even in services like dewa-
of a nozzle is a short tube having the sure is reduced), and a choked flow tered sludge, black liquor, coal slurry,
same diameter as the vena contrac- condition is reached. Such “critical fly ash slurry, taconite, and crude
ta of an equivalent orifice plate. The flow nozzles” are very accurate and oil. The minimum Reynolds number
low-beta designs range in diameter often are used in flow laboratories is only 500, and the meter requires
ratios from 0.2 to 0.5, while the high as standards for calibrating other gas only five diameters of upstream
beta-ratio designs vary between flowmetering devices. straight pipe run.
0.45 and 0.8. The nozzle should Nozzles can be installed in any The segmental wedge has a
always be centered in the pipe, position, although horizontal orien- V-shaped restriction characterized
and the downstream pressure tap tation is preferred. Vertical down- by the H/D ratio, where H is the
should be inside the nozzle exit. The flow is preferred for wet steam, height of the opening below the
throat taper should always decrease gases, or liquids containing solids. restriction and D is the diameter. The
the diameter toward the exit. Flow The straight pipe run requirements H/D ratio can be varied to match
nozzles are not recommended for are similar to those of orifice plates. the flow range and to produce the
desired d/p. The oncoming flow
creates a sweeping action through
the meter. This provides a scouring
effect on both faces of the restric-
tion, helping to keep it clean and
free of buildup. Segmental wedges
can measure flow in both directions,
but the d/p transmitter must be
calibrated for a split range, or the
flow element must be provided with
two sets of connections for two d/p
transmitters (one for forward and
one for reverse flow).
An uncalibrated wedge element
can be expected to have a 2% to 5%
slurries or dirty fluids. The most AR inaccuracy over a 3:1 range. A cali-
common flow nozzle is the flange • Segmental Wedge Elements brated wedge element can reduce
type. Taps are commonly located The segmental wedge element (Figure that to 0.5% AR if the fluid density is
one pipe diameter upstream and H 2-8A) is a proprietary device designed constant. If slurry density is variable
pipe diameter downstream from for use in slurry, corrosive, erosive, and/or unmeasured, error rises.
the inlet face. viscous, or high-temperature applica-
Flow nozzle accuracy is typically tions. It is relatively expensive and is • Venturi-Cone Element
1% AR, with a potential for 0.25% AR used mostly on difficult fluids, where The venturi-cone (V-cone) element
if calibrated. While discharge coef- the dramatic savings in maintenance (Figure 2-8B) is another proprietary
an orifice. Its flow rangeability of
3:1 (some operate at 4:1) is also simi-
lar to the capability of the orifice
plate. The main difference is that,
while an orifice measures the full
flowstream, the pitot tube detects
the flow velocity at only one point
in the flowstream. An advantage of
the slender pitot tube is that it can
be inserted into existing and pressur-
ized pipelines (called hot-tapping)
without requiring a shutdown.
• Theory of Operation
Pitot tubes were invented by Henri
Pitot in 1732 to measure the flowing
velocity of fluids. Basically a dif-
ferential pressure (d/p) flowmeter,
a pitot tube measures two pres-
sures: the static and the total impact
pressure. The static pressure is the
operating pressure in the pipe, duct,
or the environment, upstream to the
design that promises consistent perfor- exceed 0.75. For example, a 3-in meter pitot tube. It is measured at right
mance at low Reynolds numbers and is with a beta ratio of 0.3 can have a 0 to angles to the flow direction, prefer-
insensitive to velocity profile distor- 75 gpm range. Published test results on ably in a low turbulence location
tion or swirl effects. Again, however, liquid and gas flows place the system (Figure 2-9).
it is relatively expensive. The V-cone accuracy between 0.25 and 1.2% AR. The total impact pressure (PT) is
restriction has a unique geometry the sum of the static and kinetic
Pitot Tubes
that minimizes accuracy degradation Although the pitot tube is one of the
due to wear, making it a good choice simplest flow sensors, it is used in
for high velocity flows and erosive/ a wide range of flow measurement
corrosive applications. applications such as air speed in rac-
The V-cone creates a controlled ing cars and Air Force fighter jets. In
turbulence region that flattens the industrial applications, pitot tubes
incoming irregular velocity pro- are used to measure air flow in pipes,
file and induces a stable differen- ducts, and stacks, and liquid flow
tial pressure that is sensed by a in pipes, weirs, and open channels.
downstream tap. The beta ratio of While accuracy and rangeability are
a V-cone is so defined that an relatively low, pitot tubes are simple,
orifice and a V-cone with equal beta reliable, inexpensive, and suited for a
ratios will have equal opening areas. variety of environmental conditions,
including extremely high tempera-
Beta ratio = (D2 - d2).05 / D tures and a wide range of pressures.
The pitot tube is an inexpen-
where d is the cone diameter and D sive alternative to an orifice plate.
is the inside diameter of the pipe. Accuracy ranges from 0.5% to 5%
With this design, the beta ratio can FS, which is comparable to that of
In jacketed (dual-walled) pitot-
tube designs, the impact pressure
port faces forward into the flow,
while static ports do not, but are,
instead, spaced around the outer
tube. Both pressure signals (PT and
P) are routed by tubing to a d/p
indicator or transmitter. In industrial
applications, the static pressure (P)
can be measured in three ways: 1)
through taps in the pipe wall; 2) by
static probes inserted in the pro-
cess stream; or 3) by small openings
located on the pitot tube itself or
on a separate aerodynamic element.
Wall taps can measure static pres-
sures at flow velocities up to 200
ft/sec. A static probe (resembling an
L-shaped pitot tube) can have four
holes of 0.04 inches in diameter,
spaced 90° apart. Aerodynamic bod-
ies can be cylinders or wedges, with
two or more sensing ports.
Errors in detecting static pressure
pressures and is detected as the in the pipe changes from elongated arise from fluid viscosity, velocity,
flowing stream impacts on the pitot (laminar) to more flat (turbulent). and fluid compressibility. The key to
opening. To measure impact pres- This changes the point of average accurate static pressure detection is
sure, most pitot tubes use a small, velocity and requires an adjustment
sometimes L-shaped tube, with the of the insertion depth. Pitot tubes
opening directly facing the oncom- are recommended only for highly
ing flowstream. The point velocity turbulent flows (Reynolds Numbers >
of approach (VP) can be calculated 20,000) and, under these conditions,
by taking the square root of the dif- the velocity profile tends to be flat
ference between the total pressure enough so that the insertion depth is
(PT) and the static pressure (P) and not critical.
multiplying that by the C/D ratio, In 1797, G.B. Venturi developed a
where C is a dimensional constant short tube with a throat-like pas-
and D is density: sage that increases flow velocity
and reduces the permanent pres-
VP = C(PT - P)H /D sure drop. Special pitot designs are
available that, instead of providing
When the flowrate is obtained by just an impact hole for opening,
multiplying the point velocity (VP) add a single or double venturi to
by the cross-sectional area of the the impact opening of the pitot
pipe or duct, it is critical that the tube. The venturi version generates
velocity measurement be made at an a higher differential pressure than
insertion depth which corresponds does a regular pitot tube. Pitot tube shown with associated fittings and
to the average velocity. As the flow pressure transmitter.
velocity rises, the velocity profile • Static Pressure Measurement
High
to minimize the kinetic component Pressure
sating applications, the oil serves as couple, and a sampling nozzle.
Profile
in the pressure measurement. a pulsation dampening and pressure- A pitot tube also can be used
• Single-Port Pitot Tubes averaging medium. to measure water velocity in open
A single-port pitot tube can mea- Pitot tubes also can be used in channels, at drops, chutes, or over
sure the flow velocity at only a square, rectangular or circular air fall crests. At the low flow veloci-
single point in the cross-section of ducts. Typically, the pitot tube fits ties typical of laminar conditions,
a flowing stream (Figure 2-10). The through a 5/16-in diameter hole in pitot tubes are not recommended
probe must be inserted to a point in the duct. Mounting can be by a because it is difficult to find the
the flowing stream where the flow
velocity is the average of the veloci-
ties across the cross-section, and its
impact port must face directly into
the fluid flow. The pitot tube can be
made less sensitive to flow direction
if the impact port has an internal
bevel of about 15°, extending about
1.5 diameters into the tube.
If the pressure differential gen-
erated by the venturi is too low
for accurate detection, the conven-
tional pitot tube can be replaced by
a pitot venturi or a double venturi
sensor. This will produce a higher
pressure differential.
A calibrated, clean and properly
inserted single-port pitot tube can
provide ±1% of full scale flow accu-
racy over a flow range of 3:1; and,
with some loss of accuracy, it can
even measure over a range of 4:1. Its flange or gland. The tube is usually insertion depth corresponding to
advantages are low cost, no mov- provided with an external indicator, the average velocity and because
ing parts, simplicity, and the fact so that its impact port can be accu- the pitot element produces such a
that it causes very little pressure rately rotated to face directly into small pressure differential. The use
loss in the flowing stream. Its main the flow. In addition, the tube can of a pitot venturi does improve on
limitations include the errors result- be designed for detecting the full this situation by increasing the pres-
ing from velocity profile changes velocity profile by making rapid and sure differential, but cannot help the
or from plugging of the pressure consistent traverses across the duct. problem caused by the elongated
ports. Pitot tubes are generally used In some applications, such as EPA- velocity profile.
for flow measurements of secondary mandated stack particulate sampling,
importance, where cost is a major it is necessary to traverse a pitot • Averaging Pitot Tubes
concern, and/or when the pipe or sampler across a stack or duct. In Averaging pitot tubes been intro-
duct diameter is large (up to 72 these applications, at each point duced to overcome the problem of
inches or more). noted in Figure 2-11, a temperature finding the average velocity point.
Specially designed pitot probes and flow measurement is made in An averaging pitot tube is provided
have been developed for use with addition to taking a gas sample, with multiple impact and static pres-
pulsating flows. One design uses a which data are then combined and sure ports and is designed to extend
pitot probe filled with silicone oil taken to a laboratory for analysis. across the entire diameter of the
to transmit the process pressures to In such applications, a single probe pipe. The pressures detected by all
the d/p cell. At high frequency pul- contains a pitot tube, a thermo- the impact (and separately by all the
static) pressure ports are combined clean, allowing the sensor to use port. Each set of ports is connected
and the square root of their differ- smaller ports. to its own manifold, which com-
ence is measured as an indication of Averaging pitot tubes offer the bines the average static and average
the average flow in the pipe (Figure same advantages and disadvantages impact pressure signals. If plugging
2-12). The port closer to the outlet as do single-port tubes. They are is likely, the manifolds can be purged
of the combined signal has a slightly slightly more expensive and a little to keep the ports clean.
greater influence, than the port that more accurate, especially if the flow Because area-averaging pitot sta-
is farthest away, but, for secondary is not fully formed. Some averag- tions generate very small pressure dif-
applications where pitot tubes are ing pitot sensors can be inserted ferentials, it may be necessary to use
commonly used, this error is accept- through the same opening (or hot low differential d/p cells with spans
able. tap) which accommodates a single- as low as 0-0.01 in water column. To
The number of impact ports, the port tube. improve accuracy, a hexagonal cell-
distance between ports, and the type flow straightener and a flow
diameter of the averaging pitot tube • Area Averaging nozzle can be installed upstream of
all can be modified to match the Area-averaging pitot stations are the area-averaging pitot flow sen-
needs of a particular application. used to measure the large flows of sor. The flow straightener removes
Sensing ports in averaging pitot tubes low pressure air in boilers, dryers, local turbulence, while the nozzle
amplifies the differential pressure
produced by the sensor.
• Installation
Pitot tubes can be used as permanently
installed flow sensors or as portable
monitoring devices providing periodic
data. Permanently installed carbon
steel or stainless steel units can oper-
ate at up to 1400 PSIG pressures and
are inserted into the pipe through
flanged or screw connections. Their
installation usually occurs prior to
plant start-up, but they can be hot-
tapped into an operating process.
In a hot-tap installation (Figure
2-14), one first welds a fitting to the
pipe. Then a drill-through valve is
attached to the fitting and a hole is
drilled through the pipe. Then, after
partially withdrawing the drill, the
valve is closed, the drill is removed
and the pitot tube is inserted. Finally,
the valve is opened and the pitot
tube is fully inserted.
are often too large to allow the tube or HVAC systems. These units are The velocity profile of the flowing
to behave as a true averaging cham- available for the various standard stream inside the pipe is affected by
ber. This is because the oversized sizes of circular or rectangular ducts the Reynolds number of the flowing
port openings are optimized not for (Figure 2-13) and for pipes. They are fluid, pipe surface roughness, and
averaging, but to prevent plugging. In so designed that each segment of by upstream disturbances, such as
some installations, purging with an the cross-section is provided with valves, elbows, and other fittings.
inert gas is used to keep the ports both an impact and a static pressure Pitot tubes should be used only if the
minimum Reynolds number exceeds 1-in); D = probe diameter (inches); L = rotameter combination (bypass rota-
20,000 and if either a straight run of unsupported probe length in inches, meter), open-channel variable gate,
about 25 diameters can be provided which is calculated as the sum of the tapered plug, and vane or piston
upstream to the pitot tube or if pipe I.D. plus the pipe wall thickness designs.
straightening vanes can be installed. plus: 1.25 in for K-in diameter probes; Either the force of gravity or a
1.5 in for H-in; 1.56 in for I-in; and 1.94 spring is used to return the flow ele-
100
90
80
70
60
50
40
30
20
10
Scale
• Vibration Damage in for 1-in diameter probes. ment to its resting position when the
Natural frequency resonant vibra- Once the velocity limits have flow lessens. Gravity-operated meters
tions can cause pitot tube fail- been calculated, make sure that they (rotameters) must be installed in a
ure. Natural frequency vibration is do not fall within the range of oper- vertical position, whereas spring oper-
caused by forces created as vortices ating velocities. If they do, change ated ones can be mounted in any
are shed by the pitot tube. The pitot the probe diameter, or its mounting, position. All variable area flowme-
tube is expected to experience such or do both, until there is no overlap. ters are available with local indicators.
vibration if the process fluid veloc- Most can also be provided with posi-
ity (in feet per second) is between Variable Area Flowmeters tion sensors and transmitters (pneu-
a lower limit (VL) and an upper limit Variable area flowmeters (Figure 2-15) matic, electronic, digital, or fiberoptic)
(VH). The values of VL and VH can be are simple and versatile devices that for connecting to remote displays or
calculated (for the products of a given operate at a relatively constant pres- controls.
manufacturer) using the equations sure drop and measure the flow of
below. liquids, gases, and steam. The posi- • Purge-Flow Regulators
tion of their float, piston or vane If a needle valve is placed at the
VL = 5253(M x Pr x D)/L2 is changed as the increasing flow inlet or outlet of a rotameter, and a
VH = 7879(M x Pr x D)/L2 rate opens a larger flow area to pass d/p regulator controls the pressure
the flowing fluid. The position of difference across this combination,
Where M = mounting factor (3.52 the float, piston or vane provides a the result is a purge-flow regula-
for single mount); Pr = probe factor direct visual indication of flow rate. tor. Such instrumentation packages
(0.185 for K-in diameter probes; 0.269 Design variations include the rotame- are used as self-contained purge
for H-in; 0.372 for I-in; and 0.552 for ter (a float in a tapered tube), orifice/ flowmeters (Figure 2-16). These are
among the least expensive and most H2O differential. In bubbler and rangeability, and linear output. Its
widely used flowmeters. Their main purge applications, the inlet pressure operation is simple: in order to pass
application is to control small gas or (P1) is held constant and the outlet through the tapered tube, the fluid
liquid purge streams. They are used pressure (P0) is variable. Figure 2-16 flow raises the float. The greater the
to protect instruments from con- describes a configuration where the flow, the higher the float is lifted. In
tacting hot and corrosive fluids, to outlet pressure (P0) is held constant liquid service, the float rises due to a
protect pressure taps from plugging, and the inlet pressure (P1) is variable. combination of the buoyancy of the
liquid and the velocity head of the
fluid. With gases, buoyancy is negli-
gible, and the float responds mostly
to the velocity head.
In a rotameter (Figure 2-15), the
metering tube is mounted vertically,
with the small end at the bottom. The
fluid to be measured enters at the
bottom of the tube, passes upward
around the float, and exits the top.
When no flow exists, the float rests
at the bottom. When fluid enters, the
metering float begins to rise.
The float moves up and down in
proportion to the fluid flow rate
and the annular area between the
float and the tube wall. As the float
rises, the size of the annular opening
increases. As this area increases, the
differential pressure across the float
decreases. The float reaches a stable
position when the upward force
exerted by the flowing fluid equals
the weight of the float. Every float
position corresponds to a particular
to protect the cleanliness of optical They can handle extremely small flowrate for a particular fluid’s den-
devices, and to protect electrical flow rates from 0.01 cc/min for sity and viscosity. For this reason, it
devices from igniting upon contact liquids and from 0.5 cc/min for is necessary to size the rotameter
with combustibles. gases. The most common size is a for each application. When sized
Purge meters are quite useful in glass tube rotameter with G-in (6 correctly, the flow rate can be deter-
adding nitrogen gas to the vapor mm) connections, a range of 0.05- mined by matching the float position
spaces of tanks and other equip- 0.5 gpm (0.2-2.0 lpm) on water or to a calibrated scale on the outside
ment. Purging with nitrogen gas 0.2-2.0 scfm (0.3-3.0 cmph) in air ser- of the rotameter. Many rotameters
reduces the possibility of developing vice. Typical accuracy is ±5% FS over come with a built-in valve for adjust-
a flammable mixture because it dis- a 10:1 range, and the most common ing flow manually.
places flammable gases. The purge- pressure rating is 150 psig (1 MPa). Several shapes of float are avail-
flow regulator is reliable, intrinsically able for various applications. One
safe, and inexpensive. • Rotameters early design had slots, which caused
As shown in Figure 2-16, purge The rotameter is the most wide- the float to spin for stabilizing and
meters can operate in the con- ly used variable area flowmeter centering purposes. Because this
stant flow mode, where P2 - P0 is because of its low cost, simplicity, float rotated, the term rotameter
held constant at about 60 to 80 in low pressure drop, relatively wide was coined.
Rotameters are typically provided or stainless steel for corrosion resis- substantially reduces the operating
with calibration data and a direct tance. Other float materials include pressure of the tube. In general, there
reading scale for air or water (or carboloy, sapphire, and tantalum. is a linear relationship between oper-
both). To size a rotameter for other End fittings are available in metal or ating temperature and pressure.
service, one must first convert the plastic. Some fluids attack the glass Glass-tube rotameters are often
actual flow to a standard flow. For metering tube, such as wet steam used in applications where several
liquids, this standard flow is the water or high-pH water over 194°F (which streams of gases or liquids are being
equivalent in gpm; for gases, the stan- can soften glass); caustic soda (which metered at the same time or mixed
dard flow is the air flow equivalent in dissolves glass); and hydrofluoric acid in a manifold, or where a single fluid
standard cubic feet per minute (scfm). (which etches glass). is being exhausted through several
Tables listing standard water equiva- Floats have a sharp edge at the
lent gpm and/or air scfm values are point where the reading should be
provided by rotameter manufactur- observed on the tube-mounted
ers. Manufacturers also often provide scale. For improved reading accu-
slide rules, nomographs, or computer racy, a glass-tube rotameter should
software for rotameter sizing. be installed at eye level. The scale
can be calibrated for direct reading
• Design Variations of air or water, or can read percent-
A wide choice of materials is avail- age of range. In general, glass tube
able for floats, packing, O-rings, and rotameters can measure flows up to
end fittings. Rotameter tubes for such about 60 gpm water and 200 scfh air.
safe applications as air or water can A correlation rotameter has a
scale from which a reading is taken
(Figure 2-15). This reading is then
compared to a correlation table for
a given gas or liquid to get the actual channels (Figure 2-17). Multiple tube
flow in engineering units. Correlation flowmeters allow up to six rotame-
charts are readily available for nitro- ters to be mounted in the same frame.
gen, oxygen, hydrogen, helium, It also is possible to operate a
argon, and carbon dioxide. While rotameter in a vacuum. If the rota-
not nearly as convenient as a direct meter has a valve, it must be placed
reading device, a correlation meter at the outlet at the top of the
is more accurate. This is because a meter. For applications requiring a
direct-reading device is accurate for wide measurement range, a dual-ball
only one specific gas or liquid at a rotameter can be used. This instru-
particular temperature and pressure. ment has two ball floats: a light ball
A correlation flowmeter can be used (typically black) for indicating low
with a wide variety of fluids and flows and a heavy ball (usually white)
gases under various conditions. In for indicating high flows. The black
Rotameters can be specified in a wide range of the same tube, different flow rates ball is read until it goes off scale,
sizes and materials. can be handled by using different and then the white ball is read. One
floats. such instrument has a black measuring
be made of glass, whereas if breakage Small glass tube rotameters are suit- range from 235-2,350 ml/min and a
would create an unsafe condition, able for working with pressures up to white to 5,000 ml/min.
they are provided with metal tubes. 500 psig, but the maximum operating For higher pressures and tempera-
Glass tubes are most common, being pressure of a large (2-in diameter) tube tures beyond the practical range of
precision formed of safety shielded may be as low as 100 psig. The practi- glass, metal tube rotameters can be
borosilicate glass. Floats typically are cal temperature limit is about 400°F, used. These tubes are usually made
machined from glass, plastic, metal, but such high-temperature operation of stainless steel, and the position
valve opening while observing the
scale to establish the required pro-
cess flow rate. If operating conditions
remain unaltered, rotameters can be
repeatable to within 0.25% of the
actual flow rate.
Most rotameters are relatively
insensitive to viscosity variations.
The most sensitive are very small
rotameters with ball floats, while
larger rotameters are less sensitive
to viscosity effects. The limitations
of each design are published by
the manufacturer (Figure 2-18). The
float shape does affect the viscos-
ity limit. If the viscosity limit is
exceeded, the indicated flow must
be corrected for viscosity.
Because the float is sensitive to
changes in fluid density, a rotameter
can be furnished with two floats
(one sensitive to density, the other
to velocity) and used to approximate
the mass flow rate. The more closely
of the float is detected by magnetic helix that moves the pointer. The the float density matches the fluid
followers with readouts outside the transmitter can be intrinsically safe, density, the greater the effect of a
metering tube. microprocessor-based, and can be fluid density change will be on the
Metal-tube rotameters can be provided with alarms and a pulse float position. Mass-flow rotameters
used for hot and strong alkalis, fluo- output for totalization. work best with low viscosity fluids
rine, hydrofluoric acid, hot water, Plastic-tube rotameters are rela- such as raw sugar juice, gasoline, jet
steam, slurries, sour gas, additives, tively low cost rotameters that are fuel, and light hydrocarbons.
and molten metals. They also can ideal for applications involving cor- Rotameter accuracy is not affect-
be used in applications where high rosive fluids or deionized water. The ed by the upstream piping configura-
operating pressures, water hammer, tube itself can be made from Teflon® tion. The meter also can be installed
or other forces could damage glass PFA, polysulfone, or polyamide. The directly after a pipe elbow without
tubes. Metal-tube rotameters are wetted parts can be made from stain- adverse effect on metering accura-
available in diameter sizes from K in less steel, PVDF, or Teflon® PFA, PTFE, cy. Rotameters are inherently self
to 4 in, can operate at pressures up PCTFE, with Viton® or Kalrez® O-rings. cleaning because, as the fluid flows
to 750 psig, temperatures to 540°C between the tube wall and the float,
(1,000°F), and can measure flows up • Accuracy it produces a scouring action that
to 4,000 gpm of water or 1,300 scfm Laboratory rotameters can be cali- tends to prevent the buildup of for-
of air. Metal-tube rotameters are brated to an accuracy of 0.50% AR eign matter. Nevertheless, rotame-
readily available as flow transmit- over a 4:1 range, while the inaccuracy ters should be used only on clean
ters for integration with remote ana- of industrial rotameters is typically fluids which do not coat the float or
log or digital controls. Transmitters 1-2% FS over a 10:1 range. Purge and the tube. Liquids with fibrous mate-
usually detect the float position bypass rotameter errors are in the rials, abrasives, and large particles
through magnetic coupling and are 5% range. should also be avoided.
often provided with external indica- Rotameters can be used to manu-
tion through a rotatable magnetic ally set flow rates by adjusting the • Other Variable-Area Flowmeters
Major disadvantages of the rotameter design has a metallic meter body and a mechanical connection pro-
are its relatively high cost in larger for applications up to 1,000 psig. vides local flow rate indication. The
sizes and the requirement that it be One gate-type variable-area inaccuracy of such meters is 2-5%
installed vertically (there may not flow-meter resembles a butterfly FS. The meter can be used on oil,
be enough head room). The cost of valve. Flow through the meter forc- water and air, and is available in
a large rotameter installation can be es a spring-loaded vane to rotate, sizes up to 4 inches. It also is used as
reduced by using an orifice bypass
or a pitot tube in combination with
a smaller rotameter. The same-size References & Further Reading
bypass rotameter can be used to • OMEGA Complete Flow and Level Measurement Handbook and
measure a variety of flows, with Encyclopedia®, Omega Press, 1995.
the only difference between applica- • Omega Volume 29 Handbook & Encyclopedia, Purchasing Agents
tions being the orifice plate and the Edition, Omega Press, 1995.
differential it produces. • “Choices Abound in Flow Measurement”, D. Ginesi, Chemical Engineering,
Advantages of a bypass rotameter April 1991.
include low cost; its major disadvan- • “Developments in DP Flowmeters,” Jesse Yoder, Control, April 1998.
tage is inaccuracy and sensitivity to • Differential Producers - Orifice, Nozzle, Venturi, ANSI/ASME MFC,
material build-up. Bypass rotameters December 1983.
are often provided with isolation • Flow Measurement Engineers’ Handbook, R.W. Miller, McGraw-Hill, 1996.
valves so that they can be removed • Flow Measurement, D.W. Spitzer, Instrument Society of America, 1991.
for maintenance without shutting • Flow of Water Through Orifices, AGA/ASME, Ohio State Univ. Bulletin
down the process line. 89, Vol. IV, No. 3.
Tapered plug flowmeters are vari- • Fluid Meters, H.S. Bean , American Society of Mechanical Engineers, 1971.
able-area flowmeters with a station- • Fundamentals of Flow Measurement, J. P. DeCarlo, Instrument Society of
ary core and a piston that moves America, 1984.
as the flow varies. In one design, • Instrument Engineers Handbook, 3rd edition, Bela Liptak, CRC Press, 1995.
the piston movement mechanically • “Orifice Metering of Natural Gas”, AGA Report 3, 1985.
moves a pointer, while in another • “Primary Element Solves Difficult Flow Metering Problems at Water
it magnetically moves an external Waste Treatment Plant,” D. Ginesi, L. Keefe, and P. Miller, Proceedings of
flow rate indicator. The second ISA 1989, Instrument Society of America, 1989.
Mechanical Flowmeters
D iscussed in this chapter are
various types of mechanical
flowmeters that measure
flow using an arrangement
of moving parts, either by passing
isolated, known volumes of a fluid
angles to the flow, suspended in the
fluid stream on a free-running bear-
ing. The diameter of the rotor is very
close to the inside diameter of the
metering chamber, and its speed of
rotation is proportional to the volu-
is reduced and metering accuracy is
therefore increased as the viscosity of
the process fluid increases.
The process fluid must be clean.
Particles greater than 100 microns in
size must be removed by filtering. PD
through a series of gears or chambers metric flow rate. Turbine rotation can meters operate with small clearanc-
(positive displacement, or PD) or by be detected by solid state devices or es between their precision-machined
means of a spinning turbine or rotor. by mechanical sensors. Other types parts; wear rapidly destroys their
All positive displacement flowme- of rotary element flowmeters include accuracy. For this reason, PD meters
ters operate by isolating and count- the propeller (impeller), shunt, and are generally not recommended for
ing known volumes of a fluid (gas paddlewheel designs. measuring slurries or abrasive fluids.
or liquid) while feeding it through In clean fluid services, however, their
the meter. By counting the number Positive Displacement Flowmeters precision and wide rangeability make
of passed isolated volumes, a flow Positive displacement meters provide them ideal for custody transfer and
measurement is obtained. Each PD high accuracy (±0.1% of actual flow batch charging. They are most widely
design uses a different means of rate in some cases) and good repeat- used as household water meters.
isolating and counting these vol- ability (as high as 0.05% of reading). Millions of such units are produced
umes. The frequency of the result- Accuracy is not affected by pulsating annually at a unit cost of less than
ing pulse train is a measure of flow flow unless it entrains air or gas in $50 U.S. In industrial and petrochem-
rate, while the total number of pulses the fluid. PD meters do not require ical applications, PD meters are com-
gives the size of the batch. While PD a power supply for their operation monly used for batch charging of
meters are operated by the kinetic and do not require straight upstream both liquids and gases.
energy of the flowing fluid, meter- and downstream pipe runs for their Although slippage through the PD
ing pumps (described only briefly in installation. PD meters are available meter decreases (that is, accuracy
this article) determine the flow rate in sizes from G in to 12 in and can increases) as fluid viscosity increases,
while also adding kinetic energy to operate with turndowns as high as pressure drop through the meter
the fluid. 100:1, although ranges of 15:1 or lower also rises. Consequently, the maxi-
The turbine flowmeter consists of are much more common. Slippage mum (and minimum) flow capac-
a multi-bladed rotor mounted at right between the flowmeter components ity of the flowmeter is decreased
as viscosity increases. The higher Because it must be nonmagnetic, racy of these meters is required
the viscosity, the less slippage and the meter housing is usually made of to be ±2% of actual flow rate.
the lower the measurable flow rate bronze but can be made from plas- Higher viscosity can produce higher
becomes. As viscosity decreases, the tic for corrosion resistance or cost accuracy, while lower viscosity and
low flow performance of the meter savings. The wetted parts such as the wear over time will reduce accuracy.
deteriorates. The maximum allow- disc and spindle are usually bronze, The AWWA requires that residential
able pressure drop across the meter rubber, aluminum, neoprene, Buna-N, water meters be re-calibrated every
constrains the maximum operating or a fluoroelastomer such as Viton®. 10 years. Because of the intermittent
flow in high viscosity services. Nutating disc meters are designed use patterns of residential users,
for water service and the materials this corresponds to recalibrating L
• Liquid PD Meters of which they are made must be x I in residential water meters after
Nutating disc meters are the most checked for compatibility with other they have metered 5 million gallons.
common PD meters. They are used as fluids. Meters with rubber discs give In industrial applications, however,
residential water meters around the better accuracy than metal discs due these meters are likely to pass this
world. As water flows through the to the better sealing they provide. threshold much sooner. The maxi-
metering chamber, it causes a disc Nutating disc meters are available mum continuous flow of a nutating
to wobble (nutate), turning a spindle, in L-in to 2-in sizes. They are suited disc meter is usually about 60-80%
which rotates a magnet. This magnet for 150-psig operating pressures with of the maximum flow in intermittent
is coupled to a mechanical register overpressure to a maximum of 300 service.
or a pulse transmitter. Because the psig. Cold water service units are Rotating vane meters (Figure 3-1B)
flowmeter entraps a fixed quantity temperature-limited to 120°F. Hot have spring-loaded vanes that entrap
of fluid each time the spindle is water units are available up to 250°F. increments of liquid between the
rotated, the rate of flow is propor- These meters must meet American eccentrically mounted rotor and the
tional to the rotational velocity of Water Works Association (AWWA) casing. The rotation of the vanes
the spindle (Figure 3-1A). standards for accuracy. The accu- moves the flow increment from inlet
to outlet and discharge. Accuracy of mesh or -74 micron) sand, but not used between 100 and 150 psig. Some
±0.1% of actual rate (AR) is normal, large particle size or abrasive solids. industrial versions are rated to 1,500
and larger size meters on higher vis- The measurement chamber is psig. They can meter flow rates from
cosity services can achieve accuracy cylindrical with a partition plate sep- 1 gpm to 65 gpm in continuous ser-
to within 0.05% of rate. arating its inlet port from its outlet. vice with intermittent excursions to
Rotating vane meters are regularly The piston is also cylindrical and is 100 gpm. Meters are sized so that
used in the petroleum industry and punctured by numerous openings pressure drop is below 35 psid at
are capable of metering solids-laden to allow free flow on both sides of maximum flow rate. Accuracy ranges
crude oils at flow rates as high as the piston and the post (Figure 3-2A). from ±0.5 % AR for viscous fluids to
17,500 gpm. Pressure and tempera- The piston is guided by a control ±2% AR for nonviscous applications.
ture limits depend on the materials roller within the measuring cham- Upper limit on viscosity is 10,000
of construction and can be as high as ber, and the motion of the piston centipoise.
350°F and 1,000 psig. Viscosity limits is transferred to a follower magnet Reciprocating piston meters are
are 1 to 25,000 centipoise. which is external to the flowstream. probably the oldest PD meter designs.
In the rotary displacement meter, The follower magnet can be used to They are available with multiple pis-
a fluted central rotor operates in drive either a transmitter, a register, tons, double-acting pistons, or rotary
constant relationship with two wiper or both. The motion of the piston pistons. As in a reciprocating piston
rotors in a six-phase cycle. Its appli- is oscillatory (not rotary) since it is engine, fluid is drawn into one pis-
cations and features are similar to constrained to move in one plane. ton chamber as it is discharged from
those of the rotary vane meter. The rate of flow is proportional to the opposed piston in the meter.
the rate of oscillation of the piston. Typically, either a crankshaft or a
• Piston Meters The internals of this flowmeter horizontal slide is used to control the
Oscillating piston flowmeters typical- can be removed without disconnec- opening and closing of the proper
ly are used in viscous fluid services tion of the meter from the pipe- orifices in the meter. These meters
such as oil metering on engine test line. Because of the close tolerances are usually smaller (available in sizes
stands where turndown is not critical required to seal the piston and to down to 1/10-in diameter) and are
(Figure 3-2). These meters also can be reduce slippage, these meters require used for measuring very low flows of
used on residential water service and regular maintenance. Oscillating pis- viscous liquids.
can pass limited quantities of dirt, ton flow meters are available in H-in
such as pipe scale and fine (viz,-200 to 3-in sizes, and can generally be • Gear & Lobe Meters
The oval gear PD meter uses two gpm in the larger sizes. They pro- In these units, the passage of mag-
fine-toothed gears, one mounted vide good repeatability (better than nets embedded in the lobes of the
horizontally, the other vertically, 0.015% AR) at high flows and can rotating impellers is sensed by prox-
with gears meshing at the tip of the be used at high operating pressures imity switches (usually Hall-effect
vertical gear and the center of the (to 1,200 psig) and temperatures (to detectors) mounted external to the
horizontal gear (Figure 3-3A). The 400°F). flow chamber. The sensor transmits
two rotors rotate opposite to each The lobe gear meter is available a pulse train to a counter or flow
other, creating an entrapment in the in a wide range of materials of con- controller. These meters are avail-
crescent-shaped gap between the struction, from thermoplastics to able in 1/10-in to 6-in sizes and can
housing and the gear. These meters highly corrosion-resistant metals. handle pressures to 3,000 psig and
can be very accurate if slippage Disadvantages of this design include temperatures to 400°F.
between the housing and the gears a loss of accuracy at low flows. Also,
is kept small. If the process fluid the maximum flow through this meter • Helix Meters
viscosity is greater than 10 centipoise is less than for the same size oscilla- The helix meter is a positive dis-
and the flowrate is above 20% of tory piston or nutating disc meter. placement device that uses two radi-
rated capacity, accuracy of 0.1% AR In the rotating impeller meter, ally pitched helical gears to continu-
can be obtained. At lower flows and very coarse gears entrap the fluid ously entrap the process fluid as it
at lower viscosity, slippage increases and pass a fixed volume of fluid flows. The flow forces the helical
and accuracy decreases to 0.5% AR with each rotation (Figure 3-3C). gears to rotate in the plane of the
or less. These meters are accurate to 0.5% pipeline. Optical or magnetic sen-
The lubricating characteristics of of rate if the viscosity of the pro- sors are used to encode a pulse
the process fluid also affect the cess fluid is both high and constant, train proportional to the rotational
turndown of an oval gear meter. or varies only within a narrow band. speed of the helical gears. The forces
With liquids that do not lubricate These meters can be made out of required to make the helices rotate
well, maximum rotor speed must be a variety of metals, including stain- are relatively small and therefore, in
derated to limit wear. Another way less steel, and corrosion-resistant comparison to other PD meters, the
to limit wear is to keep the pressure plastics such as PVDF (Kynar). These pressure drop is relatively low. The
drop across the meter below 15 psid.
Therefore, the pressure drop across
the meter limits the allowable maxi-
mum flow in high viscosity service.
Rotating lobe and impeller type
PD meters are variations of the oval
gear flowmeter that do not share
its precise gearing. In the rotating
lobe design, two impellers rotate in
opposite directions within the ovoid
housing (Figure 3-3B). As they rotate,
a fixed volume of liquid is entrapped
and then transported toward the
outlet. Because the lobe gears
remain in a fixed relative position,
it is only necessary to measure the
rotational velocity of one of them.
The impeller is either geared to a
register or is magnetically coupled meters are used to meter paints best attainable accuracy is about
to a transmitter. Lobe meters can be and, because they are available in 3A ±0.2% or rate.
furnished in 2-in to 24-in line sizes. or sanitary designs, also milk, juices, As shown in Figure 3-4, measure-
Flow capacity is 8-10 gpm to 18,000 and chocolate. ment error rises as either the oper-
ating flowrate or the viscosity of squeeze a plastic tubing against the the basis of the displacement of the
the process fluid drops. Helical gear housing, which also serves to position piston and the required flow rate
meters can measure the flow of highly the tubing. This type of metering pump and discharge pressure. Check valves
viscous fluids (from 3 to 300,000 cP), is used in laboratories, in a variety of (or, on critical applications, double
making them ideal for extremely medical applications, in the majority check valves) are selected to protect
thick fluids such as glues and very of environmental sampling systems, against backflow.
viscous polymers. Because at maxi- and also in dispensing hypochlorite Diaphragm pumps are the most
mum flow the pressure drop through solutions. The tubing can be silicone- common industrial PD pumps (Figure
the meter should not exceed 30 psid, rubber or, if a more corrosion-resistant 3-5B). A typical configuration con-
the maximum rated flow through the material is desired, PTFE tubing. sists of a single diaphragm, a cham-
meter is reduced as the fluid viscos- Piston pumps deliver a fixed vol- ber, and suction and discharge check
ity increases. If the process fluid has ume of liquid with each “out” stroke valves to prevent backflow. The pis-
good lubricating characteristics, the and a fixed volume enters the cham- ton can either be directly coupled
meter turndown can be as high as ber on each “in” stroke (Figure 3-5A). to the diaphragm or can force a
100:1, but lower (10:1) turndowns are Check valves keep the fluid flow hydraulic oil to drive the diaphragm.
more typical. from reversing. As with all positive Maximum output pressure is about
displacement pumps, piston pumps 125 psig. Variations include bellows-
• Metering Pumps generate a pulsating flow. To mini- type diaphragms, hydraulically actu-
Metering pumps are PD meters that mize the pulsation, multiple pistons ated double diaphragms, and air-
also impart kinetic energy to the or pulsation-dampening reservoirs operated, reciprocating double-dia-
process fluid. There are three basic are installed. Because of the close phragms.
designs: peristaltic, piston, and dia- tolerances of the piston and cyl-
phragm. inder sleeve, a flushing mechanism • Gas PD Meters
Peristaltic pumps operate by hav- must be provided in abrasive appli- PD gas meters operate by counting
ing fingers or a cam systematically cations. Piston pumps are sized on the number of entrapped volumes
of gas passed, similar to the way crank mechanism for the meter regis- ally dirty (producer gas, or recycled
PD meters operate on liquids. The ter. methane from composting or digest-
primary difference is that gases are Diaphragm meters generally are ing, for example), the diaphragm
compressible. calibrated for natural gas, which has meter will operate with little or no
Diaphragm gas meters most often a specific gravity of 0.6 (relative to maintenance indefinitely.
are used to measure the flow of air). Therefore, it is necessary to re- Lobe gear meters (or lobed
natural gas, especially in metering calibrate the flow rating of the meter impeller meters, as they are also
consumption by households. The when it is used to meter other gases. known), also are used for gas service.
meter is constructed from aluminum The calibration for the new flow rat- Accuracy in gas service is ±1% of rate
castings with cloth-backed rubber ing (QN) is obtained by multiplying over a 10:1 turndown, and typical
diaphragms. The meter consists of the meter’s flow rating for natural pressure drop is 0.1 psid. Because of
four chambers: the two diaphragm gas (QC) by the square root of the the close tolerances, upstream filtra-
chambers on the inlet and outlet ratio of the specific gravities of natu- tion is required for dirty lines.
sides and the inlet and outlet cham- ral gas (0.6) and the new gas (SGN): Rotating vane meters measure the
bers of the meter body. The passage flow of gas in the same ranges as do
of gas through the meter creates QN= QC(0.6/SGN)0.5 lobe gear meters (up to 100,000 ft3/
a differential pressure between the hr) but can be used over a wider 25:1
two diaphragm chambers by com- Diaphragm meters are usually turndown. They also incur a lower
pressing the one on the inlet side rated in units of cubic feet per hour pressure drop of 0.05 in H2O for simi-
and expanding the one on the outlet and sized for a pressure drop of lar accuracy, and, because the clear-
side. This action alternately emp- 0.5-2 in H2O. Accuracy is roughly ±1% ances are somewhat more forgiving,
ties and fills the four chambers. The of reading over a 200:1 range. They upstream filtration is not as critical.
slide valves at the top of the meter maintain their accuracy for long peri-
alternate the roles of the chambers ods of time, which makes them good • High-Precision PD Systems
and synchronize the action of the choices for retail revenue metering High-precision gas meters are usually
diaphragms, as well as operating the applications. Unless the gas is unusu- a hybrid combining a standard PD
meter and a motor drive that elimi- stands for fuel flow measurement the petroleum industry, it is difficult
nates the pressure drop across the (gasoline, diesel, alcohol, etc.). Flow or impossible to remove a flow-
meter. Equalizing the inlet and outlet ranges from 0.04-40 gph are typical. meter from service for calibration.
pressures eliminates slip flows, leak- Vapor separators are usually includ- Therefore, field-mounted and in-line
age, and blow-by. In high-precision ed, to prevent vapor lock. provers have been developed. This
gas flowmeter installations, high- type of prover consists of a cali-
sensitivity leaves are used to detect • Testing, Calibration & Provers brated chamber equipped with a bar-
the pressure differential, and dis- All meters with moving parts require rier piston (Figure 3-7). Two detec-
tors are mounted a known distance
(and therefore a known volume)
apart. As the flow passes through
the chamber, the displacer piston
is moved downstream. Dividing the
volume of the chamber by the time
it takes for the displacer to move
from one detector to the other gives
the calibrated flow rate. This rate is
then compared to the reading of the
flowmeter under test.
Provers are repeatable on the
order of 0.02%, and can operate
at up to 3,000 psig and 165°F/75°C.
Their operating flow range is from
as low as 0.001 gpm to as high as
20,000 gpm. Provers are available
for bench-top use, for mounting in
placement transducers are used to periodic testing, recalibration and truck-beds, on trailers, or in-line.
measure the deflection of the leaves repair, because wear increases the
(Figure 3-6A). Designed to operate clearances. Recalibration can be • PD Meter Accessories
at ambient temperatures and at up done either in a laboratory or on line PD meter accessories include
to 30 psig pressures, this meter is using a prover. strainers, filters, air/vapor release
claimed to provide accuracy to with- Gas systems are recalibrated assemblies, pulsation dampeners,
in 0.25% of reading over a 50:1 range against a bell-jar prover—a calibrated temperature compensation sys-
and 0.5% over a 100:1 range. Flow cylindrical bell, liquid sealed in a tank. tems, and a variety of valves to
capacity ranges from 0.3-1,500 scfm. As the bell is lowered, it discharges permit dribble cut-off in batching
For liquid service, a servomotor- a known volume of gas through the systems. Mechanical registers can
driven oval-gear meter equalizes meter being tested. The volumetric be equipped with mechanical or
the pressure across the meter. This accuracy of bell-jar provers is on the electronic ticket-printers for inven-
increases accuracy at low flows and order of 0.1% by volume, and provers tory control and point-of-use sales.
under varying viscosity conditions are available in discharge volumes of Batching flow computers are readily
(Figure 3-6B). This flowmeter uses a 2, 5, 10 ft3 and larger. available, as are analog and intel-
very sensitive piston to detect the Liquid systems can be calibrated ligent digital transmitters. Automatic
meter differential and drives a vari- in the laboratory against either a meter reading (AMR) devices permit
able speed servomotor to keep it calibrated secondary standard or a the remote retrieval of readings by
near zero. This design is claimed to gravimetric flow loop. This approach utility personnel.
provide 0.25% of rate accuracy over can provide high accuracy (up to
a 50:1 range at operating pressures ±0.01% of rate) but requires remov- Turbine Flowmeters
of up to 150 psig. High precision ing the flowmeter from service. Invented by Reinhard Woltman in
flowmeters are used on engine test In many operations, especially in the 18th century, the turbine flow-
meter is an accurate and reliable permanent magnet is embedded in transistors also can be used. These
flowmeter for both liquids and gases. the rotor, or the blades of the rotor transistors change their state when
It consists of a multi-bladed rotor are made of permanently magnetized they are in the presence of a very
mounted at right angles to the flow material (Figure 3-8B). As each blade low strength (on the order of 25
and suspended in the fluid stream on passes the coil, it generates a voltage gauss) magnetic field.
a free-running bearing. The diameter pulse. In some designs, only one blade In these turbine flowmeters, very
of the rotor is very slightly less than is magnetic and the pulse represents a small magnets are embedded in the
the inside diameter of the metering complete revolution of the rotor. tips of the rotor blades. Rotors are typ-
chamber, and its speed of rotation is The outputs of reluctance and ically made of a non-magnetic mate-
proportional to the volumetric flow inductive pick-up coils are continu- rial, like polypropylene, Ryton, or PVDF
rate. Turbine rotation can be detect- ous sine waves with the pulse train’s (Kynar). The signal output from a Hall-
ed by solid state devices (reluctance, frequency proportional to the flow effect sensor is a square wave pulse
inductance, capacitive and Hall- rate. At low flow, the output (the train, at a frequency proportional to
effect pick-ups) or by mechanical height of the voltage pulse) may be the volumetric flowrate.
sensors (gear or magnetic drives). on the order of 20 mV peak-to-peak. Because Hall-effect sensors have
In the reluctance pick-up, the coil It is not advisable to transport such no magnetic drag, they can operate
is a permanent magnet and the tur- a weak signal over long distances. at lower flow velocities (0.2 ft/sec)
bine blades are made of a material Therefore, the distance between than magnetic pick-up designs (0.5-1.0
attracted to magnets. As each blade the pickup and associated display ft/sec). In addition, the Hall-effect
passes the coil, a voltage is gener- electronics or preamplifier must be sensor provides a signal of high ampli-
ated in the coil (Figure 3-8A). Each short. tude (typically a 10.8-V square wave),
pulse represents a discrete volume Capacitive sensors produce a permitting distances up to 3,000 ft.
of liquid. The number of pulses per sine wave by generating an RF sig- between the sensor and the electron-
unit volume is called the meter’s nal that is amplitude-modulated by ics without amplification.
K-factor. the movement of the rotor blades. In the water distribution industry,
In the inductance pick-up, the Instead of pick-up coils, Hall-effect mechanical-drive Woltman-type tur-
bine flowmeters continue to be the of turbine flowmeters. Class I tur- are manufactured from austenitic
standard. These turbine meters use a bine meters must register between stainless steel (301, 303, 304SS),
gear train to convert the rotation of 98-102% of actual rate at maximum whereas turbine meters intended for
the rotor into the rotation of a verti- flow when tested. Class II turbine municipal water service are bronze
cal shaft. The shaft passes between meters must register between 98.5- or cast iron. The rotor and bearing
the metering tube and the register 101.5% of actual rate. Both Class materials are selected to match the
process fluid and the service. Rotors
are often made from stainless steel,
and bearings of graphite, tungsten
carbide, ceramics, or in special
cases of synthetic ruby or sapphire
combined with tungsten carbide.
In all cases, bearings and shafts
are designed to provide minimum
friction and maximum resistance
to wear. Some corrosion-resistant
designs are made from plastic mate-
rials such as PVC.
B Small turbine meters often are
called barstock turbines because
in sizes of I in to 3 in. they are
machined from stainless steel hex-
agonal barstock. The turbine is sus-
pended by a bearing between two
hanger assemblies that also serve
section through a mechanical stuff- I and Class II meters must have to condition the flow. This design is
ing box, turning a geared mechanical mechanical registers. suited for high operating pressures
register assembly to indicate flow Solid state pickup designs are less (up to 5,000 psig).
rate and actuate a mechanical total- susceptible to mechanical wear than Similar to a pitot tube differential
izer counter. AWWA Class I and Class II meters. pressure flowmeter, the insertion tur-
More recently, the water distribu- bine meter is a point-velocity device.
tion industry has adopted a magnetic • Design & Construction Variations It is designed to be inserted into
drive as an improvement over high Most industrial turbine flowmeters either a liquid or a gas line to a depth
maintenance mechanical-drive tur-
bine meters. This type of meter has a
sealing disc between the measuring
chamber and the register. On the
measuring chamber side, the verti-
cal shaft turns a magnet instead
of a gear. On the register side, an
opposing magnet is mounted to turn
the gear. This permits a completely
sealed register to be used with a
mechanical drive mechanism.
In the United States, the AWWA
sets the standards for turbine flow-
meters used in water distribution
systems. Standard C701 provides for This innovative turbine meter trades out a transmitted signal for local LCD indication.
two classes (Class I and Class II)
at which the small-diameter rotor over a 10:1 flow range and a ±0.15% larger turbine meters. If the range
will read the average velocity in the linearity in a 6:1 range. The repeat- of 10:1 is insufficient, some turbine
line. Because they are very sensitive ability is from ±0.2% to ±0.02% over flowmeters can provide up to 100:1
to the velocity profile of the flow- the linear range. turndowns if accuracy is de-rated to
ing stream, they must be profiled at Because there are minor incon- 1% of full scale (FS).
several points across the flow path. sistencies in the manufacturing
Insertion turbine meters can be process, all turbine flowmeters are • Sizing & Selection
designed for gas applications (small, calibrated prior to shipment. The Turbine meters should be sized so
lightweight rotor) or for liquid (larg- resulting K-factor in pulses per vol- that the expected average flow is
er rotor, water-lubricated bearings). ume unit will vary within the stated between 60% and 75% of the maxi-
They are often used in large diame- linearity specification. It is pos- mum capacity of the meter. If the
ter pipelines where it would be cost- sible, however, to register several pipe is oversized (with flow velocity
prohibitive to install a full size meter. K-factors for different portions of under 1 ft/sec), one should select a
They can be hot-tapped into existing the flow range and to electronically Hall-effect pick-up and use a meter
pipelines (6 in or larger) through switch from one to the other as the smaller than the line size. Flow veloci-
a valving system without shutting measured flow changes. Naturally, ties under 1 ft/sec can be insufficient,
down the process. Typical accuracy the K-factor is applicable only to while velocities in excess of 10 ft/
of an insertion turbine meter is 1% the fluid for which the meter was sec can result in excessive wear. Most
FS, and the minimum flow velocity is calibrated. turbine meters are designed for maxi-
about 0.2 ft/sec. Barstock turbine meters typically mum velocities of 30 ft/sec.
are linear to ±0.25% AR over a 10:1 Turbine flowmeters should be
• Turbine Meter Accuracy flow range. The linearity of larger sized for between 3 and 5 psid pres-
Figure 3-9 shows a typical turbine- meters is ±0.5% AR over a 10:1 flow sure drop at maximum flow. Because
meter calibration curve describing range. Turbine meters have a typi- pressure drop increases with the
the relationship between flow and cal nonlinearity (the turbine meter square of flow rate, reducing the
K-factor (pulses/gallon). The accu- hump, shown in Figure 3-9) in the meter to the next smaller size will
racy of turbine meters is typically lower 25-30% of their range. Keeping raise the pressure drop considerably.
given in percentage of actual rate the minimum flow reading above Viscosity affects the accuracy and
(% AR). This particular meter has a this region will permit linearity to linearity of turbine meters. It is there-
linearity tolerance band of ±0.25% within 0.15% on small and 0.25% on fore important to calibrate the meter
for the specific fluid it is intended to stream of the meter. However, the plus twice the pressure drop. Small
measure. Repeatability is generally not presence of any of the following amounts of air entrainment (100 mg/l
greatly affected by changes in viscos- obstructions upstream would neces- or less) will make the meter read only
ity, and turbine meters often are sitate that there be more than 15 a bit high, while large quantities can
used to control the flow of viscous diameters of upstream straight-pipe destroy the rotor.
fluids. Generally, turbine meters per- runs Turbine meters also can be dam-
form well if the Reynolds Number is • 20 diameters for 90° elbow, tee, aged by solids entrained in the
greater than 4,000 and less than or filter, strainer, or thermowell; fluid. If the amount of suspended
equal to 20,000. Because it affects vis- • 25 diameters for a partially open solids exceeds 100 mg/l of +75
cosity, temperature variation can also valve; and micron size, a flushing y-strainer or
adversely affect accuracy and must be • 50 or more diameters if there are a motorized cartridge filter must be
compensated for or controlled. The two elbows in different planes installed at least 20 diameters of
turbine meter’s operating temperature or if the flow is spiraling or cork- straight run upstream of the flow-
ranges from -200 to 450°C (-328 to screwing. meter.
840°F). In order to reduce this straight-
Density changes do not greatly run requirement, straightening • New Developments
affect turbine meters. On low density vanes are installed. Tube bundles Dual-rotor liquid turbines increase
fluids (SG < 0.7), the minimum flow or radial vane elements are used as the operating range in small line size
rate is increased due to the reduced external flow straighteners located (under 2 in) applications. The two
torque, but the meter’s accuracy at least 5 diameters upstream of the rotors turn in opposite directions.
usually is not affected. meter (Figure 3-10). The front one acts as a conditioner,
Under certain conditions, the pres- directing the flow to the back rotor.
• Installation & Accessories sure drop across the turbine can cause The rotors lock hydraulically and
Turbine meters are sensitive to flashing or cavitation. The first causes continue to turn as the flow decreases
upstream piping geometry that can the meter to read high, the second even to very low rates.
cause vortices and swirling flow. results in rotor damage. In order to The linearity of a turbine meter is
Specifications call for 10-15 diam- protect against this, the downstream affected by the velocity profile (often
eters of straight run upstream and pressure must be held at a value dictated by the installation), viscos-
five diameters of straight run down- equaling 1.25 times the vapor pressure ity, and temperature. It is now pos-
sible to include complex linearization larger and are accurate to ±2% over in the forward direction. For smaller
functions in the preamplifier of a a range of 10:1. pipes (H" to 3"), these meters are
turbine flowmeter to reduce these available only with a fixed inser-
nonlinearities. In addition, advances Other Rotary Flowmeters tion depth, while for larger pipe
in fieldbus technology make it possi- Other types of rotary element flow- sizes (4" to 48") adjustable inser-
ble to recalibrate turbine flowmeters meters include propeller (impeller), tion depths are available. The use
continuously, thereby correcting for shunt, and paddlewheel designs. of capacitively coupled pick-ups
changes in temperature and viscosity. Propeller meters are commonly or Hall-effect sensors extends the
Flow computers are capable of used in large diameter (over 4 in) range of paddlewheel meters into
linearization, automatic temperature irrigation and water distribution the low-flow velocity region of 0.3
compensation, batching, calculation systems. Their primary trade-off is ft/sec.
of BTU content, datalogging, and low cost and low accuracy (Figure Low-flow meters (usually smaller
storage of multiple K-factors. The 3-11A). AWWA Standard C-704 sets than 1 in.) have a small jet ori-
batching controller is set with the the accuracy criterion for propeller fice that projects the fluid onto a
desired target volume and, when meters at 2% of reading. Propeller Pelton wheel. Varying the diameter
its totalizer has counted down to meters have a rangeability of about and the shape of the jet orifice
zero, it terminates the batch. Such 4:1 and exhibit very poor perfor- matches the required flow range
packages are equipped with drib- mance if the velocity drops below and provides a flowmeter that
ble flow, pre-warn, or trickle-cut- 1.5 ft/sec. Most propeller meters are is accurate to 1% FS and has a
off circuits. Whether functioning equipped with mechanical registers. rangeability of 100:1. Higher accu-
through a relay contact or a ramp Mechanical wear, straightening, and racy can be achieved by calibrat-
function, these features serve to conditioning requirements are the ing the meter and by lowering
minimize splashing or overfill and to same as for turbine meters. its range. Because of the small size
accurately terminate the batch. Paddlewheel flowmeters use of the jet orifice, these meters can
a rotor whose axis of rotation is only be used on clean fluids and
• Gas Turbine & Shunt Meters parallel to the direction of flow they incur a pressure drop of about
Gas meters compensate for the (Figure 3-11B). Most paddlewheel 20 psid. Materials of construc-
lower driving torque produced by meters have flat-bladed rotors tion include polypropylene, PVDF,
the relatively low density of gases. and are inherently bi-directional. TFE and PFA, brass, aluminum, and
This compensation is obtained by Several manufacturers, however, stainless steel. T
very large rotor hubs, very light rotor use crooked rotors that only rotate
assemblies, and larger numbers of
rotor blades. Gas turbine meters are
available from 2" to 12" and with flow References & Further Reading
ratings up to 150,000 ft3/hr. When • OMEGA Complete Flow and Level Measurement Handbook and
operating at elevated gas pressures Encyclopedia®, Omega Press, 1995.
(1,400 psig), a rangeability of 100:1 • Omega Volume 29 Handbook & Encyclopedia, Purchasing Agents
can be obtained in larger size meters. Edition, Omega Press, 1995.
Under lower pressure conditions, • Flow Measurement Engineering Handbook, Miller, McGraw-Hill, 1982.
typical rangeability is 20:1 with ±1% • Flow Measurement, D. W. Spitzer, ISA, 1991.
linearity. The minimum upstream • Flowmeters in Water Supply, Manual M33, AWWA, 1989.
straight pipe-run requirement is 20 • Industrial Flow Measurement, D. W. Spitzer, ISA 1984.
pipe diameters. • Instrument Engineer’s Handbook, Bela Liptak, editor, CRC Press, 1995.
Shunt flowmeters are used in gas • “Turbine Flowmeter Extends Flow Range”, E. Piechota, Flow Control,
and steam service. They consist of February, 1997.
an orifice in the main line and a • Water Meters—Selection, Installation, Testing and Maintenance, Manual
rotor assembly in the bypass. These M6, AWWA, 1986.
meters are available is sizes 2 in. and
Electronic Flowmeters
W hile the flow measure-
ment technologies
discussed in this chap-
ter—magnetic, vortex,
and ultrasonic—are neither exclu-
sively nor exhaustively electronic in
of voltage (E) developed across the
electrodes—as predicted by Faraday’s
law—will be proportional to the
velocity (V) of the liquid. Because the
magnetic field density and the pipe
diameter are fixed values, they can be
signal.
The K value obtained by water
testing might not be valid for non-
Newtonian fluids (with velocity-
dependent viscosity) or magnetic
slurries (those containing magnetic
nature, they do represent a logical combined into a calibration factor (K) particles). These types of fluids can
grouping of flow measurement tech- and the equation reduces to: affect the density of the magnetic
nologies. All have no moving parts field in the tube. In-line calibration
(well, maybe vibrating), are relatively E = KV and special compensating designs
non-intrusive, and are made possible should be considered for both of
by today’s sophisticated electronics The velocity differences at dif- these fluids.
technology. ferent points of the flow profile are
Magnetic flowmeters, for example, compensated for by a signal-weigh- • Magmeter Excitation
are the most directly electrical in ing factor. Compensation is also pro- The voltage that develops at the
nature, deriving their first principles vided by shaping the magnetic coils electrodes is a millivolt signal. This
of operation from Faraday’s law. such that the magnetic flux will be signal is typically converted into a
Vortex meters depend on piezoelec- greatest where the signal weighing standard current (4-20 mA) or fre-
tric sensors to detect vortices shed factor is lowest, and vice versa. quency output (0-10,000 Hz) at or
from a stationary shedder bar. And Manufacturers determine each near the flowtube. Intelligent mag-
today’s ultrasonic flowmeters owe magmeter’s K factor by water calibra- netic transmitters with digital out-
their successful application to sophis- tion of each flowtube. The K value puts allow direct connection to a
ticated digital signal processing. thus obtained is valid for any other distributed control system. Because
conductive liquid and is linear over the the magmeter signal is a weak one,
Magnetic Flowmeters entire flowmeter range. For this rea- the lead wire should be shielded and
The operation of magnetic flow- son, flowtubes are usually calibrated twisted if the transmitter is remote.
meters is based on Faraday’s law at only one velocity. Magmeters can The magmeter’s coils can be pow-
of electromagnetic induction. measure flow in both directions, as ered by either alternating or direct
Magmeters can detect the flow of reversing direction will change the current (Figure 4-2). When ac excita-
conductive fluids only. Early magme- polarity but not the magnitude of the tion is used, line voltage is applied
ter designs required a minimum flu-
idic conductivity of 1-5 microsiemens
per centimeter for their operation.
The newer designs have reduced
that requirement a hundredfold to
between 0.05 and 0.1.
The magnetic flowmeter consists
of a non-magnetic pipe lined with an
insulating material. A pair of magnetic
coils is situated as shown in Figure 4-1,
and a pair of electrodes penetrates
the pipe and its lining. If a conductive
fluid flows through a pipe of diameter
(D) through a magnetic field density
(B) generated by the coils, the amount
to the magnetic coils. As a result, zero stability problems of traditional ally meet these guidelines as well.
the flow signal (at constant flow) ac designs. These devices contain Magnetic flowtubes and liners are
will also look like a sine wave. The circuitry that periodically disrupts available in many materials and are
amplitude of the wave is propor- the ac power, automatically zeroing widely used in all the process indus-
tional to velocity. In addition to the out the effects of process noise on tries, including food, pharmaceutical,
flow signal, noise voltages can be the output signal. mining, and metals.
induced in the electrode loop. Out- Today, dc excitation is used in Some liner materials (particularly
of-phase noise is easily filtered, but about 85% of installations and ac Teflon®) can be damaged when pry
in-phase noise requires that the flow magmeters claim the other 15% when bars are used while installing it or
be stopped (with the pipe full) and removing it from process piping.
the transmitter output set to zero. They can also be damaged by over-
The main problem with ac magmeter torquing the flange bolts. Liner pro-
designs is that noise can vary with tectors are available to help prevent
process conditions and frequent re- such damage.
zeroing is required to maintain accu- Any flowtube can generally be
racy. used with any transmitter offered by
In dc excitation designs, a low fre- the same manufacturer. Depending
quency (7-30 Hz) dc pulse is used to on its construction and features, the
excite the magnetic coils. When the cost of a 2-in. magnetic flowmeter
coils are pulsed on (Figure 4-2), the can range from $1,500 to $5,000.
transmitter reads both the flow and This cost has been coming down, but
noise signals. In between pulses, the justified by the following conditions: is still higher than that of the least
transmitter sees only the noise signal. • When air is entrained in large expensive flow sensors.
Therefore, the noise can be continu- quantities in the process stream; Magnetic flowmeters also can be
ously eliminated after each cycle. • When the process stream is a slurry packaged as probes and inserted into
This provides a stable zero and and the solid particle sizes are not process pipes through taps. These
eliminates zero drift. In addition to uniform and/or the solid phase is probes contain both the electrodes
being more accurate and able to not homogeneously mixed within and magnetic coils. The flowing pro-
measure lower flows, dc meters are the liquid; or cess fluid induces a voltage at the
less bulky, easier to install, use less • When the flow is pulsating at a electrodes, which reflects the veloc-
energy, and have a lower cost of frequency under 15 Hz. ity at the probe tip and not the aver-
ownership than ac meters. One new When any of the above three age fluid velocity across the pipe.
dc design uses significantly more conditions exist, the output of a These magmeters are inexpensive
power than the earlier generations pulsed dc meter is likely to be noisy. and retractable. Therefore, the pro-
and thereby creates a stronger flow- In some cases, one can minimize the cess does not have to be shut down
tube signal. noise problem (hold the fluctuations to install or remove them. Metering
Another new design uses a unique within 1% of setpoint) by filtering and accuracy is highly dependent on the
dual excitation scheme that pulses damping the output signal. If more relationship between the measured
the coils at 7 Hz for zero stability and than 1 to 3 seconds of damping is velocity and the average velocity in
also at 70 Hz to obtain a stronger required to eliminate the noise, it is the pipe.
signal. Magmeter transmitters can be always better to use an ac meter.
supplied with either ac or dc power. • Electrodes
A two-wire, loop-powered dc mag- • Flowtubes, Liners, & Probes In conventional flowtubes, the elec-
netic flowmeter is also available in The face-to-face dimensions of trodes are in contact with the process
an intrinsically safe design, but its flanged flowtubes (lay lengths) usu- fluid. They can be removable or per-
performance is reduced because of ally meet the recommendations of manent if produced by a droplet of
power limitations. the International Organization for liquid platinum as it sinters through a
Pulsed ac meters have also been Standardization (ISO). The dimen- ceramic liner and fuses with the alu-
introduced recently, eliminating the sions of short-form magmeters usu- minum oxide to form a perfect seal.
This design is preferred due to its mately 1/10 the pipe diameter) in It is important that the conductiv-
low cost, its resistance to abrasion order to remain covered by the fluid. ity of the process fluid be uniform. If
and wear, its insensitivity to nucle- Compensation is provided for wave two fluids are mixed and the conduc-
ar radiation, and its suitability for action and calibration is provided for tivity of one additive is significantly
sanitary applications because there full pipe, no flow (static level), and different from that of the other pro-
are no cavities in which bacteria partially filled pipe operation. cess fluid, it is important that they
can grow. On the other hand, the Another recent development is a be completely intermixed before the
ceramic tube cannot tolerate bend- magnetic flowmeter with an unlined blend reaches the magmeter. If the
ing, tension, or sudden cooling and carbon steel flowtube. In this design, blend is not uniform, the output
cannot handle oxidizing acids or hot the measuring electrodes mount signal will be noisy. To prevent that,
and concentrated caustic. externally to the unlined flowtube pockets of varying conductivity can
In a more recent capacitively- and the magnetic coils generate a be eliminated by installing a static
coupled design, non-contacting elec- field 15 times stronger than in a con- mixer upstream of the magmeter.
trodes are used. These designs use ventional tube. This magnetic field Magmeter size is determined by
areas of metal sandwiched between penetrates deep into the process capacity tables or charts published
layers of liner material. They are fluid (not just around the electrode by the manufacturer. Figure 4-3 pro-
available in sizes under eight inches as with standard magmeter probes). vides a flow capacity nomograph
in diameter and with ceramic liners. The main advantage is low initial and for line sizes from 0.1 in. to 96 in.
Magmeters using these non-contact- replacement costs, since only the For most applications, flow veloci-
ing electrodes can “read” fluids hav- sensors need be replaced. ties should fall between 3 ft/sec
ing 100 times less conductivity than and 15 ft/sec. For corrosive fluids,
required to actuate conventional • Selection & Sizing the normal velocity range should be
flowtubes. Because the electrode is Magnetic flowmeters can detect the 3-6 ft/sec. If the flowtube is con-
behind the liner, these designs are flow of clean, multi-phase, dirty, cor- tinuously operated below 3 ft/sec,
also better suited for severe coating rosive, erosive, or viscous liquids and metering accuracy will deteriorate,
applications. slurries as long as their conductivity while continuous operation exceed-
exceeds the minimum required for ing the upper limit of the normal
• Recent Developments the particular design. The expected velocity range will shorten the life
When a magnetic flowmeter is pro- inaccuracy and rangeability of the of the meter.
vided with a capacitance level sen- better designs are from 0.2-1% of rate, The obstructionless nature of the
sor embedded in the liner, it can over a range of 10:1 to 30:1, if the flow magmeter lowers the likelihood of
also measure the flow in partially velocity exceeds 1 ft/sec. At slower plugging and limits the unrecovered
full pipes. In this design, the mag- flow velocities (even below 0.1 ft/s), head loss to that of an equivalent
meter electrodes are located at the measurement error increases, but the length of straight pipe. The low
bottom of the tube (at approxi- readings remain repeatable. pressure drop is desirable because
it lowers pumping costs and aids lines. Installation in horizontal lines liner wear. Liner protectors are avail-
gravity feed systems. is acceptable if the pipe section is able to protect the leading edge of
at a low point and if the electrodes the liners from the abrasive effects
• Problem Applications are not at the top of the pipe. of process fluids. If the magmeter is
The magmeter cannot distinguish This prevents air from coming into installed in a horizontal pipe exceed-
entrained air from the process fluid; contact with the electrodes. When ing 30 ft in length, the pipe should be
therefore, air bubbles will cause the process fluid is a slurry and the supported on both sides of the meter.
the magmeter to read high. If the magmeter is installed at a low point, The magnetic flowmeter must be
trapped air is not homogeneously it should be removed during long electrically grounded to the process
dispersed, but takes the form of air periods of shutdown, so that solids liquid. This is because the magmeter
slugs or large air bubbles (the size will not settle and coat the internals. is part of the path for any stray cur-
of the electrode), this will make the If it is essential to drain the mag- rent traveling down the pipeline or
output signal noisy or even disrupt meter periodically, it should be through the process liquid. Bonding,
it. Therefore, in applications where provided with an empty tube zero by grounding the meter at both ends
air entrainment is likely, the meter option. When this option is activat- to the process fluid, provides a short
should be sized so that the flow ed, the output of the transmitter will circuit for stray currents, routing
velocity under normal flow condi- be clamped to zero. Detection of them around the flowtube instead
tions is 6-12 ft/sec. empty tube conditions is by circuitry of through it. If the system is not
Coating of the electrodes is anoth- connected to extra sets of elec- properly grounded, these currents
er common magmeter problem. trodes in the flowtube. The empty can create a zero shift in the mag-
Material build-up on the inner surfac- tube zero feature can also be acti- netic flowmeter output.
es of the meter can electrically iso- vated by an external contact, such as Electrical bonding to the pro-
late the electrodes from the process
fluid. This can cause a loss of signal or
a measurement error, either by chang-
ing the diameter of the flowtube
or by causing span and zero shifts.
Naturally, the best solution is preven-
tion. One preventive step is to size
the meter such that, under normal
flow conditions, the flowing velocity
will be relatively high: at least 6-12 ft/
sec, or as high as practical considering
the possibility of erosion and corro-
sion.
Another method of prevention is
to use electrodes that protrude into
the flow stream to take advantage of
the turbulence and washing effect.
In more severe service, a mechanical
cleaning system can be installed and
used intermittently or continuously
to eliminate coating and build-ups.
• Installation a pump status contact. cess fluid can be achieved by metal

The magnetic flowmeter must Magmeters require five diameters ground straps. These straps connect
always be full of liquid. Therefore, of straight pipe upstream and two each end of the flowtube to the
the preferred location for mag- diameters downstream in order to adjacent pipeline flanges, which, in
meters is in vertical upward flow maintain their accuracy and minimize turn, are in contact with the pro-
cess liquid. Straps are used when Theodor von Karman discovered shed from a flagpole (which acts as
the piping is electrically conductive. that, when a non-streamlined object a bluff body); this is what causes the
When the pipe is non-conductive or (also called a bluff body) is placed in regular rippling one sees in a flag.
lined, grounding rings are used. The the path of a fast-flowing stream, the Vortices are also shed from bridge
grounding ring is like an orifice plate fluid will alternately separate from piers, pilings, offshore drilling plat-
with a bore equal to the nominal size the object on its two downstream form supports, and tall buildings. The
(inside diameter) of the flowtube. It is sides, and, as the boundary layer forces caused by the vortex-shed-
installed between the flanges of the becomes detached and curls back on ding phenomenon must be taken
flowtube and adjacent process pip- itself, the fluid forms vortices (also into account when designing these
ing on the upstream and downstream called whirlpools or eddies). He also structures. In a closed piping system,
sides. The flowtube is bonded to the noted that the distance between the the vortex effect is dissipated within
process fluid by being connected to vortices was constant and depended a few pipe diameters downstream of
the metallic grounding rings, and is solely on the size of the rock that the bluff body and causes no harm.
grounded by being wired to a good formed it.
conductor, such as a cold water pipe. On the side of the bluff body • Vortex Meter Design
In larger sizes and in exotic mate- where the vortex is being formed, the A vortex flowmeter is typical-
rials, grounding rings can become fluid velocity is higher and the pres- ly made of 316 stainless steel or
expensive; grounding electrodes (a sure is lower. As the vortex moves Hastelloy and includes a bluff body,
a vortex sensor assembly and the
transmitter electronics, although
the latter can also be mounted
remotely (Figure 4-5). They are typi-
cally available in flange sizes from
H in. to 12 in. The installed cost
of vortex meters is competitive
with that of orifice meters in sizes
under six inches. Wafer body meters
(flangeless) have the lowest cost,
while flanged meters are preferred
if the process fluid is hazardous or
is at a high temperature.
Bluff body shapes (square, rect-
angular, t-shaped, trapezoidal) and
dimensions have been experimented
with to achieve the desired char-
acteristics. Testing has shown that
linearity, low Reynolds number limi-
tation, and sensitivity to velocity
profile distortion vary only slightly
with bluff body shape. In size, the
third electrode placed in the flowtube downstream, it grows in strength bluff body must have a width that
for bonding with the process fluid) can and size, and eventually detaches is a large enough fraction of the
be used instead. Another cost-saving or sheds itself. This is followed by a pipe diameter that the entire flow
option is to use a plastic grounding vortex's being formed on the other participates in the shedding. Second,
ring with a metal electrode insert. side of the bluff body (Figure 4-4). the bluff body must have protruding
The alternating vortices are spaced edges on the upstream face to fix
Vortex Flowmeters at equal distances. the lines of flow separation, regard-
As a young person fishing in the moun- The vortex-shedding phenom- less of the flow rate. Third, the bluff
tain streams of the Transylvanian Alps, enon can be observed as wind is body length in the direction of the
flow must be a certain multiple of velocity is: Q = AV = (A f d B)/St
the bluff body width.
Today, the majority of vortex St = f(d/V) where B is the blockage factor,
meters use piezoelectric or capac- defined as the open area left by the
itance-type sensors to detect Where St is the Strouhal number, bluff body divided by the full bore
the pressure oscillation around f is the vortex shedding frequency, area of the pipe. This equation, in
the bluff body. These detectors d is the width of the bluff body, turn, can be rewritten as:
respond to the pressure oscilla-
tion with a low voltage output sig-
nal which has the same frequency
as the oscillation. Such sensors are
modular, inexpensive, easily replaced,
and can operate over a wide range
of temperature ranges—from cryo-
genic liquids to superheated steam.
Sensors can be located inside the
meter body or outside. Wetted sen-
sors are stressed directly by the
vortex pressure fluctuations and are
enclosed in hardened cases to with-
stand corrosion and erosion effects.
External sensors, typically piezo-
electric strain gages, sense the vortex
shedding indirectly through the force
exerted on the shedder bar. External
sensors are preferred on highly ero-
sive/corrosive applications to reduce
maintenance costs, while internal
sensors provide better rangeability
(better low flow sensitivity). They are and V is the average fluid velocity.
also less sensitive to pipe vibrations. The value of the Strouhal number Q=fK
The electronics housing usually is is determined experimentally, and is
rated explosion- and weatherproof, generally found to be constant over where K is the meter coefficient,
and contains the electronic transmit- a wide range of Reynolds numbers. equal to the product (A f d B). As
ter module, termination connections, The Strouhal number represents the with turbine and other frequency-
and optionally a flow-rate indicator ratio of the interval between vortex producing flowmeters, the K factor
and/or totalizer. shedding (l) and bluff body width (d), can be defined as pulses per unit
which is about six (Figure 4-4). The volume (pulses per gallon, pulses per
• Sizing & Rangeability Strouhal number is a dimensionless cubic foot, etc.). Therefore, one can
Vortex shedding frequency is directly calibration factor used to charac- determine flowrate by counting the
proportional to the velocity of the terize various bluff bodies. If their pulses per unit time. Vortex frequen-
fluid in the pipe, and therefore to Strouhal number is the same, then cies range from one to thousands of
volumetric flow rate. The shedding two different bluff bodies will per- pulses per second, depending upon
frequency is independent of fluid form and behave similarly. the flow velocity, the character of
properties such as density, viscosity, Because the volumetric flowrate the process fluid, and the size of the
conductivity, etc., except that the Q is the product of the average fluid meter. In gas service, frequencies are
flow must be turbulent for vortex velocity and of the cross-sectional about 10 times higher than in liquid
shedding to occur. The relationship area available for flow (A): applications.
between vortex frequency and fluid The K factor is determined by the
manufacturer, usually by water cali- In order to minimize measure- to give some indication at near zero
bration in a flow lab. Because the K ment noise, it is important to select flows, the vortex meter is provided
factor is the same for liquid, gas and a meter that will adequately handle with a cut-off point. Below this level,
vapor applications, the value deter- both the minimum and maximum the meter output is automatically
mined from a water calibration is valid process flows that will be measured. clamped at zero (4 mA for analog
for any other fluid. The calibration It is recommended that the minimum transmitters). This cut-off point cor-
factor (K) at moderate Reynolds num- flow rate to be measured be at least responds to a Reynolds number at or
bers is not sensitive to edge sharpness twice the minimum flow rate detect- below 10,000. If the minimum flow
or other dimensional changes that able by the meter. The maximum that one needs to measure is at least
affect square-edged orifice meters. capacity of the meter should be at twice the cut-off flow, this does not
Although vortex meter equations least five times the anticipated maxi- pose a problem. On the other hand,
are relatively simple compared to mum flowrate. it can still be a drawback if low flow-
those for orifice plates, there are rate information is desired during
many rules and considerations to • Accuracy & Rangeability start-up, shutdown, or other upset
keep in mind. Manufacturers offer Because the Reynolds number drops conditions.
free computer software for sizing, as viscosity rises, vortex flowmeter
wherewith the user enters the fluid's rangeability suffers as the viscosity • Recent Developments
properties (density, viscosity, and rises. The maximum viscosity limit, Smart vortex meters provide a digi-
desired flow range) and the program as a function of allowable accuracy tal output signal containing more
automatically sizes the meter. and rangeability, is between 8 and information than just flow rate. The
The force generated by the vortex 30 centipoises. One can expect a microprocessor in the flowmeter
pressure pulse is a function of fluid better than 20:1 rangeability for gas can automatically correct for insuf-
density multiplied by the square and steam service and over 10:1 for ficient straight pipe conditions,
of fluid velocity. The requirement low-viscosity liquid applications if for differences between the bore
that there be turbulent flow and the vortex meter has been sized diameter and that of the mating
force sufficient to actuate the sensor properly for the application. pipe, for thermal expansion of the
determines the meter’s rangeability. The inaccuracy of most vortex bluff body, and for K-factor changes
This force has to be high enough to meters is 0.5-1% of rate for Reynolds when the Reynolds number drops
be distinguishable from noise. For numbers over 30,000. As the below 10,000.
example, a typical 2-in. vortex meter Reynolds number drops, metering Intelligent transmitters are also
has a water flow range of 12 to 230 error increases. At Reynolds numbers provided with diagnostic subroutines
gpm. If the density or viscosity of the less than 10,000, error can reach 10% to signal component or other fail-
fluid differs from that of water, the of actual flow. ures. Smart transmitters can initiate
meter range will change. While most flowmeters continue testing routines to identify problems
with both the meter and with the normal flow, the vortex flowmeter kept open at the bottom. This can
application. These on-demand tests can still be considered. be achieved by installing the bluff
can also assist in ISO 9000 verifica- If the process fluid tends to coat body horizontally. Measurement
tion. or build-up on the bluff body, as inaccuracy in such applications is
Some recently introduced vortex in sludge and slurry service, this about 5% of actual flow, but with
flowmeters can detect mass flow. will eventually change the meter’s good repeatability.
One such design measures both the K factor. Vortex-shedding flowme- The permanent pressure loss
vortex frequency and the vortex ters are not recommended for such through a vortex meter is about
pulse strength simultaneously. From applications. If, however, a dirty half that of an orifice plate, roughly
these readings, the density of the fluid has only moderate amounts of two velocity heads. (A velocity head
process fluid can be determined and non-coating solids, the application is defined as V2/g, where V is the
the mass flow calculated to within is likely to be acceptable. This was flow velocity and g is the gravita-
2% of span. demonstrated by a 2-year test on a tional constant in consistent units.)
Another newer design is provided limestone slurry. At the end of the If the pipe and meter are properly
with multiple sensors to detect not test, the K factor was found to have sized and of the same size, the pres-
only the vortex frequency, but also changed only 0.3% from the origi- sure drop is likely to be only a few
the temperature and pressure of the nal factory calibration, although the psi. However, downsizing (install-
process fluid. Based on that data, it bluff body and flowtube were badly ing a smaller-than-line-size meter)
determines both the density and the scarred and pitted. in order to increase the Reynolds
mass flow rate. This meter offers a When measuring multi-phase flow can increase the head loss to more
1.25% of rate accuracy when measur- (solid particles in gas or liquid; gas than 10 psi. One should also make
ing the mass flow of liquids and a bubbles in liquid; liquid droplets in sure that the vena contracta pressure
2% of rate accuracy for gases and gas), vortex meter accuracy will drop does not drop below the vapor pres-
steam. If knowledge of process pres- because of the meter’s inability to sure of the process fluid, because that
sure and temperature is of value for differentiate between the phases. would cause cavitation. Naturally, if
other reasons, this meter provides a Wet, low-quality steam is one such the back-pressure on the meter is
convenient, less costly alternative to
installing separate transmitters.
• Applications & Limitations

Vortex meters are not usually rec-
ommended for batching or other
intermittent flow applications. This
is because the dribble flow-rate
setting of the batching station can
fall below the meter’s minimum
Reynolds number limit. The smaller
the total batch, the more significant
the resulting error is likely to be.
Low pressure (low density) gases
do not produce a strong enough
pressure pulse, especially if fluid application: the liquid phase should below the vapor pressure, the process
velocities are low. Therefore, it be homogeneously dispersed within fluid will flash and the meter reading
is likely that in such services the the steam, and vertical flow lines will not be meaningful.
rangeability of the meter will be should be avoided to prevent slug- The main advantages of vortex
poor and low flows will not be ging. When the pipe is horizontal, the meters are their low sensitivity to
measurable. On the other hand, if liquid phase is likely to travel on the variations in process conditions and
reduced rangeability is acceptable bottom of the pipe, and therefore low wear relative to orifices or tur-
and the meter is correctly sized for the inner area of the pipe should be bine meters. Also, initial and mainte-
nance costs are low. For these rea- down” of oversized process piping be carefully aligned to eliminate any
sons, they have been gaining wider by concentric reducers and expand- obstructions or steps.
acceptance among users. ers. Even if flow straighteners are Excessive pipe vibration can be
installed, some straight (relaxation) eliminated by supporting the pip-
• Installation Recommendations piping will still be required. ing on both sides of the meter, or
When installing a vortex flowmeter Vortex meters can be installed by rotating the meter so that the
in an existing process where the flow vertically, horizontally, or at any sensor is moved out of the plane
range is not known, it is recommend- angle, as long as they are kept flood- of the vibration. Process noise due
ed to first make some approximate ed. The meter can be kept flooded to valve chattering, steam traps, or
measurements (using portable pitot by installing it in a vertical upward pumps can result in high readings or
or clamp-on ultrasonic devices). flow line (Figure 4-6B). When install- non-zero readings under zero-flow
Otherwise, there is no guarantee that ing the flowmeter in a downward conditions. Most meter electronics
a line-size vortex meter will work at (Figure 4-6C) or horizontal (Figure allow for increasing the noise filter
all. 4-6D) flow, the downstream pip- settings, but increased noise reduc-
The vortex meter requires a well- ing should be kept elevated. Check tion usually also decreases the low-
developed and symmetrical flow valves can be used to keep the piping flow sensitivity of the meter. One
velocity profile, free from any distor- full of liquid when there is no flow. option is to relocate the meter to a
tions or swirls. This necessitates the Block and bypass valves are required less noisy part of the process.
use of straight up- and downstream if the replacement of the sensor in
piping to condition the flow. The the particular design requires the Ultrasonic Flowmeters
straight length of pipe must be the stopping of the flow and the open- The speed at which sound propa-
same size as the meter (Figure 4-6) and ing up of the process. gates in a fluid is dependent on
its length should be about the same Mating flanges (on the schedule the fluid’s density. If the density
as required for an orifice installation 40 or schedule 80 mating piping) is constant, however, one can use
with a beta ratio of 0.7 (see Chapter must have the same diameter and the time of ultrasonic passage (or
2). Most vortex flowmeter manufac- smooth bore as the flowmeter. reflection) to determine the veloc-
turers recommend a minimum of 30 Weld neck flanges are preferred, and ity of a flowing fluid.
pipe diameters downstream of con- reducing flanges should not be used. Some manufacturers produce
trol valves, and 3 to 4 pipe diameters The inner surface of the mating pipe transducer systems that operate in
between the meter and downstream should be free from mill scale, pits, the shear-mode, sending a single
pressure taps. Temperature elements holes, reaming scores and bumps for pulse and receiving a single pulse
should be small and located 5 to 6 a distance of 4 diameters upstream in return. Narrow-beam systems are
diameters downstream. and 2 diameters downstream of the commonly subject to walk-away (the
About half of all vortex meter meter. The bores of the meter, the signal completely missing the down-
installations require the “necking gaskets and the adjacent piping must stream transducer). Wide-beam
systems overcome beam refraction fied to: not be distinguished from the back-
and work better in changing liquid ground noise in the pipe.
density and temperature. With the V = (f0 - f1)K The reflected Doppler signal is
advent of digital signal processing, it shifted from the transmitted fre-
has become possible to apply digital Thus, flow velocity V (ft/sec) is quency by approximately 6 Hz for
signal coding to the transmitted sig- directly proportional to the change every foot per second of velocity.
nal. This can eliminate many of the in frequency. The flow (Q in gpm) in a Therefore, if the flow velocity is less
problems associated with noise and pipe having a certain inside diameter than 1 ft/sec, ultrasonic flowmeter-
variations in liquid chemistry. (ID in inches) can be obtained by: ing is not practical. There seems to
be no upper limit to detectable flow
• The Doppler Shift Q = 2.45V(ID)2 = 2.45[(f0 - f1)K](ID)2 velocity, as successful installations at
In 1842, Christian Doppler discov- velocities in the 40-50 ft/sec range
ered that the wavelength of sound The presence of acoustical dis- are well documented.
perceived by a stationary observer continuities is essential for the
appears shorter when the source is proper operation of the Doppler • Transit Time Measurement
approaching and longer when the flowmeter. The generally accepted In this design, the time of flight of the
source is moving away. This shift in rule of thumb is that for proper ultrasonic signal is measured between
frequency is the basis upon which all signal reflection there be a mini- two transducers—one upstream and
Doppler-shift ultrasonic flowmeters mum of 80-100 mg/l of solids with one downstream (Figure 4-7B). The
work. a particle size of +200 mesh (+75 difference in elapsed time going with
Doppler flowmeter transducers micron). In the case of bubbles, 100- or against the flow determines the
operate at 0.640 MHz (in clamp-on 200 mg/l with diameters between fluid velocity.
designs) and at 1.2 MHz in wet- +75 and +150 microns is desirable. If When the flow is zero, the time for
ted sensor designs. The transducer either the size or the concentration the signal T1 to get to T2 is the same
sends an ultrasonic pulse or beam of the discontinuities changes, the as that required to get from T2 to T1.
into the flowing stream. The sound amplitude of the reflected signal When there is flow, the effect is to
waves are reflected back by such will shift, introducing errors. boost the speed of the signal in the
acoustical discontinuities as parti- Doppler flowmeters are often used downstream direction, while decreas-
cles, entrained gas bubbles, or even to measure the flow of such fluids as ing it in the upstream direction. The
by turbulence vortices (Figure 4-7A). slurries. If the solids concentration flowing velocity (Vf) can be deter-
For clamp-on designs, measurement is too high (in excess of 45% by mined by the following equation:
inaccuracy ranges from ±1% to ±5%
full scale (FS).
The meter detects the velocity of
the discontinuities, rather than the
velocity of the fluid, in calculating
the flow rate. The flow velocity (V)
can be determined by:
V = (f0 - f1)Ct /2f0 cos(a)
Where Ct is the velocity of sound

inside the transducer, f0 is the trans-
mission frequency, f1 is the reflected
frequency, and a is the angle of weight), or if too much air or gas is
the transmitter and receiver crys- entrained (especially if the bubbles Vf = Kdt/TL
tals with respect to the pipe axis. are very fine), these discontinuities
Because Ct /2f0cos(a) is a constant will attenuate the reflected Doppler where K is a calibration factor for the
(K), the relationship can be simpli- signal to the point where it can- volume and time units used, dt is the
time differential between upstream potted into the same sensor body, cally decreases flowmeter accuracy
and downstream transit times, and TL which is clamped onto a single point (to within only ±20%), and, in most
is the zero-flow transit time. of the pipe surface (Figure 4-8). In cases, clamp-on meters will not work
Theoretically, transit-time ultra- the dual-sensor version, the transmit at all if the pipe is lined.
sonic meters can be very accurate crystal is in one sensor body, while Wetted transducer designs—both
(inaccuracy of ±0.1% of reading is the receive crystal is in another. Doppler and transit time are avail-
sometimes claimed). Yet the error Clamp-on transit time meters have able—overcome many of these signal
attenuation limitations. The full-pipe
transit-time meter originally consisted
of a flanged spool section with wet-
ted transducers mounted in the pipe
wall in transducer wells opposite to
one another but at 45-degree angles
to the flow (Figure 4-9A). Transit-time
flowmeters can be either single-path
or multiple-path designs (Figure 4-9B).
Single-path flowmeters are pro-
vided with a single pair of transduc-
ers that make a single-line velocity
measurement. They use a meter fac-
tor that is pre-determined by calibra-
tion to compensate for variations in
velocity profile and for flow section
construction irregularities.
In the design of multi-path flow-
meters, several sets of transducers
are placed in different paths across
in these measurements is limited by been available since the early 1970s. the flow section, thereby attempt-
both the ability of the signal process- Their aim is to rival the performance ing to measure the velocity pro-
ing electronics to determine the tran- of wetted spool-piece designs, but file across the entire cross-section
sit time and by the degree to which without the need to break the pipe or of the pipe. Multi-path instruments
the sonic velocity (C) is constant. The stop the process to install the meter. are used in large-diameter conduits,
speed of sound in the fluid is a func- This goal has not yet been reached. such as utility stacks, and in other
tion of both density and temperature. Clamp-on Doppler flowmeters applications where non-uniform
Therefore, both have to be compen- are subject to interference from the flow velocity profiles exist.
sated for. In addition, the change in pipe wall itself, as well as from any Transit-time meters can also be
sonic velocity can change the refrac- air space between the sensor and used to measure both very hot (e.g.,
tion angle (“a” in Figure 4-7B), which the wall. If the pipe wall is made of liquid sulfur) and very cold (liquid
in turn will affect the distance the stainless steel, it might conduct the nitrogen) fluids, and also to detect
signal has to travel. In extreme cases, transmit signal far enough so that very low flows. Wetted-transducer
the signal might completely miss the the returning echo will be shifted designs for small pipes (down to H
downstream receiver. Again, this type enough to interfere with the read- in.) are called axial or co-axial designs
of failure is known as walk-away. ing. There are also built-in acoustic (Figure 4-10). These devices permit
discontinuities in concrete-lined, transit-time measurement along a
• Design Variations plastic-lined, and fiberglass-rein- path length significantly greater than
Clamp-on ultrasonic meters come forced pipes. These are significant the diameter of the pipe, increasing
in either single or dual-sensor ver- enough to either completely scat- low-flow sensitivity.
sions. In the single-sensor version, ter the transmitted signal or attenu- Originally, ultrasonic flowmeters
the transmit and receive crystals are ate the return signal. This dramati- were divided into those using the
Doppler-shift principle and those problematic, as the solids concentra- area. This affects the accuracy of
using the transit-time principle. More tion can be too high for either transit- the meter. In addition, the flexure
recently, flowmeters are capable of time or Doppler flowmeters to work of the pipe wall can often break the
measuring the flow of both clean properly. In still other wastewater acoustic coupling of the transducer
fluids and of slurries with entrained applications, the problem is that the to the outside of the pipe, caus-
solids or other acoustical disconti- acoustical absorbency of the mostly ing failure. Another problem area is
nuities. Microprocessors have made organic solids in wastewater attenu- the measurement of slurries that are
it possible to switch automatically ates the ultrasonic signals. acoustically absorbent, such as lime
from clean fluid mode to particulate The use of multi-path flowme- or kaolin slurries. These applications
mode based on the "correlation fac- ters in raw wastewater and storm fail because the highly absorbent
tor". This figure of merit dramatically water applications is common, while solids attenuate the signal below
improves the accuracy of overall Doppler or cross-correlation hybrid usable strength. Lower frequency
performance. In some carefully engi- designs are most often used to mea- (0.45 MHz) sensors have been tried
neered applications, installed accu- sure activated sludge and digested for these applications, but success
racy to within 0.5% of reading has sludge flows. has been limited.
been reported. For mining slurries, Doppler flow- Multi-path, transit-time flowme-
meters typically work well. Among ters also measure stack gas flows in
• Applications & Performance the few problem applications are power-plant scrubbers, even in very
Doppler flowmeters are not rec- those in HDPE pipe, because the large diameter stacks. T
ommended for clean fluid applica- pipe wall flexes enough to change
tions. Transit-time flowmeters, on the diameter of the measurement
the other hand, are often used to
measure the flow of crude oils and
simple fractions in the petroleum References & Further Reading
industry. They also work well with • OMEGA Complete Flow and Level Measurement Handbook and
viscous liquids, provided that the Encyclopedia®, Omega Press, 1995.
Reynolds number at minimum flow is • Omega Volume 29 Handbook & Encyclopedia, Purchasing Agents
either less than 4,000 (laminar flow) Edition, Omega Press, 1995.
or above 10,000 (turbulent flow). • “An Intelligent Vortex Flowmeter,” T. Kamano and others, ISA/92
Serious non-linearities are present Proceedings, Instrument Society of America, 1992.
in the transition region (Figure 4-11). • “Application and Installation Guidelines for Volumetric and Mass
Transit-time flowmeters are the Flowmeters,” D. Ginesi and C. Annarummo, ISA Transactions, 1994.
standard for measuring cryogenic • “Clamp-On Leak Detectors Protect Mid-Valley Line,” S. Douglas and J.
liquids down to -300°C and are also Baumoel, Pipeline & Gas Journal, April 1993.
used in molten metal flowmetering. • “Committee Report: Transit Time Ultrasonic Flowmeters,” AWWA
Measurement of liquid argon, liquid Subcommittee on Ultrasonic Devices, AWWA Journal, July 1997.
nitrogen, liquid helium and molten • Flow Measurement Engineering Handbook, R.W. Miller, McGraw Hill, 1996.
sulfur have often been reported. • Flow Measurement, D.W. Spitzer, editor, Instrument Society of America,
Spool-section type flowmeters are 1991.
most often used for these applica- • “Flow Sensing: The Next Generation,” D. Ginesi, Control Engineering,
tions, especially the axial and co- November 1997.
axial designs. • Flowmeters in Water Supply, Manual M33, AWWA, 1989.
Raw wastewater applications usu- • Industrial Flow Measurement, D.W. Spitzer, ISA, 1984
ally have too few acoustic disconti- • Instrument Engineers’ Handbook, Bela Liptak, editor, CRC Press, 1995.
nuities for Doppler flowmeters. On • Ultrasonic Clamp-On Flowmeters: Have They Finally Arrived?,” P. Espina,
the other hand, raw wastewater is not Flow Control, January 1997.
clean enough all the time for transit- • Water Meters - Selection, Installation, Testing and Maintenance, Manual
time measurement. Other wastewa- M6, AWWA, 1986.
ter-related applications are equally
Mass Flowmeters
M ass flow measurement is
the basis of most recipe
formulations, material
balance determinations,
and billing and custody transfer
operations throughout industry. With
5-1B). It had a motor-driven impeller
that imparted angular momentum
(rotary motion) by accelerating the
fluid to a constant angular velocity.
The higher the density, the more
angular momentum was required
lower one will measure the hydro-
static head over a fixed elevational
distance. This pressure differential
yields the density of the material in
the tank. Such systems have been
used to measure the total mass flow
these being the most critical flow to obtain this angular velocity. of slurries.
measurements in a processing plant, Downstream of the driven impeller,
the reliability and accuracy of mass a spring-held stationary turbine was Coriolis Mass Flowmeters
flow detection is very important. exposed to this angular momentum. It was G.G. Coriolis, a French engi-
In the past, mass flow was often The resulting torque (spring torsion) neer, who first noted that all bodies
calculated from the outputs of a was an indication of mass flow. moving on the surface of the Earth
volumetric flowmeter and a densi- These meters all had moving parts tend to drift sideways because of
tometer. Density was either directly and complex mechanical designs. the eastward rotation of the plan-
measured (Figure 5-1A), or was calcu- First developed for the measurement et. In the Northern Hemisphere the
lated using the outputs of process of aircraft fuel, some are still in use. deflection is to the right of the
temperature and pressure transmit- However, because of their complex motion; in the Southern, it is to the
ters. These measurements were not nature and high maintenance costs, left. This drift plays a principal role in
very accurate, because the relation- they are gradually being replaced by both the tidal activity of the oceans
ship between process pressure or more robust and less maintenance- and the weather of the planet.
temperature and density are not demanding designs. Because a point on the equator
always precisely known—each sen- Mass flow also can be measured traces out a larger circle per day
sor adds its own separate error to by batch weighing or by combin- than a point nearer the poles, a body
the overall measurement error, and ing an accurate level sensor with a traveling towards either pole will
the speed of response of such cal- densitometer. Another method is to bear eastward, because it retains its
culations is usually not sufficient to mount two d/p transmitters on the higher (eastward) rotational speed as
detect step changes in flow. lower part of an atmospheric tank at it passes over the more slowly rotat-
One of the early designs of self- different elevations. In this case, the ing surface of the earth. This drift is
contained mass flowmeters oper- output of the top d/p cell will vary defined as the Coriolis force.
ated using angular momentum (Figure with the level in the tank, while the The first industrial Coriolis pat-
ents date back to the 1950s, and the tical when building a commercial
first Coriolis mass flowmeters were ar (centripetal) = w2r flowmeter, but oscillating or vibrat-
built in the 1970s. These flowmeters at (Coriolis) = 2wv ing the tube can achieve the same
artificially introduce a Coriolis accel- effect. Coriolis flowmeters can mea-
eration into the flowing stream and In order to impart the Coriolis accel- sure flow through the tube in either
measure mass flow by detecting the eration (at) to the fluid particle, a the forward or reverse directions.
resulting angular momentum. force of at (dm) has to generated by In most designs, the tube is
When a fluid is flowing in a the tube. The fluid particle reacts to anchored at two points and vibrated
pipe and it is subjected to Coriolis this force with an equal and opposite between these anchors.
acceleration through the mechani- Coriolis force: This configuration can be envi-
sioned as vibrating a spring and mass
assembly. Once placed in motion, a
spring and mass assembly will vibrate
at its resonant frequency, which
is a function of the mass of that
assembly. This resonant frequency is
selected because the smallest driv-
ing force is needed to keep the filled
tube in constant vibration.
• Tube Designs
A tube can be of a curved or straight
form, and some designs can also be
self-draining when mounted verti-
cally (Figure 5-3). When the design
consists of two parallel tubes, flow
cal introduction of apparent rota- is divided into two streams by a
tion into the pipe, the amount Fc = at(dm) = 2wv(dm) splitter near the meter’s inlet and is
of deflecting force generated by recombined at the exit. In the single
the Coriolis inertial effect will be Then, if the process fluid has density continuous tube design (or in two
a function of the mass flow rate D and is flowing at constant speed tubes joined in series), the flow is not
of the fluid. If a pipe is rotated inside a rotating tube of cross-sec- split inside the meter.
around a point while liquid is flow- tional area A, a segment of the tube In either case, drivers vibrate the
ing through it (toward or away from of length x will experience a Coriolis tubes. These drivers consist of a coil
the center of rotation), that fluid force of magnitude: connected to one tube and a magnet
will generate an inertial force (act- connected to the other. The transmit-
ing on the pipe) that will be at right Fc = 2wvDAx ter applies an alternating current to
angles to the direction of the flow. the coil, which causes the magnet to
With reference to Figure 5-2, a Because the mass flowrate is dm = be attracted and repelled by turns,
particle (dm) travels at a velocity (V) DvA, the Coriolis force Fc = 2w(dm) thereby forcing the tubes towards
inside a tube (T). The tube is rotat- x and, finally: and away from one another. The sen-
ing about a fixed point (P), and the sor can detect the position, veloc-
particle is at a distance of one radius Mass Flow = Fc/(2wx) ity, or acceleration of the tubes.
(R) from the fixed point. The particle If electromagnetic sensors are used,
moves with angular velocity (w) under This is how measurement of the the magnet and coil in the sensor
two components of acceleration, a Coriolis force exerted by the flowing change their relative positions as the
centripetal acceleration directed fluid on the rotating tube can pro- tubes vibrate, causing a change in the
toward P and a Coriolis acceleration vide an indication of mass flowrate. magnetic field of the coil. Therefore,
acting at right angles to ar: Naturally, rotating a tube is not prac- the sinusoidal voltage output from
the coil represents the motion of the of its geometry, materials of con- ing, which drastically increases the
tubes. struction, and the mass of the tube flowing velocity (from 5-10 ft/sec
When there is no flow in a two- assembly (mass of the tube plus the to more than 25 ft/sec). Designs
tube design (Figure 5-3A), the vibra- mass of the fluid inside the tube). with thin walls and high fluid veloci-
tion caused by the coil and magnet The mass of the tube is fixed. Since ties (that is, small bore tubing), may
drive results in identical displace- mass of the fluid is its density (D) require the use of exotic materials
ments at the two sensing points multiplied by its volume (which is because of erosion concerns. One
(B1 and B2). When flow is present, also fixed), the frequency of vibra- will obtain the longest meter life by
Coriolis forces act to produce a section can be related to the density of selecting the design with the thickest
ondary twisting vibration, resulting in the process fluid (D). Therefore, the wall and the slowest flow velocity
a small phase difference in the rela- density of the fluid can be deter- that can provide the required accu-
tive motions. This is detected at the mined by measuring the resonant racy and range.
sensing points. The deflection of the frequency of oscillation of the tubes. The Coriolis meter may need to be
tubes caused by the Coriolis force (Note that density can be measured made out of exotic materials because
only exists when both axial fluid at zero flow, as long as the tubes are of corrosion considerations or to
flow and tube vibration are present. filled with fluid and vibrating.) prevent pitting. Carbon or stainless
Vibration at zero flow, or flow with- Wall thickness varies considerably steel can often be used in process
out vibration, does not produce an from design to design; however, even piping, because a small amount of
output from the meter. the sturdiest tubing will be thinner pitting can be tolerated. In case of
The natural resonance frequency than the process piping. In addition, the Coriolis meter, even a small
of the tube structure is a function some designs use small bore tub- amount of pitting cannot be toler-
ated because the walls are thin, and Other features may require infor- bending forces were created at the
pitting induces stress concentrations mation to be pre-programmed into anchor points. This resulted in severe
within the tube structure. Therefore, the transmitter memory. In addition, vibration problems, which were alle-
standard corrosion tables (based on transmitters have other hardware and viated by two-tube designs (Figure
weight loss criteria) are not suitable software options which allow the 5-3A).
for selecting Coriolis tube materials, user to customize them to the appli- These designs reduced external
and the stricter guidelines of the
manufacturers must be used.
• Transmitter Designs
Transmitters can operate on either
ac or dc power and require separate
wiring for the power supply and
for their output signals. The Coriolis
flowmeter transmitter can be integrally
or remotely mounted (Figure 5-4).
The transmitter controls the opera-
tion of the driver and processes and
transmits the sensor signals. The cali-
bration factor (K) in the transmitter’s
memory matches the transmitter to
the particular flow tube. This calibra-
tion factor defines the constant of
proportionality between the Coriolis
force and the mass flow rate for the
dynamic spring constant of the par-
ticular vibrating tubes.
The transmitter does more than
convert sensor inputs into standard-
ized output signals. Most transmit-
ters also offer multiple outputs,
including mass flow rate, total mass
flow, density, and temperature.
Analog and/or pulse outputs are
both available, and intelligent trans-
mitters can generate digital outputs
for integration into DCS systems. cation. vibration interference, decreased the
Transmitters are often provided power needed to vibrate the tubes,
with a local displays and keypads to • Coriolis Evolution and minimized the vibrational ener-
allow easy access to process data. The first generation of Coriolis gy leaving the tube structure. One
Coriolis transmitters provide more meters consisted of a single curved driver was used to initiate tube vibra-
than just flow information and ancil- and a thin-walled tube, in which tion, and two sensors were used to
lary functions. Batch control func- high fluid velocities were created detect the Coriolis deflections. While
tions, percent Brix or percent HFCS by reducing the tube cross-sectional this design greatly improved perfor-
monitoring, viscosity, percent solids, area in relation to the process pipe. mance, the combination of reduced
PID, API gravity, and specific grav- The tube distortion was measured in bore, thin-walled tubing, and high
ity also are available. When viscos- reference to a fixed point or plane. fluid velocities (up to 50 ft/sec) still
ity information is desired, the meter The tubes were excited in such a resulted in premature meter failure,
pressure drop needs to be measured. way that localized high amplitude including potentially catastrophic
spills when the meter was used on are as reliable and rugged as tradi- for variations in tube elasticity.
corrosive and erosive services. In tional volumetric flowmeters. The Coriolis mass flowmeters usually
addition, the unrecovered head losses new designs operate at lower fluid are calibrated on water, because the
were high (sometimes over 50 psid), velocities (below 10 ft/sec) and at constants are valid for all other liq-
and accuracy was not high enough lower pressure drops (under 12 psid), uids. Calibration for density is usually
to allow users to convert batch pro- can be installed in any orientation, done by filling the tubes with two or
cesses into continuous ones. and provide longer service life on more (stagnant) calibration fluids of
More recent design improve- slurry, viscous, corrosive, or erosive known densities.
ments include the introduction of a services. The tubes are vibrated well
variety of new tube shapes, includ- below their endurance limits, and • Accuracy & Rangeability
ing ones that do not split the flow typically are made of stainless steel, Coriolis meters provide 0.1-2% of
(Figure 5-3B) and the use of multiple Hastelloy, and titanium. rate inaccuracy over a mass flow
drivers (Figure 5-5A). Thick-walled range of up to 100:1. In general,
tubing (five times thicker than early • Interferences curved tube designs provide wider
designs), the use of full bore diam- The effect of the Coriolis force on rangeability (100:1 to 200:1), while
eters and heavy manifolds to isolate the vibrating tube is small. Full-scale straight-tube meters are limited to
the tube structure from stresses flow might cause a deflection of 30:1 to 50:1 and their accuracy is
induced from piping connections, only 0.001 inch. To obtain a flow lower. Overall meter error is the
and flowtube housings that double rangeability of 100:1, sensors must sum of base inaccuracy and zero-
as secondary containment vessels be able to detect deflections to shift error, the error attributable to
have all contributed to improved an accuracy of 0.000001 inch in the irregular output signal generated
performance. industrial environments where the at zero flow conditions. Zero-shift
In some designs, torsional stresses process pressure, temperature, and error becomes the dominant por-
replaced bending, in order to pre- fluid density are all changing, and tion of total error at the lower end
vent the concentration of stresses where pipe vibration interferes with of the flow range, where the error
that can lead to tube cracking (Figure measurement. is between 1% and 2% of rate. Some
5-5B). In other designs, the effects of The elasticity of metal tubes manufacturers state the overall
pipeline vibration have been mini- changes with temperature; they accuracy as a percentage of rate for
mized by mounting the tube struc- become more elastic as they get the upper portion of the flow range
tures transverse to the pipeline. warmer. To eliminate the corre- and as a percentage of span for the
These improvements increased sponding measurement error, the lower portion, while others state it
the number of suppliers and contrib- tube temperature is continuously as a percentage of rate plus a zero-
uted to the development of a new measured by an RTD element and shift error. There is a fair amount of
generation of Coriolis meters that is used to continuously compensate “specmanship,” and one must read
sales literature carefully when com- of the meter may be required. process fluid is clean with a low
paring different devices. Variations in the density of the viscosity. On corrosive, viscous, or
When used for density measure- process fluid can affect the frequency abrasive slurry services, downsizing is
ment, the typical error range of transfer function of mechanical sys- not recommended. A list of accept-
a Coriolis measurement is 0.002- tems, necessitating the re-zeroing of able flow tube sizes and correspond-
0.0005 g/cc. older designs to protect them from ing pressure drops, inaccuracies, and
Errors are caused by air or gas degraded performance. Because of flow velocities can be obtained from
pockets in the process fluid. In the their tube configurations, newer software provided by the manufac-
case of homogeneously dispersed designs are unaffected by density turer.
small bubbles, more power is changes over wide ranges of specific Different Coriolis meters incur dif-
required to vibrate the tubes, where- gravity variations. ferent pressure drops, but in general
as, if the gas phase separates from they require more than tradition-
the liquid, a damping effect on tube • Sizing & Pressure Drop al volumetric meters, which usu-
vibration (and, consequently, error) Because of the wide rangeability of ally operate at less than 10 psid. (The
develops. Small voids also cause Coriolis flowmeters (30:1 to as high yearly electricity cost of pumping
noise because of the sloshing of the as 200:1), the same flow can be mea- 1 gpm across a differential of 10 psid
process liquid in the tubes. Larger sured by two or three different sized is about $5 U.S.). This higher head loss
voids will raise the energy required flow tubes. By using the smallest is due to the reduced tubing diam-
to vibrate the tubes to excessive lev- possible meter, one will lower the eter and the circuitous path of flow.
els and may cause complete failure. initial cost and reduce coating build- Besides pumping costs, head loss can
Because the flowtube is subjected up, but will increase erosion/corro- be of concern if the meter is installed
to axial, bending, and torsional forces sion rates and head loss, increasing in a low-pressure system, or if there is
during meter operation, if process pumping and operating costs. a potential for cavitation or flashing,
or ambient temperature and pressure Downsizing (using a meter that is or if the fluid viscosity is very high.
fluctuations alter these forces, perfor- smaller than the pipe) is acceptable The viscosity of non-Newtonian
mance may be affected and re-zeroing when the pipe is oversized and the fluids is a function of their flowing
velocity. Dilettante fluids, for exam- Therefore, the amount of driving in low viscosity fluids, like milk, will
ple, increase their apparent viscosity power that can be delivered to the separate at concentrations as low as
(resistance to flow) as their velocity flow tube is limited. 1%.
is increased. This apparent viscosity When fluid is unloaded from tank The cost of an average-sized
can be drastically higher than their trucks, drums, or railroad cars, slug (under 2 in.) Coriolis flowmeter is
viscosity when stagnant. In order to flow can occur, making the meter between $4,000 and $5,000. These
provide suppliers with data on the output unpredictable. If a slug-flow mass flowmeters provide short
flowing viscosity in a particular pipe, recovery feature is provided in the payback periods on applications
head loss per foot of pipe (used transmitter, it will stop the measure- where measurement accuracy low-
in pump sizing calculations) can be ment when slug flow is detected by ers production costs (bathing, bill-
used as an approximation. the excessive drive power drawn or by ing) or where multiple measurements
the drop in process density (reduction (including density, temperature,
• Applications & Limitations in sensor output amplitude). pressure) are needed. On the other
Coriolis mass flowmeters can detect The amount of air in the process hand, they may not be competitive
the flow of all liquids, including fluid that can be tolerated by a when used in simple flow measure-
Newtonian and non-Newtonian, as meter varies with the viscosity of ment applications where volumet-
well as that of moderately dense the fluid. Liquids with viscosities as ric sensors are sufficient and where
gases. Self-draining designs are avail- high as 300,000 centipoise can be repeatability is more important than
able for sanitary applications that metered with Coriolis meters. Gas precision. The ability to extract data
meet clean-in-place requirements. content in such highly viscous liq- on total mass charged, solids rate,
Most meters are provided with uids can be as high as 20% with the percent solids, and viscosity from
intrinsically safe circuits between small bubbles still remaining homo- a single instrument does lower the
the flow tube and the transmitter. geneously dispersed. Gas content total cost of measurement, improves
process control, and provides redun- curved-tube designs are usually of tube rupture, particularly if the
dancy for other instruments. washed out using cleaning solutions process fluid is likely to vaporize
Continuous tube designs are gen- at velocities in excess of 10 ft/sec. under such conditions. If that is the
erally preferred for slurry and other Straight-tube designs also are pre- case, secondary containment hous-
multi-phase fluid applications. The ferred for sanitary applications due ings can be ordered that enclose
total flow is divided by splitters in to self-draining requirements. the entire flow tube, including its
split-tube designs, and the resulting Long, bent tubes twist more eas- housing. Such secondary contain-
two streams do not have to be at ily than do short, straight tubes and ment enclosures can be provided
exactly the same mass flow rate to therefore will generate stronger sig- with rupture disks or pressure relief
maintain accuracy (they do, how- nals under the same conditions. In valves, and with drains or vents.
ever, need to have the same den- general, curved-tube designs provide
sity). Different densities in the two wider rangeability (100:1 to 200:1), • Installation Recommendations
parallel tubes imbalance the sys- while straight-tube meters are lim- There are no Reynolds number
tem and create measurement errors. ited to 30:1 to 50:1, with lower accu- limitations associated with Coriolis
Therefore, if there is a secondary racy. meters. They are also insensitive to
phase in the stream, a simple flow Straight-tube meters are more velocity profile distortion and swirl.
splitter may not evenly distribute immune to pipeline stresses and Therefore, there is no requirement
the flow between the two tubes. vibration, are easy to install, require for straight runs of relaxation pip-
Continuous tube designs are also less pressure drop, can be mechani- ing upstream or downstream of the
preferred for measuring fluids that cally cleaned, are more compact, and meter to condition the flow.
can coat and/or clog the meter. require less room for installation. The meter should be installed so
Continuous tubes, if sized to pass the They are also preferred on services that it will remain full of liquid and
largest solid particles in the process where the process fluid can solidify so air cannot get trapped inside the
fluid, are less likely to clog and are at ambient temperatures. tubes. In sanitary installations, the
easier to clean. Not all meter housings are meter must also drain completely.
Straight-tube designs can be designed to withstand and contain The most desirable installation is in
cleaned by mechanical means, while the pressurized process fluid in case vertical upward flow pipes (Figure
5-6B), but installations in horizontal crosstalk between the two units. railroad cars to drain completely;
lines (Figure 5-6A) are also accept- If air bubbles are likely to be pres- • Using the same pipe for pump-
able. Installations where the flow is ent in the process fluid, it is rec- ing different materials at different
downward in a vertical pipe are not ommended to install an air release times; and
recommended. upstream of the meter. System design • Allowing foam formation by high
In newer Coriolis designs, normal characteristics that can result in the turbulence in high velocity fluids.
pipe vibration should not affect the presence of air (and which can often It is recommended to install
performance of the Coriolis meter if be eliminated at the design stage) (upstream of the meter) strainers,
it is properly supported by the pro- include: filters or air/vapor eliminators as
cess piping (Figure 5-6C). No special • Common piping used for pumping required to remove all undesirable
supports or pads are needed for the into and out of storage tanks; secondary phases. Figure 5-7C illus-
flow tube, but standard piping sup- • Allowing the formation of a vor- trates an air eliminator installation. Its
ports should be located on either tex in stirred vessels under low- function is to slow the velocity of
side of the meter. If the installation level conditions; the liquid, thereby allowing time for
instructions require special hardware • Allowing air leakage through pack- the entrained air to separate and be
or supports, the particular meter ing glands of pumps that develop removed by venting. The rise and fall
design is likely to be sensitive to high vacuums on the suction side of the liquid level in the eliminator
vibration, and the pulsation dampen- (this can occur when pumping due to the accumulation of free air
ers, flexible connectors, and mount- from underground storage); closes and opens the vent valve and
ing/clamping attachments recom- • Vaporization of stagnant process discharges the air (Figure 5-7A&B).
mended by the manufacturer should fluid in pipes exposed to the sun; Prior to zeroing the meter, all
be carefully installed. • High valve pressure drops causing air should be removed. This can be
If your application requires that vaporization and flashing; accomplished by circulating the pro-
you install two Coriolis flowme- • Allowing the pipe to drain for any cess fluid through the meter for sev-
ters in series or mount two Coriolis reason, including lack of check eral minutes at a velocity of approxi-
meters near each other, the manufac- valves; mately 2-6 ft/sec. On batching or
turer should be consulted to prevent • Allowing storage tanks, trucks, or other intermittent flow applications,
the meter should stay flooded so tion) consist of comparing the out- trical heating tape can be added to
that it does not need to be repurged. put of the meter against a reference the housing. Jackets or heating tapes
All meters should be so installed so standard of higher accuracy, such as must be installed by the manufac-
they can be zeroed while filled with a dead-weight calibrated weigh tank. turer.
liquid. Before Coriolis meters, the refer- When flowmetering is not required,
When zeroing the meter, any ence standard was expected to be the Coriolis meter can be used solely
associated pumps or other equip- an order of magnitude more accu- as a densitometer. In that case, to min-
ment should be running so that their rate than the meter being calibrated; imize cost, usually a small (H in.) meter
noise can be zeroed out. This can however, due to the high accuracy of is installed in a by-pass line. Such a
be achieved in most cases by locat- Coriolis meters, this is rare. configuration is acceptable only in
ing a shut-off value downstream of In less critical installations (where clean services that will not clog the
the meter and either operating the weigh tanks are not used), volumet- small bore of the meter. In addition, a
pump with its discharge blocked, ric provers or master meters (typical- restriction must be placed in the main
which is acceptable with centrifu- ly another Coriolis or a turbine meter piping (between the by-pass taps) to
gal pumps for a short period, or by calibrated at a flow laboratory) are ensure a flow through the meter.
opening the pump bypass on posi- used. When a volumetric reference is
tive displacement pumps. Valves used in calibrating a mass flowmeter, Thermal Mass Flowmeters
used in zeroing the meter should the fluid density must be very pre- Thermal mass flowmeters also mea-
provide tight shut-off; double-seat- cisely determined. sure the mass flowrate of gases and
ed valves are preferred. Control valves should be installed liquids directly. Volumetric measure-
Meters that are expected to be downstream of the meter to increase ments are affected by all ambient
calibrated in-line must be provided the back-pressure on the meter and and process conditions that influ-
with block and bypass valves so lower the probability of cavitation ence unit volume or indirectly affect
that the reference standard (master) or flashing. pressure drop, while mass flow mea-
meter can be installed and discon- When the process fluid must be surement is unaffected by changes
nected without interrupting the pro- held at higher temperatures, some in viscosity, density, temperature, or
cess. The requirements for in-line Coriolis meters can be supplied with pressure.
calibration (for ISO 9000 verifica- steam jackets. As an alternative, elec- Thermal mass flowmeters are
often used in monitoring or con- • Theory of Operation (q), and the specific heat of the fluid
trolling mass-related processes such Thermal mass flowmeters are most (Cp), as follows:
as chemical reactions that depend often used for the regulation of low
on the relative masses of unreacted gas flows. They operate either by m = Kq/(Cp (T2 - T1))
introducing a known amount of heat
into the flowing stream and measuring
an associated temperature change, or • Heated-Tube Design
by maintaining a probe at a constant Heated-tube flowmeters were devel-
temperature and measuring the ener- oped to protect the heater and sen-
gy required to do so. The components sor elements from corrosion and any
of a basic thermal mass flowmeter coating effects of the process. By
include two temperature sensors and mounting the sensors externally to
an electric heater between them. The the piping (Figure 5-8B), the sens-
heater can protrude into the fluid ing elements respond more slowly
stream (Figure 5-8A) or can be external and the relationship between mass
to the pipe (Figure 5-8B). flow and temperature difference
In the direct-heat version, a fixed becomes nonlinear. This nonlinear-
amount of heat (q) is added by an ity results from the fact that the
ingredients. In detecting the mass electric heater. As the process fluid heat introduced is distributed over
flow of compressible vapors and flows through the pipe, resistance some portion of the pipe’s surface
gases, the measurement is unaffected temperature detectors (RTDs) mea- and transferred to the process fluid
by changes in pressure and/or tem- sure the temperature rise, while the at different rates along the length of
perature. One of the capabilities of
thermal mass flowmeters is to accu-
rately measure low gas flowrates or
low gas velocities (under 25 ft. per
minute)—much lower than can be
detected with any other device.
Thermal flowmeters provide high
rangeability (10:1 to 100:1) if they are
operated in constant-temperature-
difference mode. On the other hand,
if heat input is constant, the ability to
detect very small temperature differ-
ences is limited and both precision
and rangeability drop off. At normal
flows, measurement errors are usually
in the 1-2% full scale range.
This meter is available in high pres-
sure and high temperature designs,
and in special materials including
glass, Monel, and Teflon®. Flow- All-in-one mass flow controller provides both measurement and control of relatively low mass flow
through designs are used to measure
small flows of pure substances (heat amount of electric heat introduced the pipe.
capacity is constant if a gas is pure), is held constant. The pipe wall temperature is
while bypass and probe-type designs The mass flow (m) is calculated on highest near the heater (detected
can detect large flows in ducts, flare the basis of the measured tempera- as Tw in Figure 5-8B), while, some
stacks, and dryers. ture difference (T2 - T1), the meter distance away, there is no difference
coefficient (K), the electric heat rate between wall and fluid temperature.
Therefore, the temperature of the power), flowmeter geometry, ther- makes it possible to minimize elec-
unheated fluid (Tf) can be detected mal capacity, specific heat, and vis- tric power consumption and to
by measuring the wall temperature cosity of the process fluid must stay increase the speed of response of
at this location further away from constant when using this design. the measurement. On the other hand,
the heater. This heat transfer process because of the small size, filters are
is non-linear, and the corresponding • Bypass-Type Design necessary to prevent plugging. One
equation differs from the one above The bypass version of the thermal serious limitation is the high pres-
as follows: mass flowmeter was developed to sure drop (up to 45 psi) needed to
measure larger flow rates. It con- develop laminar flow. This is typically
m0.8 = Kq/(Cp (Tw - Tf)) sists of a thin-walled capillary tube acceptable only for high pressure
(approximately 0.125 in diameter) gas applications where the pressure
This flowmeter has two operating and two externally wound self-heat- needs to be reduced in any case.
modes: one measures the mass flow ing resistance temperature detectors This is a low accuracy (2% full
by keeping the electric power input (RTDs) that both heat the tube and scale), low maintenance, and low
constant and detecting the tempera- measure the resulting temperature cost flowmeter. Electronic packages
ture rise. The other mode holds the rise (Figure 5-9A). The meter is placed within the units allow for data acqui-
temperature difference constant and in a bypass around a restriction in the sition, chart recording, and computer
measures the amount of electricity main pipe and is sized to operate in interfacing. These devices are popu-
needed to maintain it. This second the laminar flow region over its full lar in the semiconductor processing
mode of operation provides for a operating range. industry. Modern day units are also
much higher meter rangeability. When there is no flow, the heaters available as complete control loops,
Heated-tube designs are generally raise the bypass-tube temperature to including a controller and automatic
used for the measurement of clean approximately 160°F above ambient control valve.
(e.g., bottled gases) and homoge- temperature. Under this condition,
neous (no mixtures) flows at moder- a symmetrical temperature distri- • Air Velocity Probes
ate temperature ranges. They are bution exists along the length of Probe-style mass flowmeters are
not recommended for applications the tube (Figure 5-9B). When flow used to measure air flows and are
where either the fluid composition is taking place, the gas molecules insensitive to the presence of mod-
or its moisture content is variable, carry the heat downstream and the erate amounts of dust. They maintain
because the specific heat (Cp) would temperature profile is shifted in the a temperature differential between
change. They are not affected by direction of the flow. A Wheatstone two RTDs mounted on the sensor
changes in pressure or temperature. bridge connected to the sensor ter- tube. The upper sensor measures
Advantages include wide rangeabil- minals converts the electrical signal the ambient temperature of the
ity (the ability to measure very low into a mass flow rate proportional to gas (Figure 5-10A) and continuously
flows) and ease of maintenance. The the change in temperature. maintains the second RTD (near the
temperature difference (or heater The small size of the bypass tube tip of the probe) at 60°F above ambi-
ent. The higher the gas velocity, the measurements in the chemical and rate of cooling corresponds to the
more current is required to maintain petrochemical industries, research mass flowrate.
the temperature differential. and development applications, gas The circuitry of the heated sens-
Another version of the velocity chromatography, and filter and leak ing element is controlled by one of
probe is the venturi-type thermal testing. While hot-wire anemom- two types of solid-state electronic
mass flowmeter, which places a eters are best suited for clean gases circuits: constant-temperature or
heated mass flow sensor at the mini- at low velocities, venturi meters can constant-power. The constant-tem-
mum diameter of a venturi flow also be considered for some liquid perature sensor maintains a constant
element and a temperature com-
pensation probe downstream (Figure
5-10B). An inlet screen mixes the flow
to make the temperature uniform.
This design is used for both gas and
liquid measurement (including slur-
ries), with flow range a function of
the size of the venturi. Pressure drop
is relatively low and precision is
dependent upon finding the proper
probe insertion depth.
A flow switch version is also avail-
able that contains two temperature
sensors in the tip. One of the sensors Air velocity probe provides 1.5% accuracy for local flow rate measurement.
is heated and the temperature dif-
ference is a measure of velocity. The (including slurry) flow applications. temperature differential between a
switch can be used to detect high or Thermal mass flowmeters are well heated sensor and a reference sen-
low flow within 5%. suited for high rangeability measure- sor; the amount of power required to
ments of very low flows, but also maintain the differential is measured
• Uses & Limitations can be used in measuring large flows as an indication of the mass flow rate.
Thermal mass flowmeters can have such as combustion air, natural gas, Constant-temperature anemom-
very high rangeability and reasonable or the distribution of compressed eters are popular because of their
accuracy, but they also have seri- air. high-frequency response, low elec-
ous limitations. Potential problems tronic noise level, immunity from
include the condensation of moisture Hot-Wire Anemometers sensor burnout when airflow sud-
(in saturated gases) on the tempera- The term anemometer was derived denly drops, compatibility with hot-
ture detector. Such condensation will from the Greek words anemos, film sensors, and their applicability
cause the thermometer to read low “wind,” and metron, “measure.” to liquid or gas flows.
and can lead to corrosion. Coating or Mechanical anemometers were first Constant-power anemometers
material build-up on the sensor also developed back in the 15th century do not have a feedback system.
will inhibit heat transfer and cause to measure wind speed. Temperature is simply proportional
the meter to read low. Additional A hot-wire anemometer consists to flowrate. They are less popular
potential sources of error include of an electrically heated, fine-wire because their zero-flow reading is
variations in the specific heat caused element (0.00016 inch in diameter not stable, temperature and velocity
by changes in the gas’s composition. and 0.05 inch long) supported by response is slow, and temperature
Some common gas-flow applica- needles at its ends (Figure 5-11). compensation is limited.
tions for thermal mass flowmeters Tungsten is used as the wire mate-
include combustion air measurement rial because of its strength and high • Air Duct Traversing
in large boilers, semiconductor pro- temperature coefficient of resis- Anemometers are widely used for air
cess gas measurement, air sampling tance. When placed in a moving duct balancing. This is accomplished
in nuclear power plants, process gas stream of gas, the wire cools; the by placing multiple anemometers in
a cross-section of the duct or gas pipe
and manually recording the velocity References & Further Reading
readings at numerous points. The mass • OMEGA Complete Flow and Level Measurement Handbook and
flow rate is obtained by calculating Encyclopedia®, Omega Press, 1995.
the mean velocity and multiplying this • Omega Volume 29 Handbook & Encyclopedia, Purchasing Agents
by the density and by the cross-sec- Edition, Omega Press, 1995.
tional area measurement of the duct. • “Air Elimination Techniques for Accurate Liquid Measurement,” J. R.
For cylindrical ducts, the log-linear Chester, Mechanical Engineering, February 1983.
method of traversing provides the • “Application and Installation Guidelines for Volumetric and Mass
highest accuracy because it takes Flowmeters,” D. Ginesi and C. Annarummo, ISA Transactions, Instrument
into account the effects of friction Society of America, 1994.
along the walls of the duct. Because • Automated Process Control Electronics, John Harrington, Delmar
of the number of measurements Publishing Inc., 1989.
(Figure 5-12), air duct traversing is a • “Coriolis for the Masses,” G. J. Blickley, Control Engineering, June 1995.
time-consuming task. Microprocessor- • “Coriolis Mass Flowmeter is Ready for the Tough Jobs,” W. Chin, I&CS,
based anemometers are available to February 1992.
automate this procedure. • “Field Proving Coriolis Mass Flowmeter,” R. Harold and C. Strawn, ISA/91
Because of the small size and fra- Proceedings, Instrument Society of America, 1991.
gility of the wire, hot-wire anemom- • Flow Measurement, D.W. Spitzer (editor), Instrument Society of America,
eters are susceptible to dirt build-up 1991.
and breakage. A positive consequence • “Flow Sensing: The Next Generation,” D. Ginesi, Control Engineering,
of their small mass is fast speed of November 1997.
response. They are widely used in • Instrument Engineers’ Handbook, Bela Liptak, CRC Press, 1995.
HVAC and ventilation applications. • Instrumentation for Process Measurement and Control, 3rd edition,
Larger and more rugged anemometers Norman A. Anderson, Chilton Co., 1980.
are also available for more demand- • Instruments of Science, Robert Bud and Deborah Jean Warner, Garland
ing industrial applications. To ensure Publishing Inc., 1998.
the proper formation of the velocity • “Metering Mass Flow,” H. van der Bent, Process Engineering, May 1993.
profile, a straight duct section is usu- • “On-line Viscosity Measurement with Coriolis Mass Flowmeters,” P.
ally provided upstream of the ane- Kalotry and D. Schaffer, ISA/91 Proceedings, Instrument Society of
mometer station (usually 10 diameters America, 1991.
long). A conditioning nozzle is used • Process/Industrial Instruments and Controls Handbook, 4th edition,
to eliminate boundary layer effects. If Douglas M. Considine, McGraw-Hill, 1993.
there is no room for the straight pipe • “Technical Application Guide to Mass Flow Measurement,” Wayne
section, a honeycomb flow straight- Shannon, Magnetrol International, 1998.
ener can be incorporated into the • The McGraw-Hill Encyclopedia of Science and Technology, 8th edition,
sensor assembly. T John H. Zifcak, McGraw-Hill, 1997.
A Level Measurement Orientation
O n the 28th of March, 1979,
thousands of people fled
from Three Mile Island
(near Harrisburg, PA) when
the cooling system of a nuclear reac-
tor failed. This dangerous situation
into the tank or should it be com-
pletely external?
• Should the sensor detect the level
continuously or will a point sensor
be adequate?
• Can the sensor come in contact
liquid layers, it is advisable to consult
not only Table 4, but other recom-
mendations, such as Table 5.
If it is found that a number of
level detector designs can satisfy the
requirements of the application, one
should also consider the traditions
or preferences of the particular plant
or the particular process industry,
because of user familiarity and the
availability of spare parts. For exam-
ple, the oil industry generally pre-
fers displacement-type level sensors,
while the chemical industry favors
differential pressure (d/p) cells. (The
petroleum industry will use d/p cells
when the span exceeds 60-80 in.)
If the tank is agitated, there is
often no space in which to insert
probe-type sensors. Plus, because
the liquid surface is not flat, sonic,
ultrasonic, or radar devices typically
cannot be used, either. Even with
developed because the level con- with the process fluid or must it displacer or d/p sensors, agitation
trols turned off the coolant flow be located in the vapor space? can cause the level signal to cycle.
to the reactor when they detected • Is direct measurement of the level These pulses can be filtered out by
the presence of cooling water near needed or is indirect detection of first determining the maximum rate
the top of the tank. Unfortunately, hydrostatic head (which responds
the water reached the top of the to changes in both level and den- PT
reactor vessel not because there sity) acceptable?
was too much water in the tank, but • Is tank depressurization or pro-
because there was so little that it cess shut-down acceptable when
boiled and swelled to the top. From sensor removal or maintenance is
this example, we can see that level required?
measurement is more complex than By evaluating the above choices,
simply the determination of the one will substantially shorten the list
presence or absence of a fluid at a of sensors to consider. The selection
particular elevation. is further narrowed by considering
only those designs that can be pro-
Level Sensor Selection vided in the required materials of
When determining what type of construction and can function at the
level sensor should be used for a required accuracy, operating tem-
given application, there are a series perature, etc. (Table 4). When the at which the level can change (due
of questions that must be answered: level to be measured is a solid, slurry, to filling or discharging) and disre-
• Can the level sensor be inserted foam, or the interface between two garding any change that occurs faster
than that. desirable to: (weight), and density, all with an
The relationship between level • Detect the true level, while either accuracy of 0.3% of full span.
and tank volume is a function of the the process temperature or den-
cross-sectional shape of the tank. sity varies; Boiling & Cryogenic Fluids
With vertical tanks, this relationship • Measure both level and density; When a d/p cell is used to measure
is linear, while with horizontal or and the level in a steam drum, a reverse-
spherical vessels, it is a non-linear • Measure the volume and the mass acting transmitter is usually installed
relationship (Figure 6-1). (weight) in the tank. (Figure 6-3). An uninsulated condens-
If the level in a tank is to be By measuring one temperature and ing chamber is used to connect the
inferred using hydrostatic pressure three pressures, the system shown in high pressure (HP) side of the d/p
measurement, it is necessary to use Figure 6-2 is capable of simultane- cell to the vapor space on the top
multi-transmitter systems when it is ously measuring volume (level), mass of the drum. The steam condenses
the steaming rate and the associated
swelling increase). It is for this reason
that a reverse acting d/p cell is
recommended for this application.
When the process fluid is liquid
nitrogen (or some other cryogenic
material), the tank is usually sur-
rounded by a thermally insulated
and evacuated cold box. Here, the
low pressure (LP) side of a direct
acting d/p cell is connected to the
vapor space above the cryogenic
liquid (Figure 6-4). As the liquid
in this chamber and fills the wet nitrogen approaches the HP side
leg with ambient temperature water, of the d/p cell (which is at ambi- the level measurement. To protect
while the low pressure (LP) side of ent temperature outside the cold against this, the liquid filled portion
the d/p cell detects the hydrostatic box), its temperature rises. When of the connecting line should be
head of the boiling water inside the the temperature reaches the boiling sloped back towards the tank. The
drum. The output of the d/p cell point of nitrogen, it will boil and, cross-section of the line should be
reflects the amount of water in the from that point on, the connect- large (about 1 inch in diameter) to
drum. Output rises as the mass of ing line will be filled with nitro- minimize the turbulence caused by
water in the drum drops (because gen vapor. This can cause noise in the simultaneous boiling and re-
Electromechanical
Gages
Sonic Echo
condensing occurring at the liquid- When the sludge or slurry level is In other processes, it is desirable to
vapor interface. detected continuously, one of the separately control both the liquid
goals is to eliminate dead-ended cavi- level beneath the foam and the thick-
Sludge, Foam, & Molten Metals ties where the sludge might settle. ness of the foam. In the paper indus-
Many process fluids are aggressive In addition, all surfaces which are try, beta radiation detectors are used
or difficult to handle and it’s best to exposed to the process fluid should for such applications (Kraft process-
avoid physical contact with them. be covered with Teflon®. Figure 6-5B ing), while other industries detect
This can be accomplished by plac- shows such an installation, employing the degree of foaming indirectly (by
measuring related variables, such as
heat input or vapor flow), or they
use capacitance, conductivity, tuning
fork, optical, or thermal switches, all
provided with automatic washers.
Measuring the level of molten
glass or metals is another special
application. The most expensive
(but also most accurate) technique
available is proximity capacitance-
based level measurement, which
can provide a resolution of 0.1 mm
over a range of 6 in. Laser-based
systems can provide even better
resolution from distances up to
2 ft. If such high resolution is not
required and cost is a concern, one
can make a float out of refractory
material and attach a linear variable
ing the level sensor outside the tank Teflon®-coated extended diaphragms differential transformer (LVDT), or
(weighing, radiation) or locating the to minimize material buildup. make a bubbler tube out of refrac-
sensor in the vapor space (ultrasonic, In strippers, where the goal is to tory material and bubble argon or
radar, microwave) above the process drive off the solvent in the shortest nitrogen through it. T
fluid. When these options are not period of time, one aims to keep
available or acceptable, one must aim the foam level below a maximum.
to minimize maintenance and physi-
cal contact with the process fluid.
When the process fluid is a sludge, References & Further Reading
slurry, or a highly viscous polymer, • OMEGA Complete Flow and Level Measurement Handbook and
and the goal is to detect the level at Encyclopedia®, Omega Press, 1995.
one point, the design shown in Figure • Omega Volume 29 Handbook & Encyclopedia, Purchasing Agents
6-5A is commonly considered. The Edition, Omega Press, 1995.
ultrasonic or optical signal source • Instrument Engineer’s Handbook, Bela G. Liptak, editor, CRC Press, 1995.
and receiver typically are separated • Instrumentation for Process Measurement and Control, Third Edition, N.
by more than six inches so that the A. Anderson, Chilton, 1980.
process fluid drains freely from the • Measurement and Control of Liquid Level, C. H. Cho, Instrument Society
intervening space. After a high-level of America, 1982.
episode, an automatic washing spray • Principles of Industrial Measurement for Control Applications, E. Smith,
is activated. Instrument Society of America, 1984.
Pressure/Density Level Instrumentation
O ne of the primary prin-
ciples underlying industrial
level measurement is that
different materials and dif-
ferent phases of the same material
have different densities. This basic
d/p cell can provide an indication
of level (accurate to better than 1%)
over wide ranges, as long as the den-
sity of the liquid is constant. When
a d/p cell is used, it will cancel out
the effects of barometric pressure
tures, are very low (Figure 7-1C). A dry
leg enables the d/p cell to compen-
sate for the pressure pushing down
on the liquid’s surface, in the same
way as the effect of barometric pres-
sure is canceled out in open tanks.
law of nature can be utilized to mea- variations because both the liquid in It is important to keep this refer-
sure level via differential pressure the tank and the low pressure side ence leg dry because accumulation
(that at the bottom of the tank rela- of the d/p cell are exposed to the of condensate or other liquids would
tive to that in the vapor space or to pressure of the atmosphere (Figure cause error in the level measurement.
atmospheric pressure) or via a float 7-1B). Therefore, the d/p cell reading When the process vapors condense
or displacer that depends on the will represent the tank level. at normal ambient temperatures or
density differences between phases. are corrosive, this reference leg can
Level measurement based on pres- Dry & Wet Leg Designs be filled to form a wet leg. If the
sure measurement is also referred to When measuring the level in pressur- process condensate is corrosive,
as hydrostatic tank gaging (HTG). It ized tanks, the same d/p cell designs unstable, or undesirable to use to fill
works on the principle that the dif- (motion balance, force balance, or the wet leg, this reference leg can be
ference between the two pressures electronic) are used as on open tanks. filled with an inert liquid.
(d/p) is equal to the height of the It is assumed that the weight of the In this case, two factors must be
liquid (h, in inches) multiplied by the vapor column above the liquid is considered. First, the specific grav-
specific gravity (SG) of the fluid (see negligible. On the other hand, the ity of the inert fluid (SGwl) and the
Figure 7-1): pressure in the vapor space cannot height (hwl) of the reference column
be neglected, but must be relayed to must be accurately determined, and
d/p = h (SG) the low pressure side of the d/p cell. the d/p cell must be depressed by
Such a connection to the vapor space the equivalent of the hydrostatic
By definition, specific gravity is the is called a dry leg, used when process head of that column [(SGwl)(hwl)].
liquid’s density divided by the den- vapors are non-corrosive, non-plug- Second, it is desirable to provide a
sity of pure water at 68° F at atmo- ging, and when their condensation sight flow indicator at the top of the
spheric pressure. A pressure gage or rates, at normal operating tempera- wet leg so that the height of that ref-
erence leg can be visually checked. in the tank. The required piping and ject to material build-up or plugging,
Any changes in leg fill level (due to valving must always be provided extended diaphragm Type 1:1 repeat-
leakage or vaporization) introduce on both the tank and the reference ers can be considered for the service
error into the level measurement. If leg side of the d/p cell, so that (Figure 7-2).
the specific gravity of the filling fluid draining and flushing operations can While repeaters eliminate the
for the wet leg is greater than that of easily be performed. When a wet errors caused by wet legs, they do
the process fluid, the high pressure reference leg is used, a low thermal introduce their own errors as a func-
side should be connected to the ref- expansion filling fluid should be tion of the pressure repeated. For
erence leg and the low to the tank. selected. Otherwise, the designer example, at 40 psig, repeater error
If the condensate can be used to must correct for the density varia- is about 2 in. At 400 psig, it is 20 in.
fill the reference leg, a condensate tions in the reference leg caused by In many applications, the former is
pot can be mounted and piped both ambient temperature variations. acceptable but the latter is not.
to the high level connection of the If smart transmitters are used and
tank and to the top of the vapor if the filling fluid data is known, wet- • d/p Cells
space. The condensate pot must be leg temperature compensation can Because the designs of the various
mounted slightly higher than the be provided locally. Alternatively, d/p cells are discussed in detail in
high level connection (tap) so that it the host or supervisory control sys- another issue of Transactions, only a
will maintain a constant condensate tem can perform the compensation brief overview is provided here.
level. Excess liquid will drain back calculations. The motion balance cell is well
into the tank. It is also desirable If it is desired to keep the process suited for remote locations where
either to install a level gage on the vapors in the tank, a pressure repeater instrument air or electric power are
condensate pot or to use a sight can be used. These devices repeat the not available. If a bellows is used as
flow indicator in place of the pot, so vapor pressure (or vacuum) and send the sensing element in a motion bal-
that the level in the pot can conve- out an air signal identical to that of ance d/p cell, an increase in the pres-
niently be inspected. the vapor space. The measurement sure on either side causes the corre-
Either method (wet or dry) side of the repeater is connected to sponding bellows to contract (Figure
assures a constant reference leg the vapor space and its output signal 7-3A). The bellows is connected to
for the d/p cell, guaranteeing that to the low pressure side of the d/p a linkage assembly that converts the
the only variable will be the level cell. If the tank connection is sub- linear motion of the bellows into a
rotary indicator motion, which can be wide as 0-1,000 psid. Some elec- performance of the cell. Flat and
calibrated to indicate the tank level. tronic d/p cells can operate at line extended diaphragm-type d/p cells,
In a force-balance type of d/p pressures up to 4,500 psig at 250°F. pressure repeaters, and chemical
cell, the sensing element (often a The drift and inaccuracy of some seals are available to protect d/p
diaphragm) does not move. A force of these units have been tested for cells under these conditions.
bar is provided to maintain the forc- periods of up to 30 months, and the Chemical seals, or diaphragm
es acting on the diaphragm in equi- errors did not exceed the ±0.5% of pressure seals, are available with
librium (Figure 7-3B). In pneumatic span limit. fill liquids such as water, glycol,
d/p cells, this is often achieved alcohol, and various oils. These seals
by the use of a nozzle and flap- • Difficult Process Fluids are used when plugging or corro-
per arrangement that guarantees When the process fluid is a sludge, sion can occur on both sides of
that the pneumatic output signal a viscous polymer or is otherwise the cell. A broad range of corro-
will always be proportional to the hard to handle, the goal is to isolate sion-resistant diaphragm and lin-
differential pressure across the cell. the dirty process from the d/p cell. ing materials is available. Teflon®
The output of pneumatic d/p cells A flat diaphragm can be bolted to lining is often used to minimize
is linear and is usually ranged from 3 a block valve on the tank nozzle so material build-up and coating. Level
to 15 psig. The levels represented by that the d/p cell can be removed measurement accuracy does suffer
such transmitted signals (pneumatic, for cleaning or replacement without when these seals are used. Capillary
electronic, fiberoptic or digital) can taking the tank out of service. If it tube lengths should be as short as
be displayed on local indicators or is acceptable to take the tank out possible and the tubes should be
remote instruments. Pneumatic trans- of service when d/p cell removal shielded from the sun. In addition,
mitters require a compressed air (or is needed, an extended diaphragm either low thermal expansion filling
nitrogen) supply. design can be considered. In this fluids should be used or ambient
Electronic d/p cells provide case, the diaphragm extension fills temperature compensation should
±0.5% of span or better precision the tank nozzle so that the dia- be provided, as discussed in con-
typically conveyed via a 4-20 mA phragm is flush with the inside sur- nection with wet legs. If the seals
signal. The range of these simple face of the tank. This eliminates leak, maintenance of these systems
and robust cells can be as narrow dead ends or pockets where sol- is usually done at the supplier’s
as a draft range of 0-H inH2O or as ids can accumulate and affect the factory due to the complex
evacuation and backfilling proce- end of the dip pipe should be located at a pressure at least 10 psi greater
dures involved. far enough above the tank bottom so than the expected maximum total
that sediment or sludge will not plug pressure required (when the tank
Bubbler Tubes it. Also, its tip should be notched is full and the vapor pressure is at
Bubbler tubes provide a simple and with a slot or “V” to ensure the for- its maximum). An alternative to
inexpensive but less accurate (±1-2%) mation of a uniform and continuous a continuous bubbler is to use a
level measurement system for cor- flow of small bubbles. An alternative hand pump (similar to a bicycle
rosive or slurry-type applications. to locating the dip pipe in the tank tire pump) providing purge air only
Bubblers use compressed air or an is to place it in an external chamber when the level is being read.
inert gas (usually nitrogen) intro- connected to the tank. Bubblers do consume inert gases,
duced through a dip pipe (Figure 7-4A). In pressurized tanks, two sets which can later accumulate and
Gas flow is regulated at a constant of dip pipes are needed to mea- blanket processing equipment. They
rate (usually at about 500 cc/min). A sure the level (Figure 7-4B). The also require maintenance to ensure
differential pressure regulator across two back-pressures on the two dip that the purge supply is always avail-
a rotameter maintains constant flow, pipes can be connected to the two able and that the system is properly
while the tank level determines the sides of a u-tube manometer, a adjusted and calibrated. When all
back-pressure. As the level drops, differential pressure gage or a d/p factors are considered, d/p cells
the back-pressure is proportionally cell/transmitter. The pneumatic pip- typically are preferred to bubblers in
reduced and is read on a pressure ing or tubing in a bubbler system the majority of applications.
gage calibrated in percent level or should be sloped toward the tank
on a manometer or transmitter. The so that condensed process vapors • Elevation & Suppression
dip pipe should have a relatively large will drain back into the tank if purge If the d/p cell is not located at an
diameter (about 2 in.) so that the pres- pressure is lost. The purge gas supply elevation that corresponds to 0%
sure drop is negligible. The bottom should be clean, dry, and available level in the tank, it must be calibrated
to account for the difference in ele- the high pressure side of the d/p cell saturated liquid layer (0.76) vary
vation. This calibration adjustment is should be connected to the tank if not only with drum pressure but
called zero elevation when the cell the specific gravity of the wet leg also with steaming rate. This causes
is located above the lower tap, and filling fluid is close to that of the light the swelling of bubbles when the
is called zero suppression or zero layer. It should be connected to the steaming rate rises (and SG2 drops),
as well as their collapse when the
steaming rate drops (and SG2 rises).
Therefore, to make an accurate
determination of both the level and
the mass of the water in the steam
drum, the calculation must consider
not only the d/p cell output, but
also the drum pressure and the pre-
vailing steaming rate.
• Tank Farms
h1
Computerized tank farm systems usu-

ally accept level signals from several
h2
LT tanks through field networks. These

systems perform the level monitoring
tasks using a variety of compensation
and conversion algorithms. The algo-
rithms provide density corrections,
volumetric or mass conversions, and
depression when the cell is located reference leg if the wet-leg fluid’s SG corrections to consider the shapes of
below the lower tap. Most d/p cells is closer to that of the heavy layer. horizontal, vertical or spherical tanks.
are available with elevation and sup- These systems can perform safety
pression ranges of 600% and 500% • Special Applications functions, such as shutting off feed
of calibrated span, respectively, as When the process fluid is boiling, pumps to prevent overfilling.
long as the calibrated span does not such as in a steam drum, a wet refer-
exceed 100% of the upper range limit ence leg is maintained by a conden- Floats & Displacers
of the cell. sate pot, which drains back into the It was more than 2,200 years ago
For example, assume that an elec- steam drum so that the level of the that Archimedes first discovered
tronic d/p cell can be calibrated for wet leg is kept constant. Changes in that the apparent weight of a float-
spans between 0-10 psid (which is its ambient temperature (or sun expo- ing object is reduced by the weight
lower range limit, LRL) and 0-100 psid sure) will change the water density of the liquid displaced. Some 2,000
(which is its upper range limit, URL). in the reference leg and, therefore, years later, in the late 1700s, the first
The cell is to be used on a 45-ft tall temperature compensation (manual industrial application of the level float
closed water tank, which requires a or automatic) is needed. appeared, when James Brindley and
hydrostatic range of 0-20 psid. The Figure 7-5 describes a typical Sutton Thomas Wood in England and
cell is located about 11 feet (5 psid) power plant steam drum level appli- I. I. Polzunov in Russia introduced
above the lower tap of the tank; cation. The differential pressure the first float-type level regulators in
therefore, a zero elevation of 5 psid is detected by the level d/p cell is: boilers.
needed. The d/p cell can handle this Floats are motion balance devices
application, because the calibrated d/p = h1SG1 + h2SG2 - h3SG3 that move up and down with liquid
span is 20% of the URL and the eleva- d/p = 0.03h1 + 0.76h2 - 0.99h3 level. Displacers are force balance
tion is 25% of the calibrated span. devices (restrained floats), whose
On interface level measurement Note that the SG of the saturated apparent weight varies in accor-
applications with a wet leg reference, steam layer (0.03) and that of the dance with Archimedes’ principle:
the buoyant force acting on an object to 80°C (-40 to 180° F) and up to 150 than the minimum expected specific
equals the weight of the fluid dis- psig for rubber or plastic floats, and gravity (SG) of the process fluid. For
placed. As the level changes around -40 to 260°C (-40 to 500°F) and up clean liquids a 0.1 SG difference might
B)
the stationary (and constant diameter) to 750 psig for stainless steel floats. suffice, while for viscous or dirty
displacer float, the buoyant force var- Standard float sizes are available from applications, a difference of at least
ies in proportion and can be detected 1 to 5 inches in diameter. Custom 0.3 SG is recommended. This provides
as an indication of level. Regular and float sizes, shapes, and materials can additional force to overcome the
displacer floats are available as both be ordered from most manufacturers. resistance due to friction and material
continuous level transmitters and The float of a side-mounted switch is build-up. In dirty applications, floats
point-sensing level switches. horizontal; a permanent magnet actu- should also be accessible for cleaning.
In industrial applications, displacer ates the reed switch in it (Figure 7-6B). Floats can be attached to mechan-
floats are often favored because they Floats should always be lighter ical arms or levers and can actuate
do not require motion. Furthermore,
force can often be detected more
accurately than position. However,
regular floats are also used, mostly
for utilities and in other secondary
applications.
• Float Level Switches

The buoyant force available to oper-
ate a float level switch (that is, its net
buoyancy) is the difference between
the weight of the displaced fluid (gross
buoyancy) and the weight of the float.
Floats are available in spherical (Figure
7-6A), cylindrical (Figure 7-6B), and a
variety of other shapes (Figure 7-6C).
They can be made out of stainless
steel, Teflon®, Hastelloy, Monel, and
various plastic materials. Typical tem-
perature and pressure ratings are -40
electrical, pneumatic, or mechanical the cable determines the actuation pumps, another in the middle (HI) to
mechanisms. The switch itself can level. One, two, or three switches start one pump, and the last at the
be mercury (Figures 7-6A and 7-6C), can be used to operate simplex and top (HI-HI) to actuate the second
dry contact (snap-action or reed type, duplex sump-pump stations. A sim- pump, as well as perhaps an audible
and/or visual alarm.
Figure 7-8A illustrates how a side-
mounted float switch might actuate
an adjacent, sealed reed switch. The
main advantage of this design is that
the lever extension tends to amplify
the buoyant force generated by the
float. Therefore the float itself can
be rather small. The main disadvan-
tage is that the tank must be opened
in order to perform maintenance
on the switch. If the buoyant force
of the float is used mechanically to
actuate a snap-action switch, a force
of only one ounce is needed.
In top (or bottom) mounted mag-
netic float switches (Figure 7-8B), the
magnet is in the cylindrical float that
travels up or down on a short vertical
guide tube containing a reed switch.
shown in Figure 7-6B), hermetically plex (one pump) system will use a The float’s motion is restrained by
sealed, or pneumatic. The switch can single switch wired in series with clips and can be only H in or less.
be used to actuate a visual display, the motor leads so that the switch These float and guide tubes are avail-
annunciator, pump, or valve. The elec- directly starts and stops the pump able with multiple floats that can
tric contacts can be rated light-duty motor (Figure 7-7). detect several levels. The switch
(10-100 volt amps, VA) or heavy-duty A duplex (two pump) application assembly itself can be either inserted
(up to 15 A @ 120 Vac). If the switch might use three switches: one at directly into the tank or side-mount-
is to operate a circuit with a greater the tank bottom (LO) to stop both ed in a separate chamber.
load than the rating of the switch con-
tacts, an interposing relay needs to be
inserted. If the switch is to be inserted
in a 4-20 mA dc circuit, gold-plated
dry contacts should be specified to
ensure the required very low contact
resistance.
• Applications & Installations

In the tilt switch (Figure 7-6C), a
mercury element or relay is mounted
inside a plastic float; the float’s elec-
trical cable is strapped to a pipe
inside the tank or sump. As the level
rises and falls, the float tilts up and
down, thus opening and closing its
electric contact. The free length of
A magnetic piston operated can be provided with level gages. the support spring.
switch also can be mounted in an Multiple switches are available for Testing the proper functioning of
external chamber (Figure 7-8C). As multiple-switching applications such a regular float switch may require
the magnet slides up and down as boiler level alarms and controls. filling the tank to the actuation level,
inside a non-magnetic tube, it oper- • Displacer Switches while a displacer switch can be test-
ates the mercury switch outside the Whereas a float usually follows the ed simply by lifting a suspension
tube. These switches are complete- liquid level, a displacer remains par- (Figure 7-10A). Displacer switches are
ly sealed and well suited for heavy tially or completely submerged. As available with heavy-duty cages and
duty industrial applications up to shown in Figure 7-10A, the apparent flanges for applications up to 5000
900 psig and 400°C (750°F), meet- weight of the displacer is reduced psig at 150°C (300°F), suitable for use
ing ASME code requirements. These as it becomes covered by more liq- on hydraulic accumulators, natural
switches can be side, top, or cage uid. When the weight drops below gas receivers, high pressure scrub-
mounted (Figure 7-9) and can serve the spring tension, the switch is bers, and hydrocarbon flash tanks.
both alarm and control functions actuated. Displacer switches are
on steam drums, feedwater heaters, more reliable than regular floats • Continuous Level Displacers
condensate pots, gas/oil separa- on turbulent, surging, frothy, or Displacers are popular as level
tors, receivers, and accumulators. foamy applications. Changing their transmitters and as local level con-
Light-duty caged float switches are settings is easy because displac- trollers, particularly in the oil and
also available for service ratings up ers can be moved anywhere along petrochemical industries. However,
to 250 psig at 200°C (400°F) and 400 the suspension cable (up to 50 ft). they are not suited for slurry or
psig at 40°C (100°F)—suitable for These switches are interchangeable sludge service because coating of
many boilers, condensate receivers, between tanks because differences the displacer changes its volume and
flash tanks, day tanks, holding tanks, in process density can be accommo- therefore its buoyant force. They are
and dump valve controls. The cages dated by changing the tension of most accurate and reliable for servic-
es involving clean liquids of constant torque tubes, the buoyant force can cesses that are operated continuous-
density. They should be temperature- also be detected by other force sen- ly, the American Petroleum Institute
compensated, particularly if varia- sors, including springs and force-bal- recommends (in API RP 550) that
tions in process temperature cause ance instruments. When the buoyant displacers be installed in external
significant changes in the density of force is balanced by a spring, there is standpipes with level gages and iso-
the process fluid. some movement, while with a force- lating valves (Figure 7-11). This way it
is possible to recalibrate or maintain
the displacer without interrupting
the process.
• Interface Applications
When measuring the interface
between a heavy liquid and a light
liquid (such as oil on water), the
top connection of the displacer
is placed into the light and the
bottom connection into the heavy
liquid layer. If the output of such a
transmitter is set to zero when the
chamber is full of the light liquid,
and to 100% when it is full with
the heavy phase, the output will
correspond to the interface level.
Naturally, when interface is being
measured, it is essential that the
two connections of the displacer
chamber be located in the two differ-
ent liquid layers and that the chamber
always be flooded. Displacer diam-
eter can be changed to match the
difference in liquid densities, and
displacer length can be set to match
the vertical range of the level inter-
face variation.
When used as a level transmitter, balance detector, the displacer stays Regular floats can also be used
the displacer, which is always heavier in one position and only the level for interface detection if the dif-
than the process fluid, is suspended over the displacer varies. ference in SG between the two
from the torque arm. Its apparent Displacer units are available with process liquids is more than 0.05.
weight causes an angular displace- both pneumatic and electronic out- In such applications, a float den-
ment of the torque tube (a torsion puts and can also be configured sity is needed that is greater than
spring, a frictionless pressure seal). as local, self-contained controllers. the lighter liquid and less than the
This angular displacement is linear- When used in water service, a 100 heavier liquid. When so selected,
ly proportional to the displacer’s cubic inch displacer will gener- the float will follow the interface
weight (Figure 7-10B). ate a buoyant force of 3.6 pounds. level and, in clean services, provide
Standard displacer volume is 100 Therefore, standard torque tubes are acceptable performance.
cubic inches and the most commonly calibrated for a force range of 0-3.6
used lengths are 14, 32, 48, and 60 lbf and thin-walled torque tubes for • Continuous Level Floats
in. (Lengths up to 60 ft are available a 0-1.8 lbf range. Of the various float sensor designs
in special designs.) In addition to For oil refineries and other pro- used for continuous level measure-
ment, the oldest and arguably most steel) pipe. The pipe is connected to pulse. The difference in the interroga-
accurate is the tape level gage (Figure flanged nozzles on the side of the tion time and the return pulse time is
7-12A). In this design, a tape or cable tank. The pipe column is provided proportional to the liquid level in
connects the float inside the tank to with a visual indicator, consisting the tank.
a gage board or an indicating take- of G-in triangular wafer elements. This tank level sensing method
up reel mounted on the outside of These elements flip over (from is highly accurate, to ±0.02 in, and
the tank. The float is guided up and green to red, or any other color) therefore is ideal for precision inven-
down the tank by guide wires or trav- when the magnet in the float reach- tory management operations. The
els inside a stilling well. These level es their level (Figure 7-12B). Alarm sensor is available in lengths of 2-25
indicators are used in remote, unat- switches and transmitter options ft and can be inserted into the tank
tended, stand-alone applications, or are available with similar magnetic from the top of the vessel through
they can be provided with data trans- coupling schemes (Figure 7-12C). In flanged, screwed, or welded con-
mission electronics for integration a similar design, a series of reed nections. For the simultaneous mea-
into plant-wide control systems. switches is located inside a stand- surement of both interface and total
To install the tape gage, an open- pipe. The change in output voltage level, a two-float system is available
ing is needed at the top of the as the individual reed switches are (Figure 7-12D). A resistance tempera-
tank and an anchor is required at its closed by the rising magnet is mea- ture detector (RTD) is also available
bottom. When properly maintained, sured, giving an indication of level. for temperature compensation. Like
tape gages are accurate to ±G in. It The operation of magnetostric- all other float level instruments, this
is important to maintain the guide tive sensors is based on the Villari design too is for clean liquids. Rating
wires under tension, clean and free effect. In the magnetic waveguide- is up to 150°C (300° F) and 300 psig.
of corrosion, and to make sure that type continuous level detector, the The transmitter output can be 4-20
the tape never touches the protec- float (or floats, when detecting inter- mA dc analog or fieldbus-compatible
tive piping in which it travels. If this face) travels concentrically up and digital.
is not done, the float can get stuck down outside a vertical pipe. Inside
on the guide wires or the tape can the pipe is a concentric waveguide • Float Control Valves
get stuck to the pipe. (This can hap- made of a magnetostrictive material. Float-operated control valves com-
pen if the level does not change for A low current interrogation pulse is bine level measurement and level
long periods or if the tank farm is sent down the waveguide, creating control functions into a single level
located in a humid region.) an electromagnetic field along the regulator. While simple and inex-
Another continuous level indicator length of the waveguide. When this pensive, they are limited to applica-
is the magnetic level gage, consist- field interacts with the permanent tions involving small flows and small
ing of a magnetic float that travels magnet inside the float, a torsional pressure drops across the valve. This
up and down on the inside of a strain pulse (or waveguide twist) is is because the force available to
long, non-magnetic (usually stainless created and detected as a return throttle the valve is limited to that
provided by the buoyant force act- way for the regulator to increase the regulator (Kc = Qmax/h), expressed
ing on the float, multiplied by the feed flow (say into a cooling tower in units of GPM/inch. The offset of
lever action of the float arm. This basin) is to first let the level drop a float regulator is the distance (in
does not suffice to close large valves so that the sinking of the float will inches) between the center of the
against high pressure differentials. further open the valve. The relation- float range and the amount of eleva-
Yet, for simple and unattended ship between the maximum flow tion of the float required to deliver
applications (like controlling the through a linear valve (Qmax) and the flowrate demanded by the pro-
make-up water supply into a cooling the range in liquid level (h) is called cess. T
tower basin or draining condensate the proportional sensitivity of the
from a trap), they are acceptable. It
is important to understand that float
regulators are simple proportional References & Further Reading
controllers: they are incapable of • OMEGA Complete Flow and Level Measurement Handbook and
holding level at a single setpoint. Encyclopedia®, Omega Press, 1995.
What they can do is open or close a • Omega Volume 29 Handbook & Encyclopedia, Purchasing Agents
valve as the float travels through its Edition, Omega Press, 1995.
control range. Therefore, instead of a • Instrument Engineer’s Handbook, Bela G. Liptak, editor, CRC Press, 1995.
setpoint, regulators have a throttling • Instrumentation for Process Measurement and Control, Third Edition, N.
range. If the range is narrow (floats A. Anderson, Chilton, 1980.
usually fully stroke their valve over • Measurement and Control of Liquid Level, C. H. Cho, Instrument Society
a few inches of float travel), it gives of America, 1982.
the impression of a constant level. • Principles of Industrial Measurement for Control Applications, E. Smith,
In fact, level will vary over the Instrument Society of America, 1984.
throttling range because the only
RF/Capacitance Level Instrumentation
C apacitance level detectors
are also referred to as radio
frequency (RF) or admittance
level sensors. They operate
in the low MHz radio frequency
range, measuring admittance of an
is capable of storing an electric
charge. The storage capability of a
capacitor is measured in farads. As
shown in Figure 8-1, the capacitor
plates have an area (A) and are sepa-
rated by a gap (D) filled with a non-
tance. The lower the dielectric con-
stant, the lower the admittance of the
material (that is, the less conductive it
is). Capacitance (C) is calculated as:
C = KA/D
alternating current (ac) circuit that conducting material (dielectric) of
varies with level. Admittance is a dielectric constant (K). The dielectric If the area (A) of and the distance
(D) between the plates of a capaci-
tor remain constant, capacitance
will vary only as a function of the
dielectric constant of the substance
filling the gap between the plates.
If a change in level causes a change
in the total dielectric of the capaci-
tance system, because (as illustrated
in Figure 8-1B) the lower part of area
(A) is exposed to a liquid (dielectric Kl)
while the upper part is in contact with
a vapor (dielectric Kv, which is close
to 1.0), the capacitance measurement
will be proportional to level.
In the case of a horizontally
mounted level switch (Figure 8-2),
a conductive probe forms one of
measure of the conductivity in an constant of a vacuum is 1.0; the the plates of the capacitor (A1), and
ac circuit, and is the reciprocal of dielectric constants of a variety of the vessel wall (assuming it is made
impedance. Admittance and imped- materials are listed in Table 7. from a conductive material) forms
ance in an ac circuit are similar to The dielectric constant of a sub- the other (A2). An insulator with a
conductance and resistance in a stance is proportional to its admit- low dielectric constant is used to
direct current (dc) circuit. In this
chapter, the term capacitance level
sensor will be used instead of RF or
admittance.
- --
Table 6 lists some of the industries -
- --
and applications where capacitance- - -- --
- + ++
-
type level sensors are used. - -- ++
++
--
-- + ++ ++
++
Theory of Operation --
-
++
++
++
++
+
A capacitor consists of two con- ++
++
ductors (plates) that are electri- +
cally isolated from one another by
a nonconductor (dielectric). When
the two conductors are at differ-
ent potentials (voltages), the system
isolate the conductive probe from the sensor probe and the vessel one coulomb of electric energy. A
the housing, which is connected to wall. Both of these values are fixed. pico-farad is one trillionth of that,
the vessel wall. The probe is con- Therefore, when the probe is no lon- and the sensitivity of an accurate
capacitance detector is 0.5 pF. This
is the minimum detectable change in
capacitance resulting from a change
in dielectric constant (K2 -K1).
In most level-sensing applications,
the reference material is air (K1 = 1.0).
Table 7 gives the K2 values of a variety
of process materials. As the dielectric
constant of the process material gets
close to that of air (K2 for plastic pel-
lets, for example, is 1.1), the measure-
ment becomes more difficult.
Probe Designs
The most common probe design is
a stainless steel rod of G in. or H
in. diameter, suitable for most non-
conductive and non-corrosive mate-
rials. The probe is insulated from
nected to the level sensor via the ger surrounded by vapors (K1), but by the housing and bin wall by an low-
conductive threads of the housing. the process material (K2), the resulting dielectric insulator, such as Nylon or
Measurement is made by applying capacitance change is directly related Ryton. These polymers have maxi-
an RF signal between the conductive to the difference in dielectric con- mum operating temperatures of 175-
probe and the vessel wall. stant between the two media: 230°C (350-450°F). Ceramics can be
The RF signal results in a minute used for higher temperature appli-
current flow through the dielectric Change in C = (K2 - K1)(A/D) cations or if abrasion resistance is
process material in the tank from the required. For applications where the
probe to the vessel wall. When the The sensitivity of a capacitance process material is conductive and
level in the tank drops and the probe sensor is expressed in pico-farads (pF). corrosive, the probe must be coated
is exposed to the even less conduc-
tive vapors, the dielectric constant
drops. This causes a drop in the
capacitance reading and a minute
drop in current flow. This change
is detected by the level switch’s
internal circuitry and translated into
a change in the relay state of the
level switch. In the case of continu-
ous level detectors (vertical probes),
the output is not a relay state, but a
scaled analog signal.
The total area is the combined
area of the level sensor probe and The capacitance unit is the farad, with Teflon® or Kynar.
the area of the conductive vessel defined as the potential created Some point level sensors are
wall (A = A1 + A2), and the distance when a one-volt battery connected available with build-up immunity, or
(D) is the shortest distance between to a capacitor causes the storage of coating rejection functionality. This
is required when the process mate- 7 to 16 in. These probes typically are In applications where the vessel
rial is wet or sticky and likely to side-mounted (Figure 8-4A). Vertical is non-conductive and unable to
cause permanent coating. Build-up probes can be extended by solid rods form the return path for the RF sig-
immunity is provided by the addition up to a length of 1.2 to 1.5 m (4 to 5 ft), nal, a second probe placed parallel
of a second active section of probe or a steel cable with a weight can be to the active one or a conductive
and a second insulator (Figure 8-3). used to suspend the probe up to 15 m strip can be installed.
This second active section (the driven (50 ft) (Figure 8-4B). Most capacitance • Electronics & Housings
The electronic circuitry of the probe
performs the functions of: 1) rectifying
and filtering the incoming power, 2)
generating the radio frequency signal,
3) measuring the changes in current
flow, and 4) driving and controlling
interface devices such as relays,
analog signal generators and display
meters. The circuitry is usually of
Capacitance probes typically are coated with Teflon® (shown), Kynar, or polyethylene solid state design and provided with
potentiometer adjustments for set-
shield) is driven at the same poten- level sensors are provided with I to ting sensitivity and time delays.
tial and frequency as the measuring 1-H in NPT mounting connectors. The Because the level sensor will ulti-
probe. Because current cannot flow matching female coupling is usually mately drive an external device, it
between equal potentials, the mea- welded to the vessel wall and the is advisable to evaluate for system
suring probe does not sense material capacitance probe is screwed into compatibility the number of relays
build-up between the probe and ves- the mating connector. Low profile required, their capacities, or the
sel wall. capacitance sensors also are available analog signals required, time delays,
Typical insertion lengths of stan- (Figure 8-4C) and are flange-mount- and power supply requirements.
dard capacitance probes range from ed. More advanced microprocessor-
based units are self-calibrating; excluding temperature and supply synthetic materials such as glass-
sensitivity and time delay adjust- voltage effects) is typically 0.25% of reinforced nylon. Most housings are
ments are under pushbutton con- range. Minimum span is 4 pF, and the suitable for outdoor installations in
trol. These units are often supplied upper range limit (URL) is 2,500 pF. dusty or wet environments.
with self-test capability and built-in Level switches are usually provided • The Dielectric Constant
The dielectric constant of the pro-
cess material is the most important
SOLIDS
aspect of the process data. The higher
the difference between the dielectric
constants (of the process material
and the vapor space or between the
two layers in the case of an interface
measurement), the easier the mea-
surement. If the difference is low (K2-
K1 < 1.0 in Figure 8-2), a high sensitivity
design (0.5 pF) must be used.
Each sensor has a capacitance
threshold, defined as the amount
of capacitance change required to
cause a change in the sensor output.
The dielectric constant of a mate-
rial can change due to variations
in temperature, moisture, humidity,
material bulk density, and particle
size. If the change in dielectric con-
stant results in a greater capacitance
change than the calibrated capaci-
tance threshold of the sensor, a false
reading will result. This condition
can usually be corrected by reducing
the sensitivity (increasing the capaci-
tance threshold) of the sensor.
As shown in connection with
Figure 8-3, sensitivity can be
increased by increasing the probe
length (A) or by decreasing the size
of the gap (D). Either or both changes
will minimize the effect of dielec-
tric constant fluctuations or increase
sensitivity to low dielectrics. It is
temperature compensation. with time delays for filtering out usually more practical to specify a
The more advanced designs are false readings caused by material longer probe than to decrease the
also two-wire, intrinsically safe, and shifts or splashing liquids. In addition, distance (D) from the vessel wall.
supply your choice of standard 4-20 the feature of failsafe selectability When the probe is installed from the
mA or digitally enhanced out- provides a predetermined state for side (Figure 8-4A), D is fixed, whereas
put using the HART (Highway the relay output in the event of a if the probe is inserted from the top
Addressable Remote Transducer) power failure or malfunction. of the tank, D can be changed (if other
protocol. Accuracy (including linear- Sensor housings are typically considerations permit) by moving the
ity, hysteresis, and repeatability, but made from cast aluminum, steel, or probe closer to the wall of the vessel.
If the same vessel will hold dif-
ferent materials at different times,
the capacitance sensor must be
A equipped with local or remote reca-
libration capability.
Light density materials under 20
lb/ft3 and materials with particle
sizes exceeding H in. in diameter can
be a problem due to their very low
dielectric constants (caused by the
large amount of air space between
particles). These applications might
B not be suited for capacitance-type
level measurement.
• Application Considerations
Materials that are conductive (water-
based liquids with a conductivity of
100 micromhos/cm or more) can
cause a short circuit between a bare
stainless steel probe and the ves-
sel wall. As the liquid level drops,
the probe remains wetted, provid-
ing a conductive path between the
probe and the vessel wall. The faster
the level changes, the more likely
this false indication is to occur. It is
advisable to use Teflon® or Kynar
insulator coating on the conductive
probe surface when the process fluid
D
is conductive.
Temperature affects both the
sensor components inside the ves-
sel (active probes and insulators)
and the electronic components and
housing outside. An active probe is
typically made from stainless steel
E and, as such (unless it is coated), it is
suitable for most applications. Probe
insulators can be Teflon®, Kynar, or
ceramic, and should be selected for
the operating temperature of the
application. The housing and the
electronics are affected by both the
internal and external vessel tempera-
tures.
Ambient temperature limits usually
are specified by the manufacturer,
but heat conduction from a high-
temperature process is more dif- operation is unaffected by incom- end of the cable does not touch the
ficult to evaluate. Heat conduction ing or outgoing material flow vessel wall. The probe should not be
can be reduced by using an extended (Figure 8-5A). Material impacts can mounted where material can form
mounting coupling or one made of a cause false readings or damage a bridge between the active probe
low thermal conductivity material. to the probe and insulator. When and the vessel wall. In addition, the
If such methods are insufficient, the measuring low-dielectric materials, probe should not be mounted at an
electronics may be mounted up to it's important that the entire probe upward angle (Figure 8-5D), to avoid
20 ft away and connected via coaxial be covered, not just the tip (Figure material build-up.
cable. The cable’s inherent capaci- 8-5C). When rod or cable exten- If more than one capacitance level
tance, however, reduces the overall sions are used, allow for 8-12 in. of sensor is mounted in the vessel, a
sensitivity of the system. active probe coverage. minimum distance of 18 in. should be
Housings must also be com- Install the probe so that it does provided between the probes (Figure
patible with the requirements for not contact the vessel wall (Figure 8-5E). Closer than that and their elec-
hazardous, wash-down, wet, and/ 8-5B) or any structural elements of tromagnetic fields might interfere.
or dusty environments. Explosion- the vessel. If a cable extension is If a capacitance probe is installed
proof environments may require the used, allow for swinging of the cable through the side wall of a vessel
housing to be certified. In addition, as the material level in the vessel and the weight of the process mate-
the active probe might need to be rises, so that the plumb bob on the rial acting on the probe is sometimes
intrinsically safe.
If the process material is corrosive References & Further Reading
to stainless steel, the probe should • OMEGA Complete Flow and Level Measurement Handbook and
be coated with Kynar or Teflon® for Encyclopedia®, Omega Press, 1995.
protection. Ryton is a good choice • Omega Volume 29 Handbook & Encyclopedia, Purchasing Agents
for abrasive materials, and, for food Edition, Omega Press, 1995.
grade or sanitary applications, stain- • Instrument Engineer’s Handbook, Bela Liptak, Third Edition, CRC Press,
less steel and Teflon® are a good 1995.
probe-insulator combination. • Instrumentation for Process Measurement and Control, Third Edition, N.
A. Anderson, Chilton, 1980.
Installation Considerations • Measurement and Control of Liquid Level, C. H. Cho, Instrument Society
The capacitance probe should be of America, 1982.
mounted in such a way that its • Principles of Industrial Measurement for Control Applications, E. Smith,
Radiation-Based Level Gages
A n entire class of level instru-
mentation devices is based
on a material’s tendency to
reflect or absorb radiation.
For continuous level gages, the most
common types of radiation used are
their power levels (radar is around 0.01
mW/cm2, while microwaves range
from 0.1-5 mW/cm2). Because micro-
waves operate at a higher energy
level, they can withstand more coat-
ing than can radar-type sensors.
of the radar pulse and the reception
of the return echo. It is determined
by the radar detector, which is simul-
taneously exposed to both the sent
and the reflected signal. The detec-
tor output is based on the difference.
radar/microwave, ultrasonic, and Radar sensors consist of a trans- The frequency-modulated (FM) signal
nuclear. Optical electromagnetic mitter, an antenna, a receiver with varies from 0 to 200 Hz as the dis-
radiation also can be used, but this signal processor, and an operator tance to the process fluid surface
has found its way primarily into the
point-switch applications discussed
in the next chapter.
The main advantage of a radia-
tion-based level gage is the absence
of moving parts and the ability to
detect level without making physi-
cal contact with the process fluid.
Because they can in effect “see”
through solid tank walls, nuclear
radiation gages are perhaps the
ultimate in non-contact sensing.
Because they require a gamma
radiation source and are relatively
expensive, however, nuclear gages
are often considered the level gage
of last resort.
Radar & Microwave interface. The transmitter is mount- varies between 0 and 200 ft. Because
In 1925, A. Hoyt Taylor and Leo Young ed on top of the vessel. Its solid- this measurement takes place in the
of the U.S. Navy used radar (RAdio state oscillator sends out an elec- frequency domain, it is reasonably
Detection And Ranging) to measure tromagnetic wave (using a selected free of noise interference.
the height of the earth’s ionosphere. carrier frequency and waveform) The depth of the vapor space (the
By 1934, they were developing aimed downward at the surface of distance between the datum point
radar for Navy ships. In 1935, Robert the process fluid in the tank. The and the level in the tank, identified
Watson-Watt of England used radar frequency used is typically 10 GHz. as “d” in Figure 9-1B) is calculated
to detect aircraft. The first radar level The signal is radiated by a par- from the time of flight (t) and the
sensors were introduced in 1976, but abolic dish or horn-type antenna speed of light (c = 186,000 miles/
they did not become economically (Figure 9-1A) toward the surface of sec):
competitive until a decade later. the process liquid (Figure 1B). A por-
Both radar signals and microwaves tion is reflected back to the antenna, d = t/2c
travel at the speed of light, but where it is collected and routed to
are distinguished by their frequen- the receiver. Here, a microproces- The level (L in Figure 9-1B) is calculated
cies (FM radio broadcast frequency sor calculates the time of flight and by figuring the difference between
is from 88 to 108 MHz, while micro- calculates the level. Time of flight is the total tank height (E) and the
waves range from 1-300 GHz) and by the period between the transmission vapor space depth (d):
pulsed radar waves or frequency- The unreflected portion travels on to
L = E-d modulated continuous waves the end of the probe and provides a
(FMCW). In the first, short-duration zero-level reference signal. Contact
Knowing the signal velocity (c) and the radar pulses are transmitted and the radar technology can be used on liq-
dielectric constant (dc) of the vapor target distance is calculated using uids and on small-grained bulk solids
(that is, the relative ability of the vapor the transit time. The FMCW sen- with up to 20-mm grain size.
to oppose and reflect electromagnetic sor sends out continuous frequency- Reflection-type microwave switch-
waves), the velocity of the radar wave modulated signals, usually in succes- es measure the change in amplitude
transmission (V) can be calculated: sive (linear) ramps. The frequency of a reflected signal (Figure 9-3A). Air
difference caused by the time delay and vapors return a small percent-
V = c/(dc)0.5 between transmittal and reception age of the signal because of their
• Antenna Designs and Mounting

The two commonly used antennas
are the horn and the parabolic dish
antenna. When the radar level gage
sends out its signal, the microwaves
spread out. The larger the antenna
diameter, the smaller the divergence
angle and the greater the signal
strength (Figure 9-1A). The disadvan-
tages of smaller antennas include
higher beam spreading and the cor-
respondingly increased possibility of
reflection from obstacles within the
tank. On the positive side, there is a
greater chance that the emitted beam
will be reflected back to the detector. indicates the distance. low dielectric constants, while high
Therefore, alignment of the sensor is Radar beams can penetrate plas- dielectric materials such as water
not as critical. tic and fiberglass; therefore, non- return almost all the signal. More sen-
Large antennas generate a more contact radar gages can be isolated sitive switches can distinguish liquid-
focused signal, helping to eliminate from the process vapors by a seal. liquid or liquid-solid interfaces having
noise interference from flat and hori- The seal can be above the parabolic as little as 0.1 difference in dielectric
zontal metal surfaces. On the other disc (Figure 9-1A) or can totally iso- constant. Low dielectric materials like
hand, they are more prone to errors late the sensor (Figure 9-2A). The plastic pellets (dielectric 1.1) can be
caused by unwanted reflections beam’s low power allows for safe measured if the particle diameter is
from turbulent or sloping surfaces. installation in both metallic and less than 0.1 in (larger than that, exces-
A fully isolated antenna mounted non-metallic vessels. Radar sensors sive beam scattering occurs).
outside the tank (Figures 9-2 and 9-4) can be used when the process mate- The beam-breaker switch sends a
provides both sealing and thermal rials are flammable or dirty and microwave beam from a transmitter
isolation. If the antenna is positioned when the composition or tempera- to a receiver located on the oppo-
below the process seal, it is exposed ture of the vapor space varies. site side of the tank. When the beam
to the process vapors, but gains Contact radar gages send a pulse is blocked, the signal is weakened
the advantages of stronger signal down a wire to the vapor-liquid inter- (Figure 9-3B). Beam-breaker align-
amplitudes and suitability for higher face. There, a sudden change in the ment is not critical, and separation
operating pressures. dielectric constant causes the signal distance can be up to 100 ft.
• Contact & Non-Contact Radar to be partially reflected. The time-of- Both reflection and beam-breaker
Non-contact radar gages either use flight is then measured (Figure 9-2B). microwave switches are typically
used in applications where it is desir- sensors are affected by the com- 762 mph. At that same temperature,
able not to penetrate the tank. These position of the vapor space. On an ultrasonic pulse travels through
non-intrusive sensors send electro- the other hand, ultrasonic sensors water at 1,496 m/s or 3,353 mph. If
magnetic radio waves through plastic, perform better in dirty applications, the air is heated to 100°C, the speed
ceramic or glass windows, or through or with solids when the grain size is of sound rises to 386 m/s. Indeed,
fiberglass or plastic tank walls. larger than 20 mm. the speed of sound is proportional
to the square root of temperature.
• Advantages & Limitations Ultrasonic Level Gages At near ambient temperatures, the
The reflective properties of the pro- The origin of ultrasonic level instru- speed rises by 0.6 m/s per each 1°C
cess material affect the returned mentation goes back to the echom- increase, corresponding to an increase
radar signal strength. Whereas liquids eters used in measuring the depth of of 0.18%/°C.
have good reflectivity characteristics, wells by firing a blank shell and tim- Ultrasonic level switches (point
solids do not. Radar can detect the ing the return of the echo. SONAR sensors) operate by detecting either
liquid level under a layer of light dust detectors used in naval navigation dampening of ultrasonic oscillation or
or airy foam, but if the dust particle also predate industrial applications by sensing the absorption or transmis-
size increases, or if the foam or dust of this principle. sion of an ultrasonic pulse. Ultrasonic
gets thick, it will no longer detect the The frequency range of audible level transmitters measure actual dis-
liquid level. Instead, the level of the sound is 9-10 kHz, slightly below the tance by issuing an ultrasonic pulse
foam or dust will be measured. 20-45 kHz range used by industrial and measuring the time required for
Internal piping, deposits on the level gages. The velocity of an ultra- the reflected echo to be received.
antenna, multiple reflections, or sonic pulse varies with both the sub-
reflections from the wall can all stance through which it travels and • Ultrasonic Transducers
interfere with the proper operation with the temperature of that sub- The transducer that generates the
of a radar sensor. Other sources stance. This means that if the speed ultrasonic pulse is usually piezoelec-
of interference are rat-holing and of sound is to be used in measuring a tric, although in the past electrostatic
bridging of solids, as well as angled level (distance or position), the sub- units also were used. An electro-
process material surfaces that can stance through which it travels must static transducer is constructed of
reflect the radar beam away from the be well known and its temperature a thin, flexible gold-plated plastic
receiver. variations must be measured and foil, stretched over an aluminum

In comparison to other radiation compensated for. back-plate and held in place by a
reflection sensors, radar has some At room temperature, the speed of leaf spring. This design was used in
advantages. For example, ultrasonic sound in atmospheric air is 340 m/s or early Polaroid auto-focus cameras
and is still utilized in clean envi- Most often, however, a single trans- liquid.
ronments. Piezoelectric transducers ducer is cycled on and off at regular If it is desired to measure the
utilize ceramic or polymer crystals intervals to listen for the reflected height of the liquid column directly,
vibrated at their natural frequency. echo (Figure 9-4A). When mounted the transducer can be mounted in
These units are much more rugged, on the top of the tank, the sensor the bottom of the tank (Figure 9-4A).
can withstand wash-down pressures detects the depth of the vapor space. However, this configuration exposes
of 1,200 psig and can conform to Accurate knowledge of the shape of the transducer to the process fluid
NEMA-6P (IEC IP67) standards. and limits accessibility for mainte-
Generally, the larger the diameter nance. Alternately, the transducer
of the transducer, the longer the can be mounted on the outside
range and the lower the frequen- of the wall of the vessel bottom,
cy. This is because, after releasing but the ultrasonic pulse is likely to
an ultrasonic pulse, the transducer be substantially weakened by the
needs time for the vibration to settle. absorbing and dispersing effects of
The oscillation frequency is inversely the tank wall (Figure 9-4A).
proportional to the element’s diam- Stagnant, unagitated liquids and
eter, so smaller diameter transducer solids consisting of large and hard
elements generate higher frequen- particles are good reflectors, and
cies. Standard transducers have a therefore good candidates for
beam angle of about 8°, require a ultrasonic level measurement. Fluff,
connection size between G in and 2.5 foam, and loose dirt are poor reflec-
in NPT, and are suited for operating tors, and dust, mist, or humidity
temperatures in the range of -20 to in the vapor space tend to absorb
60°C (-30 to 140°F). Accuracy is typi- the ultrasonic pulse. The ultrasonic
cally within 0.25-0.5% of full range, signal also is attenuated by distance.
up to about 30 ft. Output typically is If a 44-kHz sound wave is traveling
4-20 mA with a 12-amp relay output. in dry, clean ambient air, its sound
power drops by 1-3 decibels (dB)
• Level Transmitter Configurations for each meter of distance traveled.
The ultrasonic level sensor assembly Therefore it is important, particular-
can consist of separate transmitter the tank’s cross-section is required in ly when measuring greater depths,
and receiver elements (Figure 9-4A). order to determine the volume of that the transducers generate a
strong and well-focused ultrasonic is an indication of the location of reduce the unit costs of obtaining
pulse (Figure 9-4B). the interface (Figure 9-4C). level measurements.
It is also desirable that the sur-
face be both flat and perpendicular • Special Features • Level Switches
to the sound wave. In liquid-level Most modern ultrasonic instru- When it is sufficient to detect the pres-
applications, the aiming angle must ments include temperature compen- ence or absence of level at a particular
be within 2 degrees of the vertical. sation, filters for data processing elevation, dampened or absorption-
If the surface is agitated or sloping and response time, and some even type level switches can be considered.
(as in the case of solids), the echo provide self-calibration. Figure 9-5 In the dampened design, a piezoelec-
is likely to be dispersed. Therefore, illustrates a fixed target assembly tric crystal vibrates the sensor face
the key to successful ultrasonic level that provides a point reference to at its resonant frequency. The vibration
sensor installations is the careful automatically recalibrate the level is dampened when the probe face is
analysis of the reflection, propaga- sensor. Multiple calibration targets submerged in process fluid. As shown
tion, and absorption characteristics can be provided by calibration ridges in Figure 9-3A, these switches can be
of the tank’s contents. in sounding pipes. This can guaran- mounted outside or inside the tank,
When detecting the interface tee measurement accuracy of within above or below the liquid level. The
between two liquids, such as the 5 mm over a distance of 30 meters. probe can be horizontal or vertical.
hydrocarbon/brine interface in a Intelligent units can perform auto- These switches are limited to clean
salt dome storage well, the trans- matic self-calibration or convert the liquid installations because coating can
ducer is lowered down to the bot- level in spherical, irregular, or hori- dampen the vibration. Solids may not
tom of the well. The ultrasonic zontal cylindrical tanks into actual provide sufficient dampening effects
pulse is sent up through the heavy volume. They can also be used in to actuate the switch.
brine layer to the interface. The multi-tank or multi-silo installations, In the absorption-type level
time it takes for the echo to return which, through multiplexing, can switch, one piezoelectric crystal
serves as a transmitter and another as current outputs are also used. a seemingly solid, impenetrable mass
the receiver. When the gap between The presence or absence of an of matter. In the passage, however,
them is filled with liquid, the sonic interface between clean liquids can the gamma rays lost some of their
wave passes from one crystal to be measured by inserting an absorp- intensity. The rays were predictably
the other. When vapors fill the gap, tion (gap) probe at a 10° angle below affected by the specific gravity and
however, the ultrasonic pulse does the horizontal. In this configuration, total thickness of the object, and by
not reach the receiver. The crystals as long as the probe is immersed in the distance between the gamma ray
source and the detector.
For example, Figure 9-6 shows
that, if radiation from Cesium 137 is
passing through an 3-in thick steel
object, 92% of the radiation energy
will be absorbed and only 8% will be
transmitted. Therefore, if the observ-
er can hold all variables except thick-
ness constant, the amount of gamma
transmission can be used to mea-
sure the thickness of the object.
Assuming that the distance between
the source and detector does not
change, one can make accurate mea-
surements of either thickness (level),
or, if thickness is fixed, then of the
density of a process material.
• Radiation Sources
The development of nuclear level
sensors began when this technology
can be mounted on opposite sides of the heavy or light liquid, the ultra- moved from the lab to the industrial
the tank, contained in the fingers of sonic pulse will reach the receiver. environment. This necessitated the
a fork-shaped sensor, or located on When the interface moves into the design and manufacture of suitable
the two sides of one or more 0.5-in gap, however, it is reflected away and detectors and the mass production
gaps in a horizontal or vertical probe. does not reach the receiver. of radioisotopes. Both occurred in
When the process fluid is a sludge When a sludge or slurry interface the 1950s.
or slurry, it is desirable to provide a is to be detected or when the thick- The penetrating power of nuclear
large gap between the transmitter ness of the light layer is of interest, an radiation is identified by its photon
and receiver in order to make sure ultrasonic gap sensor can be attached energy, expressed in electron volts
that sticky or coating fluids will drain to a float. As long as the absorption (eV) and related to wavelength (Figure
completely from the gap when the characteristics of the two layers dif- 9-7). The most common isotope used
level drops. fer, the sensor will signal if the layer is for level measurement is Cesium 137,
Typical accuracy of these switches thicker or thinner than desired. which has a photon energy level of
is H-in or better. Connection size 0.56 MeV. Another isotope that is
is I-in NPT. Operating temperature Nuclear Level Sensors occasionally used is Cobalt 60, which
range is 40-90°C (100 to 195°F) (with In 1898 Marie Curie discovered radi- has an energy level of 1.33 MeV. While
special units capable of readings up um by observing that certain ele- the greater penetrating power of
to 400°C/750°F) and operating pres- ments naturally emit energy. She this higher energy radiation appears
sure to 1000 psig. Standard output is named these emissions gamma rays. attractive at first, the penalty is that
a 5 or 10 amp double-pole/double- Gamma rays exhibited mysterious it also has a shorter half-life. As any
throw (DPDT) relay, but voltage and properties—they could pass through isotope decays, it loses strength—
the time it takes to lose half of its the vapor space above the liquid, as detector output when the level chang-
strength is called its half-life. the level rises in the tank, the intensity es.
The half-life of Cobalt 60 is 5.3 at the detector drops. When the tank This can be illustrated by an example:
years. This means that, in 5.3 years, is full, radiation intensity is practically
the activity of a 100 millicurie (mCi) zero. • Source Sizing
Cobalt 60 source will be reduced to When used as a tank level sensor, A point source of 10 mCi Cesium 137
50 mCi. (One mCi is defined as the radiation must pass through several (source constant for Cesium 137 is
rate of activity of one milligram of layers of material before reaching the K=0.6) is installed on a high-pressure
Radium 226.) When used for level detector. At the detector, the maximum water tank having H-in steel walls
measurement, the continuous loss radiation must be less than some safety (Figure 9-9). Usually, two criteria need
of source strength requires not only limit (such as 5 mr/hr) to avoid the need to be satisfied: First, the radiation
continuous compensation, but, even- for “posting.” Other criteria can be used, intensity at the detector must drop
tually (in the case of Cobalt 60, in such as keeping a yearly dosage under by at least 50% as the level rises from
about 5 years), the source must be 5 rems (roentgen + equivalent + man). 0-100%. The second and more impor-
replaced. This means not only the If somebody is exposed to radiation tant criterion is that the maximum
expense of purchasing a new source, throughout the year, such a dosage will radiation dose at the detector (when
but also the cost of disposing of the result from exposure to radiation at the tank is empty) must not exceed
old one. an intensity of 0.57 mr/hr, while if an the safety limit (say, 2.4 mr/hr). It must
In contrast, the 33-year half-life of operator is exposed for only 40 hrs/wk, exceed 1.0 mr/hr, however, in order
Cesium 137 is long enough that the
source may well outlive the process.
Another likelihood is that technologi-
cal advances will increase the sensi-
tivity of the detector faster than the
rate at which the source is decaying.
This provides users the option of
replacing or upgrading the detector
while keeping the source in place for
the future.
• Radiation Safety
The Nuclear Regulatory Commission
(NRC) limits radiation intensity to a
maximum of 5 milliroentgens per hour
(mr/hr) at a distance of 12 in from the
nuclear gage. If it is more, the area
requires Radiation Area posting. The
distance of 12 in is critical, because
radiation intensity decreases by the
inverse square of distance. Nuclear 5 rem/yr will correspond to what that to actuate the intended ion chamber
level gages are sized to provide radia- person would receive if exposed to 2.4 detector.
tion intensity at the detector that mr/hr in the work area. As it is the total First the in air intensity (Da in mr/
exceeds the minimum required, but is lifetime dosage of radiation exposure hr) is calculated at the detector, for
under the 5 mr/hr maximum. For ion that really matters (maximum of 250 the condition when there is no tank
chamber detectors, the minimum is 1 rems), the acceptability of the 5 rem/yr, between the source and receiver.
mr/hr. For Geiger-Mueller switches, or any other limit, is also a function of Assume distance (d) is 48 in:
it is 0.5 mr/hr. And for scintillation age (Figure 9-8). On the other hand, the
detectors, it is 0.1-0.2 mr/hr. Because radiation at the detector must still be Da = 1000 K(mCi)/d2 =
the nuclear gage is basically measuring sufficient to produce a usable change in 1000(0.6)(10)/482 = 2.6 mr/hr
accurately determined. In short, the are used as insulators, and a high volt-
Because the source is shielded in all performance of a nuclear gage instal- age (700-1000 vdc) nearly sufficient to
directions except towards the tank, lation is very much a function of the cause current flow between the elec-
the operator who is working near accurate knowledge of the installation trodes is applied. When the tube is
the detector will receive the maxi- details. exposed to gamma radiation, the gas
mum dosage when the tank is empty. ionizes and the ionized particles carry
The two H-in steel walls will reduce • Detector Options the current from one electrode to
Da (% transmission of 1-in steel in The simplest and oldest type of radia- the other. The more gamma radiation
Figure 1 is 49%) to 0.49 x 2.6 = 1.27 tion detector is the Geiger-Muller reaches the gas in the tube, the more
mr/hr. This is below the allowable tube. This instrument is most often pulses are generated. The resulting
maximum but above the minimum identified with the Geiger counters pulse rate is counted by the associ-
needed by the detector. that make a loud and dramatic clicking ated electronic circuitry, which makes
When the tank is full, the pres- sound when exposed to radiation. The measurements in pulses per second.
ence of 30 in of water in the radia- working component of this detector This detector can be used as a
tion path will reduce this maximum is a metal cylinder that acts as one level switch if it is calibrated to
intensity to 0.045 mr/hr (0.035 x 1.9
= 0.045). This reduction in intensity
well exceeds the required 50% drop
needed for sensitive measurement.
Note that the source size could
have been cut in half if a Geiger-
Mueller detector were used. A
scintillation detector would reduce
source size 5- to 10-fold.
The source size can also be
reduced by locating the source in
the tip of a probe inside the tank
and moving it relatively close to the
wall. When large level ranges are to
be measured, a strip source can be
used instead of a point source. The
accuracy of most nuclear level gages
is around 1% of range. If accounting
accuracy is desired, the source and
the detector can both be attached to
motor driven tapes and positioned at
the level (or at the interface level, if
the tank contains two liquids).
Fortunately, today’s comput-
ers can easily crunch the numbers
and formulas of any combination of
geometry and design criteria. The
biggest challenge is not the calcula-
tion, but the obtaining of accurate
inputs for the calculations. Therefore,
it is very important that your vessel’s
wall materials, thicknesses, other tank of the electrodes and is filled with an engage or disengage a relay when
components such as baffles, agitator inert gas. A thin wire down the center radiation intensity indicates a high or
blades or jackets, and all distances be acts as the other electrode. Glass caps low level condition. The G-M tube
1 00 Volume 4 TRANSACTIONS
detector can only be used as a single is converted into visible flashes com- ment, or could do major damage to
point detection device. Its advan- prised of light photons (particles of property. The liquids and bulk solids
tages include its relatively low cost, light). measured by nuclear gages are among
small size, and high reliability. These photons increase in number the most dangerous, highly pres-
The ion chamber detector is a as the intensity of gamma radia- surized, toxic, corrosive, explosive,
continuous level device. It is a 4 to tion increases. The photons travel and carcinogenic materials around.
6-in diameter tube up to twenty feet through the clear plastic scintillator Because the nuclear gage “sees”
long filled with inert gas pressurized medium to a photo multiplier tube, through tank walls, it can be installed
to several atmospheres. A small bias which converts the light photons and modified while the process is
voltage is applied to a large elec- into electrons. The output is directly running—without expensive down
trode inserted down the center of proportional to the gamma energy time or chance accidental release.
the ion chamber. As gamma energy that is striking the scintillator. Because the installation of nuclear
strikes the chamber, a very small Scintillators are available in a mul- sensors requires a Nuclear Regulatory
signal (measured in picoamperes) is titude of shapes, sizes, and lengths. Commission (NRC) license, associ-
detected as the inert gas is ionized. One of the latest is a fiber optic cable ated procedures are designed to
This current, which is proportional that allows one to increase detector guarantee that the installation will
to the amount of gamma radiation sensitivity by installing more fila- be safe. The best way to look at the
received by the detector, is ampli- ments in the bundle. Another advan- safety aspects of radioactive gag-
fied and transmitted as the level tage of the fiber optic cable is that ing is to compare the well defined
measurement signal. it is manufactured in long lengths and understood risk represented by
In level measurement applications, flexible enough to form-fit to the exposing the operators to radiation
the ion chamber will receive the most geometry of the vessel. This simpli- against the possibly larger risk of
radiation and, therefore, its output fies the measurement of levels in having an unreliable or inaccurate
will be highest when the level is low- spherical, conical, or other oddly level reading on a dangerous pro-
est. As the level rises and the greater shaped vessels. cess.
quantity of measurand absorbs more As detectors become more sen-
gamma radiation, the output current • Nuclear Applications sitive and are aided by comput-
of the detector decreases proportion- Radiation gages typically are consid- ers, radiation source sizes and the
ally. The system is calibrated to read ered when nothing else will work, or resulting radiation levels continue to
0% level when the detector current when process penetrations required drop. Therefore, the safety of these
output is its highest. 100% level is by a traditional level sensor present instruments is likely to continue to
set to match the lowest value of a risk to human life, to the environ- improve with time. T
the output current. Non-linearities
in between can usually be corrected References & Further Reading
with the use of linearizing software. • OMEGA Complete Flow and Level Measurement Handbook and
This software can correct for the Encyclopedia®, Omega Press, 1995.
effects of steam coils, agitator blades, • Omega Volume 29 Handbook & Encyclopedia, Purchasing Agents
baffles, stiffening rings, jackets and Edition, Omega Press, 1995.
other components inside or outside • Automated Process Control Electronics, John Harrington, Delmar
the tank. Publishing Inc., 1989.
Scintillation counter detectors are • Fundamentals of Radar Techniques for Level Gauging, Detlef Brumbi,
five to ten times more sensitive than Krohne Metechnik GmbH & Co. KG, 1995.
ion chambers. They also cost more, • Industrial Applications of Radar Technology for Continuous Level
yet many users are willing to accept Measurement, W. L. Hendrick, Instrument Society of America, 1992.
the added expense because it allows • Instrument Engineer’s Handbook, Bela Liptak, Third Edition, CRC Press, 1995.
them either to use a smaller source • Process/Industrial Instruments and Controls Handbook, 4th Edition,
size or to obtain a more sensitive Douglas M. Considine, McGraw-Hill, 1993.
gage. When gamma energy hits a • Theoretical Nuclear Physics Volume I: Nuclear Structure, New York, A.
scintillator material (a phosphor), it deShalit, H. Feshback,: John Wiley & Sons, 1974.

Specialty Level Switches
T hermal, vibrating, and opti-
cal level switches are spe-
cialty devices developed to
solve specific level detection
problems. Typically, they are used in
applications that either cannot be
and can be mounted from the tops
or sides of tanks. The low thermal
conductivity of solids and the dusty
atmospheres that are likely to exist in
the vapor space of solids bins tend to
exclude the use of optical and ther-
washers to remove the build-up of
coating after each high level episode.
Thermal switches can continue to
work when lightly coated, but build-
up does usually add a thermally
insulating layer, ultimately slowing
handled by the more common float mal switches from most solids level response time.
and probe-type devices, or when measurement applications. Of the three level-switch designs
ultrasonic, nuclear, radar or microwave When solid materials rat-hole or discussed in this chapter, only the
designs would be too sophisticated, bridge, few level sensors (except load laser-based optical level switch is
expensive, or otherwise unsuited for
the task.
All three types can be used to
detect liquid levels or interfaces
between liquids. The optical level
switch is also suited for detecting
high foam levels, if it is spray washed
after each event. In some special-
ized applications, all three of these
switches have been tuned to identify
specific materials or to determine
when a material reaches a particular
viscosity, density, opacity, or thermal
conductivity condition.
All three level switch designs are
simple, straightforward, and reliable.
Although some can detect other
process properties besides level,
their main purpose is to measure the
presence or absence of material at a
particular level in a tank.
These switches are good candi-
dates for use in multiple purpose cells or radiation devices) work well. appropriate for use in molten metal
processing equipment where they The performance of vibrating probe level detection. Of the other level
must be compatible with a variety of and tuning-fork sensors is also ques- sensor technologies, refractory
process materials and process condi- tionable in such services, but their floats, refractory bubbler tubes, and
tions. They do not require recalibra- vibrating nature can help to collapse proximity-type capacitance detectors
tion between batches and can be the bridges or to break up the rat- also are used in molten metal service.
cleaned in place. holes.
Vibrating probe-type sensors are Vibrating and tuning fork probes Thermal Switches
often used to detect solid materials can tolerate a fair amount of mate- Thermal level switches sense either
such as powders, bulk solids, grain, rial build-up, or, if coated with the difference between the tempera-
flour, plastic granules, cement, and fly Teflon®, can be self-cleaning in some tures of the vapor space and the liq-
ash. They provide excellent perfor- less difficult services. Optical level uid or, more commonly, the increase
mance as high or low level switches switches are available with automatic in thermal conductivity as a probe
becomes submerged in the process detecting water level, response time tify the place where the temperature
liquid. is typically 0.5 second and accuracy is on the outside of the mold suddenly
One of the simplest thermal level within 2 mm. In general, thermal level increases. This is the level inside the
switch designs consists of a tempera- switches work best with non-coating mold. Using multiple sensors spaced
ture sensor heated with a constant liquids and with slurries having 0.4-1.2 vertically, the system can determine
amount of heat input. As long as specific gravity and 1-300 cP viscosity. the level of molten metal in the
the probe is in the vapor space, the A third type of thermal switch mold to within a fraction of an inch.
probe remains at a high temperature, also uses two sensors inside the
because low-conductivity vapors same vertical probe. One is mount- Vibrating Switches
do not carry much heat away from ed above the other and both are Vibrating level switches detect
the probe. When the probe is sub- connected to a voltage source. the dampening that occurs when a
merged, the liquid absorbs more heat When both are in the vapor or both vibrating probe is submerged in a
and the probe temperature drops. process medium. The three types
The switch is actuated when this of vibrating sensors—reed, probe,
change in temperature occurs. and tuning fork—are distinguished
Another type of thermal sensor by their configurations and operat-
uses two resistance temperature ing frequencies (120, 200-400, and
detectors (RTDs), both mounted 85 Hz, respectively). Their methods
at the same elevation. One probe of operation and applications are
is heated and the other provides similar. The reed switch consists of a
an unheated reference. The out- paddle, a driver and a pickup (Figure
puts of both sensors are fed into 10-2). The driver coil induces a 120-
a Wheatstone bridge (Figure 10-1). Hz vibration in the paddle that is
While the sensor is in the vapor damped out when the paddle gets
phase, the heated probe will be covered by a process material. The
warmer than the reference probe, in the liquid phase, the current flow switch can detect both rising and
and the bridge circuit will be unbal- through the two sensors is the same. falling levels, and only its actuation
anced. When both probes are sub- If, on the other hand, the lower one depth (the material depth over the
merged in the process liquid, their is in liquid and the upper in vapor, paddle) increases as the density of
temperatures will approach that of more current will flow through the the process fluid drops. The varia-
the liquid. Their outputs will be nearly lower sensor. A current comparator tion in actuation depth is usually
equal and the bridge will be in balance. can detect this difference and signal less than an inch. A reed switch can
This level switch is actuated when a that the sensor has reached the detect liquid/liquid, liquid/vapor,
change in bridge balance occurs. vapor/liquid interface. and solid/vapor interfaces, and can
Since all process materials have One interesting feature of this also signal density or viscosity varia-
a characteristic heat transfer coef- design is that the sensor capsule can tions.
ficient, thermal level switches can be be suspended by a cable into a tank When used on wet powders, the
calibrated to detect the presence or or well, and the sensor output can vibrating paddle has a tendency to
absence of any fluid. Therefore, these be used to drive the cable take-up create a cavity in the granular sol-
switches can be used in difficult ser- motor. In this fashion, the level switch ids. If this occurs, false readings will
vices, such as interfaces, slurry, and can be used as a continuous detector result, because the sensor will con-
sludge applications. They can also of the location of the vapor/liquid fuse the cavity with vapor space.
detect thermally conductive foams if interface. It is best to use a reed switch on
spray-cleaned after each operation. Thermometers also can be used non-coating applications or to pro-
Thermal level and interface switches to detect level in higher temperature vide automatic spray washing after
have no mechanical moving parts and processes, such as measuring the level each immersion in a sludge or slurry.
are rated for pressures up to 3,000 of molten steel in casting molds. The Probe-type vibrating sensors are less
psig and process temperatures from thermometers do not actually touch sensitive to material build-up or coat-
-75 to 175°C (-100 to 350°F). When the molten metal; instead, they iden- ing. The vibrating probe is a round
stainless steel element (resembling Tuning-fork sensors can be con- properties of the process material
a thermowell) that extends into structed with components made of when measuring its level. The optical
the material. If Teflon® coated and PVDF, polypropylene, stainless steel, level switch can be of a contacting
inserted at an angle, these devic- carbon steel, and aluminum. They or non-contacting design.
es tend to be self-cleaning. Both are available with Teflon® coatings In a non-contacting, reflecting
the drive and the sensor are piezo- or in hygienic versions for sanitary optical sensor, a beam of light is
electric elements: one causes the applications. aimed down at the surface of the
vibration and the other measures Vibrating sensors can be used to process material. When the level of
this surface rises to the setpoint of
the switch, the reflected light beam
is detected by a photocell. Both the
LED light source and photodetector
are housed behind the same lens.
By adjusting the photocell or the
detection electronics, the sensor can
be calibrated to detect levels at dis-
tances 0.25 to 12 in below the sensor.
These reflective switches can measure
the levels of clear as well as translu-
cent, reflective, and opaque liquids.
Some solids also can be detected. By
using multiple photocells, a sensor
can detect several levels.
it. When the probe is buried under ascertain liquid, solid, and slurry lev- Laser light also can be used when
the process material, its vibration is els. Reed switches can operate at making difficult level measurements,
dampened and this decrease triggers pressures up to 3,000 psig, while such as of molten metals, molten
the switch. tuning forks and vibrating probes are glass, glass plate, or any other kind
Vibrating probe sensors can be limited to 150 psig. Operating tem- of solid or liquid material that has
used to monitor powders, bulk sol- peratures range from -100 to 150°C a reflecting surface. If the receiver
ids, and granular materials such as (-150 to 300°F) and response time is module is motor driven, it can track
grain, flour, plastic pellets, cement, about 1 second. the reflected laser beam as the level
and fly ash. Their vibrating nature rises and falls, thereby acting as a
tends to minimize the bridging that Optical Switches continuous level transmitter.
occurs in solid materials. Tuning fork Using visible, infrared, or laser light, A refracting sensor relies on the
sensors are vibrated at about 85 Hz optical sensors rely upon the light principle that infrared or visible
by one piezoelectric crystal, while transmitting, reflecting, or refracting light changes direction (refracts)
another piezoelectric crystal detects
the vibration. As the process fluid
rises to cover the tuning forks, the
vibration frequency changes.
Like vibrating probes, tuning-
fork designs can be self-cleaning if
Teflon® coated and installed at an
angle. They can also be calibrated
to detect a wide range of materials,
including lubricating oils, hydraulic
fluids, water, corrosive materials,
sand, thick and turbulent fluids, pow- Ultrasonic liquid level switches provide a 300:1 signal ratio from dry to wetted state.
ders, light granules, and pastes.
when it passes through the inter- encounter the sludge layer. Response time is virtually imme-
face between two media. When Other transmission sensors rely on diate, and detection accuracy of
the sensor is in the vapor phase, the refraction principle utilizing an most designs is within 1 mm. Optical
most of the light from the LED is unclad, U-shaped fiber optic cable. A level switches are also designed
reflected back within a prism (Figure light source transmits a pulsed light for specific or unique applications.
10-3). When the prism is submerged, beam through the fiber cable, and For example, Teflon® optical level
most of the light refracts into the the sensor measures the amount of switches are available for sens-
liquid, and the amount of reflected light that returns. If liquid covers the ing the level of ultra-pure fluids.
light that reaches the receiver drops cable, it will cause light to refract Other unique designs include a level
substantially. Therefore, a drop in away from the cable. The use of switch that combines an optical
the reflected light signal indicates fiber-optics makes the system imper- with a conductivity-type level sen-
contact with the process liquid. vious to electrical interference, and sor to detect the presence of both
A refracting sensor cannot be used some designs are also intrinsically water (conductive) and hydrocar-
with slurries or coating liquids, unless safe. bons (nonconductive). T
it is spray-washed after each submer- Optical sensors can operate
sion. Even a few drops of liquid on the at pressures up to 500 psig and
prism will refract light and cause erro- temperatures up to 125°C (260°F).
neous readings. Refracting sensors are
designed to be submerged in liquids; References & Further Reading
therefore, any number of them can be • OMEGA Complete Flow and Level Measurement Handbook and
installed on a vertical pipe to detect a Encyclopedia®, Omega Press, 1995.
number of level points. • Omega Volume 29 Handbook & Encyclopedia, Purchasing Agents
Transmission optical sensors send Edition, Omega Press, 1995.
a beam of light across the tank. A • Industrial Control Handbook, E.A. Parr, Butterworth-Heinemann Ltd., 1995.
sludge level sensor of this design uses • Instrument Engineer’s Handbook, Bela Liptak, Third Edition, CRC Press,
an LED and a photocell at the end of 1995.
a probe, located at the same eleva- • Process/Industrial Instruments and Controls Handbook, 4th Edition,
tion and separated by a few inches. Douglas M. Considine, McGraw-Hill, 1993.
To find the sludge level, a mechanism • The McGraw-Hill Encyclopedia of Science and Technology, 8th Edition,
(or an operator, manually) lowers the John H. Zifcak, McGraw-Hill, 1997.
probe into the tank until the sensors

Information Resources
ORGANIZATIONS
NAME/ADDRESS PHONE WEB ADDRESS
American Institute of Chemical Engineers (AIChE)
345 East 47 Street, New York NY 10017-2395 (212)705-7338 www.aiche.org
American Gas Association (AGA)
400 N. Capitol St., NW Washington DC 20001 (202)824-7000 www.aga.org
American National Standards Institute (ANSI)
11 West 42 Street, New York NY 10036 (212)642-4900 web.ansi.org
American Petroleum Institute (API)
1220 L Street, NW, Washington DC 20005 (202)682-8000 www.api.org
American Society of Mechanical Engineers (ASME)
345 East 47 Street, New York NY 10017 (212)705-7722 www.asme.org
American Society for Testing and Materials (ASTM)
100 Barr Harbor Drive, West Conshohocken PA 19428-2959 (610)832-9585 www.astm.org
American Water Works Association (AWWA)
6666 West Quincy Ave., Denver CO 80235 (303)794-7711 www.awwa.org
Canadian Gas Association (CGA)
243 Consumers Road, Suite 1200,
North York Canada M2J 5E3 ON (416)498-1994 www.cga.ca
Electric Power Research Institute (EPRI)
3412 Hillview Avenue, Palo Alto CA 94303 (415)855-2000 www.epri.com
Electronic Industries Association (EIA)
2500 Wilson Boulevard, Arlington VA 22201-3834 (703/907-7500 www.eia.org
Factory Mutual
1151 Boston-Providence Turnpike, Norwood MA 02062 (781)762-4300 www.factorymutual.com
Gas Research Institute
8600 West Bryn Mawr Ave., Chicago IL 60631-3562 (773)399-8100 www.gri.org
International Electrotechnical Commission (IEC)
3, rue de Varembé, P.O. Box 131,
CH - 1211 Geneva 20, Switzerland +41 22 919 02 11 www.iec.ch
International Organization for Standardization (ISO)
1, rue de Varembe, Case postale 56,
CH-1211 Geneve 20 Switzerland +41 22 749 01 11 www.iso.ch
Institute of Electrical & Electronics Engineers (IEEE)
445 Hoes Lane, Piscataway NJ 08855-1331 (732)981-0060 www.ieee.org
Institute of Gas Technology (IGT)
1700 South Mount Prospect Road, Des Plaines IL 60018 (847)768-0500 www.igt.org

organizations (cont'D.)
ISA—The International Society for Measurement and Control
67 Alexander Drive, Research Triangle Park NC 27709 (919)549-8411 www.isa.org
National Electrical Manufacturers Association (NEMA)
1300 North 17th Street, Suite 1847, Rosslyn VA 22209 (703)841-3200 www.nema.org
National Fire Protection Association (NFPA)
1 Batterymarch Park, Quincy MA 02269-9101 (617)770-3000 www.nfpa.org
National Institute of Standards and Technology
Gaithersburg MD 20899-0001 (301)975-3058 www.nist.gov
Society of Automotive Engineers (SAE)
400 Commonwealth Drive, Warrendale PA 15096-0001 (724)776-4841 www.sae.org
Underwriters Laboratories
333 Pfingsten Road, Northbrook IL 60062 (847)272-8800 www.ul.com
Water Environment Federation (WEF)
601 Wythe Street, Alexandria VA 22314-1994 (703)684-2452 www.wef.org
flow and level Products

For the Latest Omega Engineering, Inc.
Information on One Omega Drive
Flow and Level P.O. Box 4047
Products: Stamford CT 06907-0047
Phone: 800-82-66342
(800-TC-OMEGA®)
Email: info@omega.com
Website: www.omega.com
omega press references

The Temperature Handbook™ Voume MM™ 21st Century™ Edition,
OMEGA Press, 1999.
The OMEGA Complete Flow and Level Measurement Handbook and Encyclopedia®,
Omega Press, 1995.
The Pressure, Strain and Force Handbook™,
Omega Press, 1995.
Book of Books®: Scientific & Technical Books, Software & Videos,
Omega Press, 1998.
Omega Volume 29 Handbook & Encyclopedia, Purchasing Agents Edition,
Omega Press, 1995
21st Century™ Preview Edition,
Omega Press, 1997

OTHER REFERENCE BOOKS
Applied Fluid Flow Measurement,
N.P. Cheremisinoff, Marcel Decker, 1979.
Automated Process Control Electronics,
John Harrington, Delmar Publishing Inc., 1989.
Differential Producers - Orifice, Nozzle, Venturi,
ANSI/ASME MFC, December 1983.
Electrical Measurements and Measuring Instruments,
E.W. Goldin, Pitman and Sons, 1948.
Electrical Measurements,
F.K. Harris, Wiley, 1952.
Flow Measurement Engineering Handbook,
R.W. Miller, McGraw Hill, 1996.
Flow Measurement for Engineers and Scientists,
N.P. Cheremisinoff, Marcel Dekker, 1988.
Flow Measurement,
Bela Liptak, CRC Press, 1993.
Flow Measurement,
D.W. Spitzer, Instrument Society of America, 1991.
Flow of Water Through Orifices,
AGA/ASME, Ohio State Univ. Bulletin 89, Vol. IV, No.3.
Flowmeters,
F. Cascetta, P. Vigo, ISA, 1990.
Fluid Meters,
H.S. Bean, American Society of Mechanical Engineers, 1971.
Fundamentals of Flow Measurement,
J. P. DeCarlo, Instrument Society of America, 1984.
Fundamentals of Radar Techniques for Level Gauging,
Detlef Brumbi, Krohne Metechnik GmbH & Co. KG, 1995.
Incompressible Flow,
Donald Panton, Wiley, 1996.
Industrial Applications of Radar Technology for Continuous Level Measurement,
W. L. Hendrick, Instrument Society of America, 1992.
Industrial Control Handbook,
E.A. Parr, editor, Butterworth-Heinemann Ltd., 1995.
Industrial Flow Measurement,
D. W. Spitzer, ISA 1984.
Instrument Engineer’s Handbook,
Bela Liptak, Third Edition, CRC Press, 1995.
Instrumentation and Control,
C.L. Nachtigal, Wiley, 1990.
Instrumentation and Process Control,
Nicholas P. Chopey, McGraw-Hill, 1996.

OTHER REFERENCE BOOKS
Instrumentation for Engineering Measurements,
J. Dally, Wiley, 1993.
Instrumentation for Process Measurement and Control, 3rd ed.,
Norman A. Anderson, Chilton Co., 1980.
Instrumentation Reference Book, 2nd ed.,
B.E. Noltingk, editor, Butterworth-Heinemann, 1995.
Instruments of Science,
Robert Bud and Deborah Jean Warner, Garland Publishing Inc., 1998.
Measurement and Control Basics, 2nd ed.,
T.A. Hughes, ISA, 1995.
Measurement and Control of Liquid Level,
C. H. Cho, Instrument Society of America, 1982.
Modern Physics,
New York, P. Tipler, Worth Publishers, 1978.
National Electrical Safety Code,
IEEE, 1993.
Principles of Industrial Measurement for Control Applications,
E. Smith, Instrument Society of America, 1984.
Process /Industrial Instruments and Controls, 4th ed.,
Douglas M. Considine, McGraw-Hill, 1993.
Sensor and Analyzer Handbook,
H.N. Norton, Prentice-Hall, 1982.
Sensors and Control Systems in Manufacturing,
S. Soloman, McGraw-Hill, 1994.
The McGraw-Hill Encyclopedia of Science and Technology, 8th ed.,
John H. Zifcak, McGraw-Hill, 1997.
Theoretical Nuclear Physics Volume I: Nuclear Structure,
New York, A. deShalit, H. Feshback, John Wiley & Sons, 1974.
Van Nostrand’s Scientific Encyclopedia,
Douglas M. Considine, Van Nostrand, 1995.
Water Meters - Selection, Installation, Testing and Maintenance,
Manual M6, AWWA, 1986.
Teflon®, Viton® and Kalrez® are registered trademarks of DuPont.

A
Glossary
American National Standards Institute (ANSI): A profes-
Accuracy: Degree of conformity of a measured value to sional organization in the United States responsible for
an accepted standard value; or closeness of a reading or accepting and designating the standards developed by
indication of a sensor to the actual value of the quantity other organizations as national standards.
being measured. Ampere (A or amp): The unit of electric current flow,
Accuracy rating: A number that defines a limit that the defined as the rate at which one coulomb of electric
measurement errors will not exceed under some refer- charge (6.25 x 1018 electrons) is transferred in a second.
ence operating conditions. It includes the combined Amplifier: A device that generates an output which is
effects of conformity, hysteresis, deadband and repeat- stronger than and bears some predetermined relation-
ability errors. ship (often linear) to its input. It generates the amplified
Accuracy, units: The maximum positive or negative output signal while drawing power from a source other
deviation (inaccuracy) observed in testing a device. It can than the signal itself.
be expressed in terms of the measured variable (±1°C), or Analog signal: A signal that continuously represents a
as a percentage of the actual reading (%AR), of the full variable or condition.
scale (%FS), of upper range value (%URL), of the span or Analog-to-digital (A/D) conversion: A generic term
of scale length. referring to the conversion of an analog signal into a
Admittance: Admittance of an ac circuit is analogous to digital form.
conductivity of a dc circuit; it is the reciprocal of the Analog-to-digital converter (ADC): An electronic device
impedance of an ac circuit. that converts analog signals to an equivalent digital
Air consumption: The maximum rate at which air is form.
consumed by an instrument while operating within its Attenuation: The reciprocal of gain; a dimensionless
operating range, usually expressed in units of standard ratio defining the decrease in magnitude of a signal as
cubic feet per minute. it passes between two points or two frequencies. Large
Alphanumeric: A character set containing both letters values of attenuation are expressed in decibels (dB).
and numbers. B
Alternating current (ac): A flow of electric charge (elec- Backlash: The relative movement of interlocked mechan-
tric current) that undergoes periodic reverses in direc- ical parts that occurs when motion is reversed.
tion. In North America, household current alternates at a Baud rate: Serial communications data transmission rate
frequency of 60 times per second. expressed in bits per second (bps).
Ambient pressure: The atmospheric pressure of the Bipolar: A signal range that includes both positive and
medium surrounding a particular sensor. When no spe- negative values (i.e., -10 V to +10 V).
cific information is available, it is assumed to be 14.7 psia. Bode diagram: A plot of log amplitude ratio and phase
Ambient temperature: The average or mean temperature angle values used in describing transfer functions.
of the atmospheric air which is surrounding a sensor or Breakdown voltage: Threshold voltage at which circuit
instrument. If the sensor is a heat generator, this term components begin to be damaged.
refers to the temperature of the surroundings when the Byte (B): Eight related bits of data or an eight-bit binary
sensor is in operation. The ambient temperature is usu- number. Also denotes the amount of memory required
ally stated under the assumption that the sensor is not to store one byte of data.
exposed to the sun or other radiant energy sources. C
Ambient temperature compensation: An automatic cor- Calibrate: To ascertain that the output of a device
rection which prevents the reading of a sensor or instru- properly corresponds to the information it is measuring,
ment from being affected by variations in ambient tem- receiving or transmitting. This might involve the location
perature. The compensator specifications state the tem- of scale graduations, adjustment to bring the output
perature range within which the compensation is effective. within specified tolerance or ascertaining the error by

comparing the output to a reference standard. observable response.
Calibration: The process of adjusting an instrument or Decibel (dB): Unit for expressing a logarithmic measure
compiling a deviation chart so that its reading can be of the ratio of two signal levels.
correlated to the actual values being measured. Dielectric: A non-conductor of dc current.
Calibration curve: A graphical representation of the Dielectric constant: A numeral that expresses the degree
calibration report, which report can be in the form of a of non-conductivity of different substances, with full
table or chart. vacuum defined as 1.0.
Calibration cycle: The application of known values of a Distributed control system (DCS): Typically, a large-
measured variable and the recording of the correspond- scale process control system characterized by a distrib-
ing output readings over the range of the instrument in uted network of processors and I/O subsystems that
both ascending and descending directions. encompass the functions of control, user interface, data
Calibration traceability: The relationship of the calibra- collection, and system management.
tion process to the calibration steps performed by a Dither: A useful oscillation of small magnitude, intro-
national standardizing laboratory. duced to overcome the effects of friction, hysteresis,
Capacitance: The capability of a device to store electric or clogging.
charge. The unit is the farad, which expresses the ratio Drift: Undesired change in the input-output relationship
of stored charge in coulombs to the impressed potential over a period of time.
difference in volts. Dynamic range: Ratio of the largest to the smallest signal
Capacitor: A device designed to store electric charge. It level a circuit can handle, normally expressed in dB.
usually consists of two conductors that are electrically iso- E
lated by a nonconductor (dielectric). The plates of a per- Electromotive force: Force that causes the movement
fect capacitor are isolated by vacuum (dielectric constant of electricity, such as potential difference of voltage. A
of 1.0), in which case no current flows between the plates. measure of voltage in an electrical circuit.
Common mode rejection: The ability of a circuit to dis- Elevation: A range in which the zero value of the mea-
criminate against a common mode voltage. sured variable exceeds the lower range value.
Common mode voltage: A voltage of the same polarity Error: The difference between the measured signal value
on both sides of a differential input relative to ground. or actual reading and the true (ideal) or desired value.
Compensator: A device that eliminates the effect of an Error, common mode: Error caused by interference
unmeasured variable or condition on the measurement that appears between both measuring terminals and
of interest. ground.
Compound detector: A detector whose measurement Error, normal mode: Error caused by interference that
range extends both above and below zero. appears between the two measuring terminals.
Conductance; Conductivity: The reciprocal of resistance Error, random: The amount of error that remains even
in a dc circuit is conductance. The unit is the mho. The after calibrating a sensor. Also called "precision," while
unit of conductivity is cm-mho or cm/ohm. "repeatability" is defined as twice that: the diameter
Controller: A device that operates automatically to (instead of the radius) of the circle within which the
regulate a controlled variable. readings fall.
Coulomb: The amount of electric charge transferred in Error, systematic: A repeatable error, which either
one second by a current flow of one ampere. remains constant or varies according to some law, when
D the sensor is measuring the same value. This type of error
Damping: The suppression of oscillation. The viscosity of can be eliminated by calibration.
a fluid is used in viscous damping, while the induced cur- F
rent in electrical conductors is used to effect magnetic Farad: The unit of capacitance, equivalent to one cou-
damping. lomb of stored charge per volt of applied potential dif-
Deadband: The range through which an input can be ference. As this is a very large unit, one trillionth of it, the
changed without causing an observable response. picofarad (pf), is commonly used.
Dead time: The interval between the initiation of Fieldbus: All-digital communication network used to
a change in the input and the start of the resulting connect process instrumentation and control systems.

Designed to replace systems based on 4-20 mA analog Input/output (I/O): The analog or digital signals enter-
signals with bi-directional, multivariable data communi- ing or leaving a DCS or other central control or comput-
cation capability. er system involving communications channels, operator
Frequency: The number of cycles over a specified interface devices, and/or data acquisition and control
time period during which an event occurs. Normally interfaces.
expressed in cycles per second (hertz, Hz). Integral control: A control mode which generates a cor-
Frequency response: The frequency-dependent charac- rective output signal in proportion to the time integral
teristic that determines the phase and amplitude rela- of the past error. It eliminates the offset inherent in
tionship between sinusoidal input and output. proportional control.
G Intrinsically safe: Equipment or wiring which is incapable
Gain (magnitude ratio): For a linear system or element, of releasing sufficient electrical or thermal energy to
the ratio of the magnitude (amplitude) of a steady-state ignite a hazardous mixture of hydrocarbon vapors and
sinusoidal output relative to a causal input. In an electri- air. In such equipment, the electrical energy is limited
cal circuit, the amount of amplification used, sometime so that it cannot generate a spark or otherwise ignite a
expressed in decibels (dB). flammable mixture.
Gain accuracy: Measure of deviation of the gain (of an ISA: Formerly, The Instrument Society of America; now
amplifier or other device) from the ideal gain. referred to as the International Society for Measurement
Gain, dynamic: For a sinusoidal signal, the magnitude & Control.
ratio of the steady-state amplitude of an output signal L
to the amplitude of the input. Laser: Narrow, intense beam of coherent light.
Gain, static: The ratio of change of steady-state value Linearity: The closeness to which a curve approximates a
to a step change in input, provided that the output does straight line, or the deviation of an instrument’s response
not saturate. from a straight line.
Ground: The electrical neutral line having the same Linear stroke: For a transducer, the calibrated mechani-
potential as the surrounding earth; the negative side of a cal movement over which its electrical output linearity
direct current power system; the reference point for an meets its specifications.
electrical system. Loop gain characteristics: Of a closed loop, the character-
H istic curve of the ratio of the change in the return signal
Hertz (Hz): The unit of frequency, defined as one cycle to the change in the error signal for all real frequencies.
per second. Loop transfer function: Of a closed loop, the transfer
Hunting: An undesirable oscillation which continues for function obtained by taking the ratio of the Laplace
some time after an external stimulus has disappeared. transform of the return signal to the Laplace transform
Hysteresis: The property of an element or sensor, where- of its corresponding error signal.
by output is dependent not only on the value of the Lower range limit (LRL): The lowest value of the
input, but on the direction of the current traverse. (That measured variable that a device can be adjusted to
is, the reading of the same value differs as a function of measure.
whether the measurement is rising or falling.) Lower range value (LRV): The lowest value of the mea-
I sured variable that a device is adjusted to measure.
Impedance: Opposition to the flow of ac current; the M
equivalent of resistance in dc circuits. The unit is the Manipulated variable: A quantity or condition which is
ohm. The impedance of an ac circuit is one ohm if a varied as a function of an actuating error signal so as to
potential difference of one volt creates a current flow change the value of the directly controlled variable.
of one ampere within it. Measurement signal: The electrical, mechanical, pneu-
Inductance: The property by which an electromotive matic, digital or other variable applied to the input of
force (emf) is induced in a conductor when the mag- a device. It is the analog of the measured variable pro-
netic field is changing about it. This is usually caused by duced by the transducer.
changes in the current flow in the circuit or in a neigh- Measurement variable: A quantity, property or condition
boring circuit. which is being measured, sometimes referred to as the

measurand. poles, one positive and one negative.
Milliamp (mA): One thousandth of an ampere. Port: A communications connection on an electronic or
Millivolt (mV): One thousandth of a volt. computer-based device.
Multiplexer (Mux): A switching device that sequentially Power supply: A separate unit or part of a system that
connects multiple inputs or outputs in order to process provides power (pneumatic, electric, etc.) to the rest of
several signal channels with a single A/D or D/A con- a system.
verter. Pressure, ambient: The pressure of the medium sur-
N rounding a device.
Noise: Any undesirable electrical signal, from external Pressure, design: The pressure used in the design of a
sources such as ac power lines, motors, electrical storms, vessel or other item of equipment for the purpose of
radio transmitters, as well as internal sources such as determining the minimum permissible wall thickness
electrical components. or size of parts for a given maximum working pressure
Non-linearity: The deviation from the best fit straight (MWP) at a given temperature.
line that passes through zero. Pressure, maximum working: The maximum permissible
Normal-mode rejection ratio: The ability of an instru- operating pressure at a specified temperature. This is the
ment to reject electrical interference across its input highest pressure to which the device will be subjected
terminals, normally of line frequency (50-60 Hz). during regular use.
Nyquist theorem: The law that is the basis for sampling Pressure, operating: The actual (positive or negative)
continuous information. It states that the frequency of pressure at which a device operates under normal con-
data sampling should be at least twice the maximum fre- ditions.
quency at which the information might vary. This theo- Pressure, rupture: The burst pressure of a device (deter-
rem should be observed in order to preserve patterns in mined by testing).
the information or data, without introducing artificial, Pressure, static: The steady-state pressure applied to a
lower frequency patterns. device.
O Pressure, supply: The pressure at which a utility (such as
Ohmmeter: A device used to measure electrical resis- air) is supplied to a device.
tance. Pressure, surge: Operating pressure plus the increment
One-to-one repeater: A diaphragm-operated device to which a device can be subjected for a very short time
which detects process pressure and generates an air (or during temporary pressure surges caused by such phe-
nitrogen) output signal of equal pressure. nomena as pump start-up or valve shut-off.
Optical isolation: Two networks or circuits in which an Pretravel: That part of a stroke which falls below the
LED transmitter and receiver are used to maintain elec- calibrated range, between zero and the travel stop.
trical discontinuity between the circuits. Primary element: An element that converts a measured
Output settling time: The time required for an analog out- variable into a force, motion or other form suitable for
put voltage to reach its final value within specified limits. measurement.
Output signal: A signal delivered by a device, element Process: Physical or chemical change of matter or con-
or system. version of energy.
Output slew rate: The maximum rate of change of ana- Process measurement: The acquisition of information
log output voltage from one level to another. that establishes the magnitude of process quantities.
Overtravel: That part of a stroke which falls between the Programmable logic controller (PLC): Computer-based
end of the calibrated range and the travel stop. industrial monitoring and control package with applica-
P tions mostly in the areas of safety, sequential or logical
Phase: A time-based relationship between a periodic operations, where control actions are based on equip-
function and a reference. ment and alarm status.
Phase shift: The angle in degrees between an energizing Proportional control: A control mode which generates
voltage waveform and an output signal waveform. an output correction in proportion to error (the process
Polarity: In electricity, the quality of having two charged variable’s deviation from setpoint).

Proportional-integral-derivative (PID): Also referred to its unit is the ohm/cm.
as a 3-mode controller, combining proportional, integral, Resolution: The smallest change in input which produces
and derivative control actions. a detectable change in output. This is the smallest incre-
psia: Pounds per square inch absolute; the unit of pres- ment of change that can be detected by a measurement
sure used when the zero reference is full vacuum. system. Resolution can be expressed in bits, in propor-
psig: Pounds per square inch gauge; the unit of pressure tions, in percent of actual reading or in percent of full
used when the zero reference is the barometric pressure scale. For example, a 12-bit system has a resolution of
of the atmosphere. one part in 4,096 or 0.0244% of full scale.
R Resonance: A condition of oscillation caused when
Radio frequency: The frequency range between ultrasonic a small amplitude of periodic input has a frequency
and infrared. AM broadcast frequencies range from 540 approaching one of the natural frequencies of the driven
to 1,800 kHz, while FM broadcasts from 88 to 108 MHz. system.
Radio frequency interference (RFI): Noise induced upon Response time: An output expressed as a function of
signal wires by ambient radio-frequency electromagnetic time, resulting from the application of a specified input
radiation with the effect of obscuring an instrument signal. under specified operating conditions.
Ramp: The total (transient plus steady-state) time response S
resulting from a sudden increase in the rate of change from Sampling period: The time interval between observations.
zero to some finite value of input stimulus. Scale factor: The factor by which the number of scale
Range: The region between the limits within which a divisions indicated or recorded by an instrument must be
quantity is measured, received or transmitted, expressed multiplied to compute the value of a measured variable.
by stating lower and upper range values. Sensing element: The element that is directly responsive
Reactance: The opposition to the flow of ac current, to the value of a measured variable.
which is created by either inductance or capacitance. In Sensitivity: The minimum change in a physical variable
such a circuit, total impedance is therefore the sum of to which an instrument can respond; the ratio of the
reactance and resistance. The unit is the ohm. change in output magnitude to the change of the input
Reference input: An external signal serving as a setpoint which causes it after the steady-state has been reached.
or as a standard of comparison for a controlled variable. Sensor: An element or device that detects a variable
Reliability: The probability that a device will perform its by receiving information in the form of one quantity
objective adequately for the period of time specified, and converting it to information in the form of that or
under the operating conditions specified. another quantity.
Repeatability: The maximum difference between output Setpoint: A variable, expressed in the same units as the
readings when the same input is applied consecutively; measurement, which sets either the desired target for
the closeness of agreement among consecutive measure- a controller, or the condition at which alarms or safety
ments of an output for the same value of input under the interlocks are to be energized.
same operating conditions, approaching from the same Settling time: The time required after a stimulus for the
direction, usually measured as non-repeatability and output to center and remain within a specified narrow
expressed as percent of span. band centered on its steady-state value.
Reproducibility: The closeness of agreement among Signal: A variable that carries information about another
repeated measurements of an output for the same value variable that it represents.
of input made under the same operating conditions over Signal-to-noise ratio: Ratio of signal amplitude to noise
a period of time, approaching from both directions. It amplitude. The ratio of overall rms signal level to rms
includes hysteresis, deadband, drift and repeatability. noise level, expressed in dB. For sinusoidal signals, ampli-
Resistance; Resistivity: The opposition to the flow of tude may be peak or rms.
current in a dc circuit. The unit is the ohm, which is Span: The algebraic difference between the upper and
defined as the resistance that will give a one-ampere lower range values expressed in the same units as the
current flow if a one-volt potential difference is applied range.
in a circuit Resistivity is the reciprocal of conductivity; Span shift: Any change in slope of the input-output

curve. Transducer: An element or device that receives informa-
Stability: The ability of an instrument or sensor to tion in the form of one quantity and converts it to informa-
maintain a consistent output when a constant input is tion in the same or another quantity or form. Primary ele-
applied. ments and transmitters are also referred to as transducers.
Steady-state: A characteristic of a condition, such as Transfer function: Mathematical, graphical, or tabular
value, rate, periodicity, or amplitude, exhibiting only statement of the influence which a system or element
negligible change over an arbitrary, long period of time. has on a signal or action compared at input and at out-
Stiffness: The ratio of change of force (or torque) to the put terminals.
resulting change in deflection of a spring-like element, Transient: The behavior of a variable during transition
the opposite of compliance. between two steady-states.
Strain: The ratio of the change in length to the initial Transmitter: A transducer which responds to a measured
unstressed reference length of an element under stress. variable by means of a sensing element, and converts it
Subsidence: The progressive reduction or suppression of to a standardized transmission signal which is a function
oscillation in a device or system. only of the values of the measured variable.
Suppressed range: A range in which the zero value of a U
measured variable is greater than the lower-range value Upper range limit (URL): The highest value of a measured
(LRV). The terms "elevated zero," "suppression" or "sup- variable that a device can be adjusted to measure. (This
pressed span" are also used to express the condition when value corresponds to the top of the range.)
the zero of the measured variable is greater than the LRV. Upper range value (URV): The highest value of a mea-
Suppressed span: The span in which the zero of the measured variable that a device is adjusted to measure. (This
sured variable is greater than the LRV. value corresponds to the top of the span.)
Suppressed zero: The range in which the zero value of a V
measured variable is less than the lower range value. The Vapor pressure: The pressure exerted by a vapor which is
terms "elevation," "elevated range" and "elevated span" in equilibrium with its own liquid.
are frequently used to express the condition in which Variable: Any condition that is measured, controlled
the zero of the measured variable is less than the lower (directly or indirectly) or manipulated.
range value. Velocity limit: A limit on the rate of change which a
Suppression ratio: The ration of the lower-range value to particular variable may not exceed.
the span. If range is 20-100 and, therefore, span is 80 and Vibration: The periodic motion or oscillation of an ele-
LRV is 20, the suppression ratio is 20/80 = 0.25 or 25%. ment, device, or system.
Synchronous: An event or action that is synchronized to Volt (V): The electrical potential difference between two
a reference clock. points in a circuit. One volt is the potential needed to
System noise: Measure of the amount of noise seen by move one coulomb of charge between two points while
an analog circuit or an ADC when the analog inputs are using one joule of energy.
grounded. W
T Warm-up period: The time required after energizing a
Temperature coefficient: The amount of drift, in percent device before its rated performance characteristics start
of full scale output, that might result from a 1°C change to apply.
in ambient temperature. Z
Thermal shock: An abrupt temperature change applied Zero offset: The non-zero output of an instrument,
to a device. expressed in units of measure, under conditions of true
Time constant: The value "T" in an exponential term A(-t/T). zero.
For the output of a first-order system forced by a step or Zero suppression: For a suppressed-zero range, the
an impulse, T is the time required to complete 63.2% of amount by which a measured variable’s zero is less than
the total rise or decay. For higher order systems, there is the lower-range value; can be expressed as a percentage
a time constant for each of the first-order components of either the measured variable or of the span.
of the process. Zone, neutral: A predetermined range of input values
that do not result in a change in the previously existing
output value.

Notice of
Intellectual Property Rights
The OMEGA® Handbook Series is based upon original
intellectual property rights that were created and
developed by OMEGA. These rights are protected
under applicable copyright, trade dress, patent and
trademark laws. The distinctive, composite appear-
ance of these Handbooks is uniquely identified with
OMEGA, including graphics, product identifying pings,
paging/section highlights, and layout style. The front,
back and inside front cover arrangement is the sub-
ject of a U. S. Patent Pending.
©2001 Putman Publishing Company and OMEGA Press LLC.

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www.omega.com • e-mail: flow@omega.

VOLUME 4—FLOW & LEVEL MEASUREMENT

• The Flow Pioneers

• Flow Sensor Selection 08

• Primary Element Options

• Positive Displacement Flowmeters

• Coriolis Mass Flowmeters

VOLUME 4—FLOW & LEVEL MEASUREMENT

• Level Sensor Selection

• Dry & Wet Leg Designs

• Radar & Microwave

The Flow Pioneers

Where D is the pipe diameter, V is

Flow Sensor Selection

• Installation a pump status contact. cess fluid can be achieved by metal

• Applications & Limitations

V = (f0 - f1)Ct /2f0 cos(a)

Where Ct is the velocity of sound

Computerized tank farm systems usu-

LT tanks through field networks. These

• Float Level Switches

• Applications & Installations

• Antenna Designs and Mounting

receiver. variations must be measured and foil, stretched over an aluminum

TRANSACTIONS Volume 4 101

TRANSACTIONS Volume 4 105

106 Volume 4 TRANSACTIONS

flow and level Products

omega press references

TRANSACTIONS Volume 4 107

108 Volume 4 TRANSACTIONS

Teflon®, Viton® and Kalrez® are registered trademarks of DuPont.

TRANSACTIONS Volume 4 109

110 Volume 4 TRANSACTIONS

TRANSACTIONS Volume 4 111

112 Volume 4 TRANSACTIONS

TRANSACTIONS Volume 4 113

114 Volume 4 TRANSACTIONS

TRANSACTIONS Volume 4 115

©2001 Putman Publishing Company and OMEGA Press LLC.

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