Chapter-3 Transducers and Measuring Instruments: Current and Voltage Measurement

Instrumentation and Measurement Mizan-Tepi University
CHAPTER-3
TRANSDUCERS AND MEASURING INSTRUMENTS
CURRENT AND VOLTAGE MEASUREMENT:

CURRENT MEASURMENT:
An ammeter (from Ampere Meter) is a measuring instrument used to measure the
current in a circuit. Electric currents are measured in amperes (A), hence the name. Instruments
used to measure smaller currents, in the milliampere or microampere range, are designated as
milliammeters or microammeters. Early ammeters were laboratory instruments which relied on
the Earth's magnetic field for operation. By the late 19th century, improved instruments were
designed which could be mounted in any position and allowed accurate measurements in
electric power systems. It is generally represented by letter 'A' in a circle.
The majority of ammeters are either connected in series with the circuit carrying the
current to be measured (for small fractional amperes), or have their shunt resistors connected
similarly in series. In either case, the current passes through the meter or (mostly) through its
shunt. Ammeters must not be connected directly across a voltage source since their internal
resistance is very low and excess current would flow. Ammeters are designed for a low voltage
drop across their terminals, much less than one volt; the extra circuit losses produced by the
ammeter are called its "burden" on the measured circuit.
VOLTAGE MEASURMENT:
A voltmeter is an instrument used for measuring electrical potential difference between
two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion
to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an
analog to digital converter.
A voltmeter in a circuit diagram is represented by the letter V in a circle. Voltmeters are
made in a wide range of styles. Instruments permanently mounted in a panel are used to monitor
generators or other fixed apparatus. Portable instruments, usually equipped to also measure
current and resistance in the form of a multimeter, are standard test instruments used in
electrical and electronics work. Any measurement that can be converted to a voltage can be
Prepared by: Belachew G. Page 1

displayed on a meter that is suitably calibrated; for example, pressure, temperature, flow or level
in a chemical process plant.
General purpose analog voltmeters may have an accuracy of a few percent of full scale,
and are used with voltages from a fraction of a volt to several thousand volts. Digital meters can
be made with high accuracy, typically better than 1%. Specially calibrated test instruments have
higher accuracies, with laboratory instruments capable of measuring to accuracies of a few parts
per million. Meters using amplifiers can measure tiny voltages of microvolts or less.
DISPLACEMENT MEASUREMENT:
Linear displacement may be measured by the following transducers:
1. Resistive potentiometers 2. Strain gauges
3. Variable inductance transducers 4. Linear variable differential transducers (LVDT)
5. Capacitive transducers 6. Piezoelectric transducers
7. Hall effect transducers 8. Electronic transducers
9. Ionization transducers 10. Digital transducers
11. Acoustic transducers
 Mechanical displacement may be converted into an electrical variable by the simple
expedient of adjusting resistance in an electrical circuit. A slide-wire resistor, having a movable
contact attached to the part whose displacement is to be measured, may be connected through a
2 – conductor circuit to a steady – voltage source in series with an ammeter (or milliammeter)
calibrated in terms of the displacement. If the resistor is connected as a voltage divider, the need
for a regulated supply is eliminated and with a 3 – conductor circuit the display instrument may
be a ratio – meter or potentiometer. Such combinations are common and are available for both
D.C and A.C. operation.
 Where deflections are small of the order of 0.25 mm, measurement may be made by use
of a differential transformer.
Measurement of small displacements:
The linear or differential transformer is the popular means for measuring small
displacement of the order of 0.25 mm. This device is generally produced with a single primary
winding and two secondaries, all disposed along a common axis and having in the common
magnetic circuit a moveable iron core longitudinally displaceable with the motion to be
measured, The secondaries may be connected additively or differentially and may be included
in the circuit of a null type instrument based either by shifting the core of a similar transformer
excited from the same source or by the use of a slide wire potentiometer.

Linear transformers are regularly supplied for operation at all frequencies upto 30000
Hz. The sensitivity, of course, increases with the frequency.
Linear transformers may be interconnected in a great variety of arrangement to perform
computations or to express desired mathematical functions of measured variables.
Schematic diagram of differential transformer transducer with servo – activated receiver
Measurement of Angular Displacement:

The following transducers are used for angular displacement:
1. Resistive potentiometer 2. Variable inductive transducers
3. Synchros 4. Capacitive transducers
5. Rotary variable differential transformer (RVDT) 6. Variable reluctance transducer
Variable reluctance transducer:
Figure shows a variable reluctance transducer for measurement of angular displacement.
L1 and L2 are the two identical coils connected in series opposition. These coils are so
placed/positioned with respect to a cam shaped iron core that, when the core is in the null
position, the impedances of both the coils are same. Hence, voltages across the two coils are the
same, though they are 180 out of phase. Any angular displacement from the null position
results in a differential voltage output proportional to the amount of displacement. The
clockwise and anticlockwise angular displacements produce different output which are opposite
in phase. Thus the angular displacement may be measured in magnitude and direction.
STRAIN MEASURMENT:
Strain is defined as the change in length of a line segment between two points divided by
the original length of the line segment. For the strain measurement it is the usual practice to

make measurements over shortest possible gauge lengths. This is owing to the fact the
measurement of a change in a finite length does not give the strain at any fixed point, but rather
the average value over the length. The strain at various points might be different depending
upon the strain gradient along the gauge length. When the strain gradient is constant over a
small length, the average stain will be point strain at the middle point of the gauge length.
A strain gauge is a device which is used to measure dimensional change on the surface
of a structural member under test.
Strain gauge gives indication of strain at only one point.
The strain measurement is of significant importance in a variety of applications due to the
following reasons:
 To avoid the use of large factors of safety in the design of aircraft and automatic control
equipment due to consideration of mass/inertia.
 Strain measurement utility as a means to determine maximum stress values or in
specialized transducers to measure pressure, acceleration, force, torque etc.
 Necessity to verify experimentally the strain in complex physical systems (e.g. human
skills and legs) where strain only be estimated approximately even by the use of most
rigorous analytical methods.
Techniques of Strain Measurement:
The various techniques available for the measurement of mechanical strain are given
below:
1. Photo – elasticity
2. Strain gauges
i. Non-electrical (good for static loads only)
 Mechanical (10 micro-strain)
 Optical (2 micro-strain)
 Photo-elastic (40 micro-strain)
ii. Electrical (good even under dynamic load conditions)
 Resistance
- Metallic (0.5 micro-strain)
- Semiconductor (0.01 micro-strain)
 Capacitance/Inductance (Large mass and size, rugged in construction and retain
calibration over large periods of time)
3. Brittle lacquers (600 micro-strains)
Requirements of a Strain Gauge:
A strain gauge should posses the following properties/characteristics:
1. Cheaper, reliable and readily available
2. Negligible mass and extremely small size
3. High speed of response, negligible time lag
4. Good response in unison with changes in surface to which it is fixed
5. Availability of gauges in variety of types and sizes suitable for a wide range of
applications
6. Capability to indicate static, transient and dynamic strain
7. High sensitivity in the direction of measured strain but low sensitivity in the transverse
plane.
8. Insensitiveness to ambient conditions (e.g., temperature, humidity, vibration etc)
9. Simply and easy attachment to the specimen under test.

10. Non-interference with the stiffness and other characteristics of the member over which it
is mounted.
1. Mechanical Strain Gauges:
In these gauges the magnification is carried out by mechanical means. In the initial stage
an extensometer of single mechanical lever type having a magnification of 10:1 was developed;
it used to work on a long gauge length. Later on, extensometers employing compound levers
(dial gauges) having a magnification of 2000:1 were introduced; these operated on small gauge
lengths.
The most commonly used mechanical strain gauges are of Berry-type and Huggen
berger type.
 These are best suited only for use in static tests
Advantages:
1. Self-contained magnification system
2. No auxiliary equipment required (as in case of electrical strain gauges)
Disadvantages:
1. Owing to high inertia of the gauge, it is unsuitable for dynamic measurements and
varying strains
2. Slow response (due to high inertia)
3. No arrangement for recording the readings automatically
4. Non availability of adequate surface area on the test specimen and clearance above it to
accommodate the gauge together with its mounting fixture.
2. Optical Strain Gauges:

In these gauges some form of optical lever is employed to measure small displacements.
 The most commonly used gauge of this type was developed by L.B. Tuckerman. It
consists of an “extensometer” (made of steel or aluminium alloy) and an “autocollimator”.
The gauge length of extensometer is adjustable one varying from 6 to 380 mm in steps of
6 mm.
- The main advantage of this instrument is that the position of collimator need not
be fixed relative to extensometer and reading can be taken by holding the collimator in
hand.
- This gauge is satisfactory only for static measurements and suffers from the
drawbacks inherent in all mechanical systems, if used for dynamic measurements. Also,
it cannot be used at those places where large strain gradients are encountered.

 An interferometer type of gauge is the true optical gauge. Its working principle is based on
the fact that interference fringes are produced when two rays of some wavelength and
intensity undergo a path difference of half wavelength of light, before combining.
This type of gauge is generally not used due to its high sensitivity
This gauge can be used:
1. To find change in thickness of photo elastic models
2. To determine Young’s modulus and Poisson’s ratio in laboratories
3. Semiconductor or peizo resistive gauges:

The semiconductor gauges are produced in wafers from silicon or germanium crystals in
which exact amount of special impurities such as boron, phosphorus etc, have been added to
impart certain desirable properties. The process is called doping and the crystals are known as
doped crystals. The resistance of strain gauges constructed of n-type silicon decreases with an
increase in tensile strain; p-type gauges undergo increase in resistance as tensile strain increases.
Since the breaking stress of the material rises with the decrease in cross-sectional area,
therefore, these gauges are constructed in the thinnest possible cross-section and as such can be
bent to much smaller radius of curvature without fracture. The gauge is in the form of a single
rectangular element about 0.05 mm thick by 0.25 mm wide and 1.5 mm to 12 mm in length.
Characteristics of semiconductor gauges:
 Very high sensitivity (in comparison to metal gauges)
 High gauge factor in the range of 100 to 200. By proper doping of the silicon semiconductor
crystal, gauge factor can be varied from – 100 to 150. The gauge factor however is
generally not constant and varies with strain and temperature.
 Low hysteresis
 Range 1/4% of strain with accuracy of 2.3%; linearity is poor 0.75% over 1/10 full scale
and 20% over full scale.
 Brittle and unsuitable for large strain measurement.
Uses: These gauges find their greatest usage in high output transducers such as load cells and
pressure cells where circuit compensation techniques can be used to minimize the adverse
effects of temperature sensitivity and non-linearity.

VELOCITY AND ACCELERATION MEASUREMENT:

Velocity Measurement:
1. Measurement of Linear Velocity:
The following types of transducers are used (in the analogue methods) for measurement
of linear velocity (first derivative of displacement).
1. Electro-magnetic transducers
 Moving magnet type
 Moving coil type
2. Seismic type transducers
Electro-magnetic transducers:
An electro-magnetic transducer utilizes the voltage produced in a coil on account of
change in flux linkages resulting from change in reluctance. This is the most commonly used
transducer for measurement of linear velocities.
 Moving magnet type: A moving type electro-magnetic transducer uses permanent magnet
which provides a constant polarizing field. Figure shows such a transducer. It consists of rod
rigidly coupled to the device whose velocity is being measured. The rod is a permanent
magnet which surrounded by a coil.
Working: When the magnet moves a voltage is induced in the coil which is directly
proportional to the velocity. The voltage induced in the coil (placed in a magnetic field) is given
by
e = BANV
eV
Where,
B = flux density, Wb/m2
A = Area of coil, m2
N = Number of turns of coil
V = Relative velocity of magnet with respect to coil
The direction of motion is determined by the polarity of the output voltage.

Moving magnet type transducers

Advantage:
 The output voltage is linearly proportional to velocity
 Negligible maintenance (due to absence of mechanical surfaces/contacts)
 Can be employed as event markers which are robust and inexpensive to manufacture.
Disadvantages:
 Limited frequency response
 Unsuitable for measurement of vibrations
 Stray magnetic fields adversely affect their performance.
 Moving coil type: This type of transducer operates essentially through the action of a coil
moving in a magnetic field; the voltage generated in the coil being proportional to the
velocity of the coil.
The advantages of this type of transducer are:
 Reduced stray magnetic fields effects due to the whole device being contained in an
antimagnetic case.
 More satisfactory arrangement, as the system forms a closed magnetic circuit with a
constant air gap.
2. Measurement of Angular Velocity:
Angular velocity may be measured with the help of a “tachometer”
A tachometer may be defined as follows:
“An instrument which either continuously indicates the value of rotary speed or continuously
displays a reading of average speed over rapidly operated short-intervals of time”
(Or)
“An instrument used for measurement of angular velocity, as of shaft either by registering the
total number of revolutions during the period of contact or by indicating directly the number of
revolutions per minute”
Classification of Tachometers:
The tachometers are classified as follows:
1. Mechanical tachometers:

a) Hand speed indicator

b) Revolution counter and timer
c) Tachoscope
d) Centrifugal tachometer
e) Vibrating reed tachometers
2. Electrical tachometers:
a) D.C. tachometer generators
b) A.C. tachometer generators
c) Photoelectric tachometers (or Speed – meter)
d) Eddy current or drag – cup tachometer
e) Capacitive tachometer
f) Stroboscopic tachometers
Mechanical tachometers:
1. Hand speed indicator:
This tachometer has an integral stopwatch and counter with automatic disconnect.
- The spindle operates when brought in contact with the shaft; the counter, however, does not
function until the start and wind button is pressed to start the watch and engage the
automatic clutch. Depressing of the starting button also serves to wound the starting watch.
- The revolution counter automatically gets disengaged after a fixed interval, usually 3 to 6
seconds.
The instrument indicates the average speed over the short interval; the dial is designed to
indicate the rotational speed directly in r.p.m.
These tachometers can be used to measure speeds within range of 20000 to 30000 r.p.m.
These units have an accuracy of 1 percent of the full scale.
Acceleration Measurement:
The acceleration of a moving body is generally measured by means of sensors called
accelerometers.
For measurement of acceleration, generally following types of accelerometers are used:
a) Piezoelectric type
b) Seismic type
Piezoelectric Accelerometer:
Figure shows; A piezoelectric accelerator is probably the simplest and most commonly
used transducer for measuring acceleration.

Piezoelectric accelerometer
Construction: It consists of a piezoelectric crystal sandwitched between two electrodes and has
mass placed on it. The unit is fastened to the base whose acceleration characteristics are to be
obtained. The “can” threaded to the base acts as a spring and squeezes the mass against the
crystal. Mass exerts a force on the crystal and a certain voltage output is generated.
Working: When the base is accelerated downward inertial reaction force on the base acts
upward against the top of the “can”. This relieves stress on the crystal. According to Newton’s
second law of motion, force = mass x acceleration, since the mass is a fixed quantity, the
decrease in force is proportional to the acceleration. Similarly, acceleration in the upward
direction would increase the force on the crystal in proportion to the acceleration. The resulting
change in the output voltage is recorded and correlated to the acceleration imposed on the base.
Advantages:
1. Small size and a small weight
2. High output impedance
3. “Can” measure acceleration from a fraction of ‘g’ to thousands of ‘g’.
4. High sensitivity
5. High frequency response (10 Hz to 50 KHz)
Disadvantage:
1. Unsuitable for applications where the input frequency is lower than 10 Hz.
2. Subject to hysteresis errors
3. Sensitive to temperature changes.
ROTATIONAL SPEED MEASUREMENT:
1. Photoelectric Tachometer:
Photoelectric tachometer
It consists of an opaque disc mounted on the shaft whose speed is to be measured. The
disc has a number of equivalent holes around the periphery. On one side of the disc there is a
source of light (L) while on the other side there is a light sensor (may be a photosensitive device
or photo-tube) in line with it (light-source).
On the rotation of the disc, holes and opaque portions of the disc come alternate in
between the light source and the light sensor. When a hole comes in between the two, light
passes through the holes and falls on the light sensor, with the result that an output pulse is
generated. But when the opaque portion of the disc comes in between, the light from the source

is blocked and hence there is no pulse output. Thus whenever a hole comes in line with the light
source and sensor, a pulse is generated. These pulses are counted/measured through an electric
counter.
The number of pulses generated depends upon the following factors:
 The number of holes in the disc
 The shaft speed
Since the numbers of holes are fixed, therefore, the number of pulses generated depends
on the speed of the shaft only. The electronic counter may therefore be calibrated in
terms of speed (r.p.m)
Advantage: It is a digital instrument
Disadvantage: It is required to replace the light source periodically and if the grating period is
small then errors might creep in the output.
TORQUE MEASUREMENT:
Introduction:
To transmit energy by rotation it necessary to apply a turning force. In case of a shaft it
the force is applied tangentially and in the plane of transverse cross-section the torque or
twisting moment may be calculated by multiplying the force with the radius of the shaft. If the
shaft is subjected to two opposite turning moments it is said to be in pure torsion and it will
exhibit the tendency of shearing off at every cross-section which is perpendicular to the
longitudinal axis.
The measurement of torque is carried out because of the reasons: (i) It is of considerable
interest for its own sake; (ii) It is required to obtain load information necessary for stress and
deflection analysis; (iii) It is often associated with determination of mechanical power.
Torque Measurement Methods:
Torque may be measured by the following methods:
a. Gravity balance method
b. Mechanical torsion meter
c. Optical torsion meter
d. Electrical torsion meter
e. Strain-gauge torsion meter
1. Gravity balance method:
This method is illustrated in figure; A mass ‘m’ is moved along an arm until the value of
the torque exerted by the mass balances the unknown torque.
Torque (T) = F x r
Where, F = mg; Force exerted by the mass.
This method utilizes the movement of a constant mass, m, over a variable distance.
Alternatively, magnitude of the mass may be varied, keeping the radius (r) constant.
In both of the above cases, the arm must be kept horizontal so that arm distance is
perpendicular to the line of action of force. Since the shaft is supported at the bearing, there may
be a friction torque (due to the force acting on the bearing) leading to error in the measurement
of torque. This error may be eliminated by arranging to apply equal and opposite force.

Gravity balance method

2. Strain – gauge torsion meter:
Figure shows a general configuration of a strain gauge bridge circuit widely employed
for torque measurement from a rotating shaft.
Construction: Four bonded – wire strain gauges are mounted on a 45 helix with the axis of
rotation; and are placed in pairs diametrically opposite. When the gauges are accurately placed
and have matched characteristics, the systems is temperature compensated and insensitive to
bending and thrust or pull effects. Any change in the gauge circuit then results only from
torsional deflection.
Working: When the shaft is under torsion, gauges 1 and 4 will elongate as a result of the tensile
component of a pure shear stress on one diagonal axis, while 2 and 3 will contract due to
compressive component on the other diagonal axis. The tensile and compressive principal
strains can be measured and hence shaft torque calculated.
Strain gauge torsion-meter
Advantages: This arrangement has the following advantages:

 Fully temperature compensated
 Gives the maximum sensitivity for a given torque
 Provides automatic compensation for bending and axial loads.
Limitation:
The main limitation/difficulty associated with the use of this arrangement is the
connection of the bridge to its power source and display arrangement. Slip rings are used for

this purpose. These are conducting rings attached to the shaft, but insulated from it, with one of
the slip rings connected to each of the bridge terminals. Slip rings are mercury filled and
transmit the signal to a stationary member where it is amplified and displayed or recorded.
 Good results from the strain gauge method are available only when the shaft experiences
sufficient strain to produce a measurable output signal. In case the torque is small the gauge
bridge will not give adequate output signal for measurement. In such cases a flexible coupling is
introduced in the shaft. The coupling incorporates one or more elastic members (elastic member
may be a different shaft or a commercial torque meter) to which the gauge may be attached. The
elastic members are so designed that they produce sufficiently large deflection even under light
load conditions. This large deflection produces large strains resulting in large output of the
strain gauge bridge.
PRESSURE MEASUREMENT:
The pressure measurement instruments can be categorized as follows:
1. Instruments for measuring low pressures (below 1 mm of Hg):
 Manometers
 Low pressure gauges
2. Instruments for medium and high pressures (between 1 mm of Hg to 1000 atmospheres):
 Bourdon tube
 Diaphragm gauges
 Bellow pressure gauge
 Dead – weight pressure gauge
3. Instrument for measuring low vacuum and ultra high vacuum (760 torr to 10-9 torr and
beyond: 1 torr = 1 mm of Hg):
 Mcleod
 Thermal conductivity
 Ionization gauges
4. Instruments for measuring very high pressures (1000 atmospheres and above):
 Bourdon tube
 Diaphragm gauges
 Electrical resistance pressure gauges
5. Instruments for measuring varying pressure:
 Engine indicator
 Cathode ray oscilloscope (CRO)
1. U – Tube manometer:
Piezometers cannot be employed when large pressures in the lighter liquids are to be
measured, since this would require very long tubes, which cannot be handled conveniently.
Furthermore gas pressures cannot be measured by the piezometers because a gas forms no free
atmospheric surface. These limitations can be overcome by the use of U-tube manometers.
A U-tube manometer consists of a glass tube bend in U-shape one end of which is
connected to a point at which pressure is to be measured and other end remains open to the
atmosphere as shown in figure.

(i) For positive pressure:

[
Let A be the point at which pressure is to be measured X-X is the datum line as shown
in figure (a).
Let
h1 = Height of the light liquid in the left limb above the datum line
h2 = Height of the heavy liquid in the right limb above the datum line
h = Pressure in pipe, expressed in terms of head
S1 = Specific gravity of the light liquid
S2 = Specific gravity of the heavy liquid
The pressure in the left limb and right limb and datum X-X are equal (as the pressure at two
points at the same level in a continuous homogeneous liquid are equal).
Pressure head above X-X in the left limb = h + h1S1
Pressure head above X-X in the right limb = h2S2
Equating these two pressure, we get
h + h1S1 = h2 S2
h = h2S2 – h1S1
(ii) For negative pressure:
Refer the figure (b)
Pressure head above X-X in the left limb = h + h1S1 + h2S2
Pressure head above X-X in the right limb = 0
Equating these two pressures, we get
h + h1S1 + h2S2 = 0
(Or) h = - (h1S1 + h2S2).
2. Bellow gauges/elements:
The flexibility of bellows is directly proportional to:
 The number of convolutions
 The square of outside diameter of the bellows (inversely proportional to the cube of wall
thickness)
 Young’s modulus of elasticity of material.

Bellow gauges
Advantages:
1. Simple and rugged construction
2. Useful for measurement of low and medium pressures
3. Can be used for measurement of absolute, gauge and differential pressure
Disadvantages:
1. Need spring for accurate characterization
2. Greater hysteresis and zero drift problems
3. Unsuitable for transient measurement due to longer relative motion and mass
3. Thermocouple vacuum gauge:
A thermocouple vacuum gauge operates on the principle that at low pressure the thermal
conductivity of a gas is a function of pressure.
Figure shows basic elements of a thermocouple vacuum gauge. It consists of a heater
element (heated to a temperature of 50 to 400C by a known constant current) having a
thermocouple is contact with its centre. The heater element and thermocouple are enclosed in a
glass or metal envelope which is sealed into the vacuum system. The heater elements are
supplied with a constant electric energy and its temperature (which is a function of the heat loss
and hence thermal conductivity or pressure of the surrounding gas) is measured by a
thermocouple. The voltage measuring instrument can be directly calibrated to read the pressure
of the gas.
 The thermocouple gauges of one type or another are available to measure pressure in the
range 10-4 to 1 torr.
thermocouple gauges
Advantages:
1. Inexpensive and rugged construction

2. No departure from linearity in the range 0.02 to 1 mm of Hg.

3. Convenient and continuous reading
4. Possibility of remote reading from the system.
5. Possibility of process control with meter relay.
Disadvantages:
1. Required electric power
2. Narrow reading range
3. Need for individual and frequent calibration for different gases.
4. Diaphragm gauge:
This type of gauge employs a metallic disc or diaphragm instead of a bent tube. This
disc or diaphragm is used for actuating the indicating device.
Figure shows; when pressure is applied on the lower side of the diaphragm it is deflected
upward. This movement of the diaphragm is transmitted to a rack and pinion. The latter is
attached to the spindle of needle moving on a graduated dial. The dial can again be graduated in
a suitable scale.
Diaphragm gauge
Advantages:
i. Minimum hysteresis and no permanent zero shift.
ii. Can withstand high overpressures
iii. Can maintain good linearity over a wide range
iv. Gauge are available for absolute and differential-pressure measurements
v. Relatively small size and moderate cost.
Disadvantages:
i. Difficult to repair
ii. Needs protection from shock and vibration
5. Recent trends – smart pressure transmitters:
The microprocessors are now being used in transmitter also; as a consequence of the
availability of computing power the transmitters have become more intelligent.
The output in case of smart transmitters is 4 – 20 mA on 2 – wire but with added
capability of digital communication from a hand-held interface connected anywhere on 4 – 20
mA signal, the remote adjustment of the transmitter data base and acquisition diagnostic
information to minimize loop downtime is possible. It has high rangeability and much better
performance.

The transmitter senses all the three parameters, ‘differential pressure’, ‘static pressure’
and temperature effects and it computes a highly repeatable and accurate pressure measurement.
These characteristics are held in PROM memory and being specific to one meter are kept with
the meter body. The combination of characterized meter body and digital electronic has enabled
a quantum leap forward in performance.
 The rangeability of smart transmitters is very high (400:1). Thus only 3 sensors would
be required to cover the entire range of 2.5 milli-bars to 700 bars.
 The reliability is very high due to use of minimum number of components and
protection against all foreseeable damping influences like radio frequency, reverse
polarity, overpressure, surge voltage and lightning.
Advantages of digital transmitters:
The major advantages of digital (so called ‘smart’) transmitters over their conventional analog
counterparts are:
i. Increased rangeability (400:1 against 6:1 of analog transmitters)
ii. Higher accuracy
iii. Self-diagnostic facilities
iv. Almost no drift with time
v. Reduced cabling cost due to the use of a field bus cuts.
FLUID VELOCITY AND FLOW RATE MEASUREMENT:
In almost of the flow situation of engineering importance measurements of fluid
velocity, flow rate and flow quantity with varying degree of accuracy are a fundamental
necessity. Flow of material in a process or system can be measured by a variety of methods of
depending upon the material and its condition, the type of flow, the volume, the pressure and
temperature range, the accuracy and the control required.
The accurate measurement of flow presents many and varied problems. The flowing
medium may be liquid, a gas and a granular solid. The flow may be a laminar or turbulent,
steady – state or transient; in flow measurement.
Flow measurement methods/devices:
The flow measurement devices are classified as follows:
1. Rate meters
2. Quantity meters
1. Rate meters: They measure either the volumetric flow rate directly or use meters that measure
velocity and the volume flow rate can then be calculated with the help of cross-section.
(i) Inferential type: Inferential methods imply that the flow is not directly measured but is inferred
from the measurement of other quantities (e.g. pressure, temperature etc.)
These meters basically capture and release a fixed volume
(ii) Absolute or positive displacement type:
of fluid by some type of pumping action. They normally count the number of cycles that occur
and indicate or register an integrated flow volume. The flow is determined by the frequency of
the cycle.
These meters may be designed for the measurement of either weight or
2. Quantity meters:
volume. They may be absolute or displacement type. These are generally cited as positive
meters.
 Rotameter:
A rotameter is a constant-pressure drop, variable area flow meter.

Construction: It consists of a tapered metering glass tube, inside of which is located a rotor or
active element (float) of the meter. The tube is provided with inlet and outlet connections. The
specific gravity of the float or bob material is higher than that of the fluid to be metered. On a
part of the float spherical slots are cut which cause it (float) to rotate slowly about the axis of
the tube and keep it centered. Owing to this spinning accumulation of any sediment on the top
or sides of float is checked. However, the stability of the bob may also be ensured by using a
guide along which the float-would slide.
Working: When the rate of flow increases the float rises in the tube and consequently there is an
increase in the annular area between the float and the tube. Thus, the float rides higher or lowers
depending on the rate of flow.
The discharge through a rotameter is given by
As the flow area Aann is a function of height of float in the tube, the flow rate scale can
be engraved on the tube corresponding to a particular float.
Advantages:
1. Simpler in operation.
2. Handling and installation easy
3. Wide variety of corrosive fluids can be handled.
4. Possibility of convenient and visible flow comparisons by mounting several
rotameters side by side
5. Easily equipped with data transmission, indicating and recording devices.

Limitations:
1. Mounted vertically, limited to small pipe sizes and capacities
2. Less accurate, compared to venturimeter and orificemeter.
3. Glass tubes subject to breakage.
 Turbine Meter:
A turbine type flow meter operates on the principle that when a turbine wheel is placed
in a pipe containing a flowing fluid, its rotary speed depends on the flow rate of fluid. The speed
of turbine varies linearly with flow rate, if the bearing friction is reduced and the losses are kept
to a minimum.
This flow meter is known as inferential meter because the operation depends upon the
rotating form of the turbine and the stream velocity of the liquid in which it rotates. These
meters are volumetric flow meter and are available in wide ranges.
The turbine flow meter (shown in figure) consists of freely rotating wheel (or propeller)
with multiple blades. The rotor (supported by ball or sleeve bearings) is located centrally in the
pipe along which flow occurs. The flowing fluid impinging on the turbine blades imparts a force
on the blade surfaces and sets the rotor in motion with an angular speed proportional to the fluid
velocity. The rotor speed is measured with a mechanical counter or with an electromagnetic
pick up and associated counter. The electromagnetic pick up would be a small permanent
magnet mounted at the tip of one of the rotor blades with a coil being placed just outside the
tube. As the magnet moves past the coil, an e.m.f. is induced. Faster the fluid flow the greater
the counter per second. Each pulse represents a definite flow quantity and the total number of
pulse may be taken as an indication of total flow.

Advantages:
1. Good dynamic response
2. Easy to install and maintain
3. Low pressure drop, good temperature and pressure ratings
4. Good accuracy and excellent repeatability
5. Wide range commencing from 0.5 lpm to 15000 lpm for liquids and 3 lpm to 500000 lpm for
air.
Disadvantages:
1. Relatively high cost
2. Errors may be caused by excessive frictional torques
3. Errors arise on account of wear and corrosion of bearing and this calls for special design of
bearings.
TEMPERATURE MEASUREMENT:
The temperature measuring instruments are based on changes in a broad range of
physical properties, among which are the following:
1. Change in physical dimensions:
a) Liquid – in – glass thermometers
b) Bimetallic elements
2. Changes in gas pressure or vapour pressure:
a) Constant – volume gas thermometers
b) Pressure thermometers (gas, vapour and liquid filled)
3. Change in electrical properties:
a) Resistance thermometers (RTD, PRT)
b) Thermistors
c) Thermocouples
d) Semiconductor – junction sensors

4. Changes in emitted thermal radiations:

a) Thermal and photon sensors
b) Total-radiation pyrometers
c) Optical and two – colour pyrometers
d) Infrared pyrometers
5. Changes in chemical phase:
a) Fusible indicators
b) Liquid crystals
c) Temperature – reference (fixed points) cells.
 Bimetallic Thermometers:
In a bimetallic thermometer differential expansion of bimetallic strips is used to indicate
the temperature.
In this type of thermometer two flat strips of different metals are placed side by side and
are welded together. Many different metals can be used for this purpose. Generally one is low
expanding metal and the other is high expanding metal. The bimetal strip is coiled in the form
of a spiral or helix. Due to rise in temperature, the curvature of the strip changes. The
differential expansion of a strip causes the pointer to move on the dial of the thermometer.
The range over which a linear relationship exists between deflection and temperature
depends upon the combination of metals used for the bimetallic strip.
Bimetallic strip fixed at one end

Figure shows the industrial type bimetallic thermometers. The helical strip is used for
most process application because of its ability to be managed into a small diameter sheath while
maintaining ruggedness. The spiral strip often used in ambient temperature measurement
devices and air – control thermostats.
(a) Helical type b) Spiral type

 The bimetallic thermometers are made with metals which have widely different thermal
expansion coefficients.

- Invar (an alloy of nickel and iron) is the most commonly used low expansion
material. Its thermal expansion coefficient remains stable over a wide
temperature range.
- Nickel – iron alloys with chromium and manganese added are often used for
thermal expansion materials.
 In order to protect the bimetallic thermometers against wear and corrosion, they are
usually mounted in wells.
Advantages:
a) Simple and robust
b) Relatively less costly
c) Can withstand, in general, about 50% over range
d) Their accuracy range from 0.5% for laboratory type to 2% for process type
instruments
Disadvantages:
a) Temperatures indicated are not correct
b) Not suitable for use at temperature above 400C for continuous duty or above 550C for
intermittent duty.
Applications:
a) The bimetallic elements find wide applications in simple thermometers in which
deflection of the element is made to open or close electrical contacts in the electrical
heat supply to control a gas flow.
b) As switching devices used in domestic ovens, electric irons, car winkler lamps and the
refrigerators.
c) Used as compensator for ambient temperature change in the filled system thermometers,
aneroid barometers and in some watches as balance wheel compensators.
 Optical Pyrometers:
An optical pyrometer works on the principle that matters glow above 480C and the
colour of visible radiation is proportional to the temperature of the glowing matters. The amount
of light radiated from the glowing matter (solid or liquid) is measured and employed to
determine the temperature.
Figure shows a disappearing filament pyrometer.
Optical pyrometer
Operation:
 The optical pyrometer is sighted at the hot body and focused

 In the beginning filament will appear dark as compared to the background which is
bright (being hot)
 By varying the resistance (R) in the filament circuit more and more current is fed into it,
till filament becomes equally bright as the background and hence disappears.
 The current flowing in the filament at this stage is measured with the help of an ammeter
which is calibrated directly in terms of temperature.
 If the filament current is further increased, the filament appears brighter as compared to
the background which then looks dark.
 An optical pyrometer can measure temperature ranging from 700 to 4000C.
Uses:
The optical pyrometer is widely used for accurate measurement of temperature of
 Furnaces
 Molten metals
 Other heated material
This pyrometer has been accepted as the standard means for determining temperatures on the
International Temperature Scale from the gold point and upwards.
Advantages:
a) Excellent accuracy within 5C for the operating range 700 - 3000c
b) No direct contact is necessary with the object whose temperature is to the measured.
Thus this type of pyrometer can be used in situations where the measuring target is
remote and inaccessible such as furnace interiors, molten metal’s etc.
c) Measurement is independent of the distance between the target and measuring
instrument.
d) The skill in operating the thermometer can be acquired readily.
Disadvantages:
 The lower measuring temperature is limited to 700C (The eye is insensitive to
wavelength characteristics below this temperature)
 Owing to the manual mull-balance operation of this pyrometer it is not suitable for
continuous reading or automatic control applications.
FLUE – GAS COMPOSITION AND RADIATION MEASUREMENT:
1. ORSAT APPARATUS:
Introduction:
 To have proper control on combustion process, an idea about complete or complete
combustion of fuel is made by the analysis of flue gas. Thus,
(i) If the gases contain considerable amount of carbon monoxide, it indicates that incomplete
combustion is occurring (i.e. considerable wastage of fuel is taking flue).
 Also indicates the short supply of oxygen for combustion
(ii) If the flue gases contain a considerable amount of oxygen, it indicates the oxygen supply is
in excess, though the combustion may be complete.
 The analysis of flue gases made with the help of ORSAT’S APPARATUS.

Orsat Apparatus
Construction:
 Consists of a water-jacketed measuring burette, connected in series to a set of three
absorption bulbs, each through a stop-cock.
 The other end is provided with a three-way stop-cock, the free end of which is further
connected to a U-tube packed with glass wool (for avoiding the incoming of any smoke
particles, etc.)
 The graduated burette is surrounded by a water-jacket to keep the temperature of the gas
constant during the experiment.
 The lower end of the burette is connected to a water reservoir by means of a long rubber
tubing.
 The absorption bulbs are usually filled with glass tubes, so that the surface area of contact
between the gas and the solution is increased.
 The absorption bulbs have solutions for the absorption of CO2, O2 and CO respectively.
 First bulb has ‘potassium hydroxide’ solution (250g KOH in 500mL of boiled distilled
water), and it absorbs only CO2.
 Second bulb has a solution of ‘alkaline pyrogallic acid’ (25g pyrogallic acid+200g KOH in
500 mL of distilled water) and it can absorb CO2 and O2.
 Third bulb contains ‘ammonical cuprous chloride’ (100g cuprous chloride + 125 mL liquor
ammonia+375 mL of water) and it can absorb CO2, O2 and CO.
 Hence, it is necessary that the flue gas is passed first through potassium hydroxide bulb,
where CO2 is absorbed, then through alkaline pyrogallic acid bulb, when only O2 will be
absorbed ( because CO2 has already been removed) and finally through ammonical
cuprous chloride bulb, where only CO will be absorbed.
Working:
STEP 1:
 The whole apparatus is thoroughly cleaned, stoppers greased and then tested for air-
tightness.

 The absorption bulbs are filled with their respective solutions to level just below their rubber
connections.
 Their stop-cocks are then closed. The jacket and levelling reservoir are filled with water.
 The three-way stop-cock is opened to the atmosphere and reservoir is raised, till the burette
is completely filled with water and air is excluded from the burette.
 The three-way stop-cock is now connected to the flue gas supply and the reservoir is
lowered to draw in the gas, to be analyzed, in the burette.
 The sample gas mixed with some air is present in the apparatus. So the three-way stop-cock
is opened to the atmosphere, and the gas expelled out by raising the reservoir.
 This process of sucking and exhausting of gas is repeated 3-4 times, so as to expel the air
from the capillary connecting tubes, etc.
 Finally, gas is sucked in the burette and the volume of the flue gas is adjusted to 100 mL at
atmospheric pressure.
 For adjusting final volume, the three-way stop-cock is opened to atmosphere and the
reservoir is carefully raised, till the level of water in it is the same as in the burette, which
stands at 100 mL mark.
 The three-way stop-cock is then closed.
STEP 2:
 The stopper of the absorption bulb, containing caustic potash solution, is opened and all the
gas is forced into this bulb by raising the water reservoir.
 The gas is again sent to the burette.
 This process is repeated several times to ensure complete absorption of CO2 [by KOH
solution].
 The unabsorbed gas is finally taken back to the burette, till the level of solution in the CO2
absorption bulb stands at the constant mark and then, its stop-cock is closed.
 The levels of water in the burette and reservoir are equalized and the volume of residual gas
is noted.
 The decrease in volume-gives the volume of CO2 in 100 mL of the flue gas sample.
STEP 3:
 The volumes of O2 and CO are similarly determined by passing the remaining gas through
alkaline pyrogallic acid bulb and ammonical cuprous chloride bulb respectively.
 The gas remaining in burette after absorption of CO2, O2 and CO is taken as nitrogen.
Precautions:
 The reagents in the absorption bulb 1, 2 and 3 are brought to the etched mark levels one-by-
one by operating the reservoir bottle and the valve of each bulb. Then their respective valves
are closed.
 All the air in the reservoir bottle is expelled to atmosphere by lifting the reservoir bottle and
opening the three-way to atmosphere.
 The three-way is then connected to the flue gas supply and the reservoir bottle is brought
down, until the level in the burette becomes zero (i.e., 100 mL of gas is taken in the burette).
The gas in the burette is expelled to the atmosphere to remove any air left in the joints,
tubes, etc. This procedure is repeated 2-3 times to ensure a right sample of the gas taken for
analysis.
 It is quite necessary to follow the order of absorbing gases: CO2 first, O2 second and CO
last. This is because the absorbent used for O2 (i.e., alkaline pyrogallic acid) can absorb

only some CO2 and the percentage CO2 left would be less; while the percentage of O2 thus-
detected would be more. The absorbent used for CO2, however, does not absorb O2 or CO2.
 The % CO in the flue gas is very small and this should be measured quite carefully.
2. GAS CHROMATOGRAPHY:
• In gas chromatography (GC), the sample is vaporized and injected onto the head of a
chromatographic column. Elution is brought about by the flow of an inert gaseous
mobile phase.
• The mobile phase does not interact with molecule of the analyst; its only function is to
transport the analyst through the column.
• Gas-liquid chromatography is based upon the partition of the analyst between a gaseous
mobile phase and a liquid phase immobilized on the surface of an inert solid.
Gas chromatography
INSTRUMENTS FOR GAS CHROMATOGRAPHY:
Carrier Gas-Supply:
Carrier gases, which must be chemically inert, include helium, nitrogen, and hydrogen.
Associated with the gas supply are pressure regulators, gauges, and flow meters. In addition, the
carrier gas system often contains a molecular sieve to remove water or other impurities.
Sample Injection System:
• Column efficiency requires that the sample be of suitable size and be introduced as a
“plug” of vapor; slow injection of oversized samples causes band spreading and poor
resolution.
• The most common method of sample injection involves the use of micro syringe to
inject a liquid or gaseous sample through a self-sealing, silicone-rubber diaphragm or
septum into a flash vaporizer port located at the head of the column (the sample port is
ordinarily about 50oC above the boiling point of the least volatile component of the
sample).

Sample Injection System:

 For quantitative work, more reproducible sample sizes for both liquids and gases are
obtained by means of a rotary sample valve.
 Errors due to sample size can be reduced to 0.5% to 2% relative.
 The sampling loop is filled by injection of an excess of sample.
Rotation of the valve by 45 deg then introduces the reproducible volume ACB into the mobile
phase.
Column Configurations:
• Two general types of columns are encountered in gas chromatography, packed and
open tubular, or capillary.
• Chromatographic columns vary in length from less than 2 m to 50 m or more. They are
constructed of stainless steel, glass, fused silica, or Teflon. In order to fit into an oven
for thermostating, they are usually formed as coils having diameters of 10 to 30 cm.
Column Ovens:
• Column temperature is an important variable that must be controlled to a few tenths of a
degree for precise work. Thus, the column is ordinarily housed in a thermostated oven.
The optimum column temperature depends upon the boiling point of the sample and the
degree of separation required.
• Roughly, a temperature equal to or slightly above the average boiling point of a sample
results in a reasonable elution time (2 to 30 min). For samples with a broad boiling

range, it is often desirable to employ temperature programming, whereby the column

temperature is increased either continuously or in steps as the separation proceeds.
Detection Systems:
Characteristics of the Ideal Detector: The ideal detector for gas chromatography has the
following characteristics:
1. Adequate sensitivity
2. Good stability and reproducibility.
3. A linear response to solutes that extends over several orders of magnitude.
4. A temperature range from room temperature to at least 400oC.
5. A short response time that is independent of flow rate.
6. High reliability and ease of use.
7. Similarity in response toward all solutes or a highly selective response toward one or
more classes of solutes.
8. Nondestructive of sample.
Flame Ionization Detectors (FID):
The flame ionization detector is the most widely used and generally applicable detector
for gas chromatography.
• The effluent from the column is mixed with hydrogen and air and then ignited
electrically.
• Most organic compounds, when pyrolyzed at the temperature of a hydrogen/air flame,
produce ions and electrons that can conduct electricity through the flame.
• A potential of a few hundred volts is applied.
• The resulting current (~10-12 A) is then measured.
• The flame ionization detector exhibits a high sensitivity (~10-13 g/s), large linear
response range (~107), and low noise.
• A disadvantage of the flame ionization detector is that it is destructive of sample.

Gas Chromatographic Columns:

a) Open tubular Columns:
Open tubular, or capillary, columns are of two basic types, namely, wall—coated open
tubular (WCOT) and support-coated open tubular (SCOT). Wall-coated columns are simply
capillary tubes coated with a thin layer of the stationary phase. In support-coated open tubular
columns, the inner surface of the capillary is lined with a thin film (~30 m) of a support
material, such as diatomaceous earth. This type of column holds several times as much
stationary phase as does a wall-coated column and thus has a greater sample capacity.
Solid Support Materials:
The most widely used support material is prepared from naturally occurring
diatomaceous earth, which is made up of the skeletons of thousands of species of single-celled
plants (diatoms) that inhabited ancient lakes and seas. Such plants received their nutrients and
disposed of their wastes via molecular diffusion through their pores. As a consequence, their
remains are well-suited as support materials because gas chromatography is also based upon the
same kind of molecular diffusion.
Particle Size of Supports:
The efficiency of a gas-chromatographic column increases rapidly with decreasing
particle diameter of the packing. The pressure difference required to maintain a given flow rate
of carrier gas, however, varies inversely as the square of the particle diameter.
Qualitative Analysis:
Gas chromatograms are widely used as criteria of purity for organic compounds.
Contaminants, if present, are revealed by the appearance of additional peaks; the areas under
these peaks provide rough estimates of the extent of contamination. The technique is also useful
for evaluating the effectiveness of purification procedures. Retention times should be useful for
the identification of components in mixtures. Gas chromatography provides an excellent means
of confirming the presence or absence of a suspected compound in a mixture.
Quantitative Analysis:
The detector signal from a gas-liquid chromatographic column has had wide use for
quantitative and semi-quantitative analyses. An accuracy of 1% relative is attainable under
carefully controlled conditions. Reliability is directly related to the control of variables; the
nature of the sample also plays a part in determining the potential accuracy.

Interfacing Gas Chromatography with Spectroscopic Methods:

Gas chromatography is often coupled with the selective techniques of spectroscopy, thus
giving so-called hyphenated methods that provide the chemist with powerful tools for
identifying the components of complex mixtures.
Gas Chromatography/Mass Spectrometry (GC/MS):
The flow rate from capillary columns is generally low enough that the column output
can be fed directly into the ionization chamber of the mass spectrometer. For packed columns
and mega bore capillary columns however, a jet separator must be employed to remove most of
the carrier gas from the analyst.

Chapter-3 Transducers and Measuring Instruments: Current and Voltage Measurement

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Instrumentation and Measurement Mizan-Tepi University

CURRENT AND VOLTAGE MEASUREMENT:

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Schematic diagram of differential transformer transducer with servo – activated receiver

Measurement of Angular Displacement:

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2. Optical Strain Gauges:

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3. Semiconductor or peizo resistive gauges:

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VELOCITY AND ACCELERATION MEASUREMENT:

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Moving magnet type transducers

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a) Hand speed indicator

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Gravity balance method

Strain gauge torsion-meter

Advantages: This arrangement has the following advantages:

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(i) For positive pressure:

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2. No departure from linearity in the range 0.02 to 1 mm of Hg.

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4. Changes in emitted thermal radiations:

Bimetallic strip fixed at one end

(a) Helical type b) Spiral type

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Sample Injection System:

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range, it is often desirable to employ temperature programming, whereby the column

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Gas Chromatographic Columns:

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Interfacing Gas Chromatography with Spectroscopic Methods:

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