Level Measurement: By: Ramir M. Salenga Bataan Peninsula State University

LEVEL
MEASUREMENT
By: Ramir M. Salenga

Bataan Peninsula State University
Level Measurement
What?
Level measurement is the determination
of the linear vertical distance between a
reference point (usually the base of a
holding container) and the surface of a
liquid or the top of a solid.
Precise control of the level of liquid or dry

matter in a tank, reactor, or other vessel is
important in many process applications.
To provide good control, accurate
measurement is essential.
Level Measurement
Level Measurement and Control
 The following five sections are included:

 Why Measure Level?
 Level Terminology
 Device Selection
 Classifying Level Technologies
 Level Measurement Technologies
WHY?
• Inventory
• Custody transfer
• Efficiency
• Safety
• Consistent supply
Level Terminologies
Level measurements are typically

expressed in terms of feet or meters.
Level may also be given in terms of
percent full or percent of measured
span.
Level Terminologies
Level Terminologies
INTERFACE
An interface is the boundary between
two immiscible (incapable of being
mixed) fluids with different densities
(e.g., oil and water).
Level Terminologies
Level Terminologies
DENSITY
Density is the mass of a material per unit
of volume. Density is often expressed in
terms of grams per cubic centimeter
(g/cm3) or pounds per cubic foot (lb/ft3).
Specific gravity is often used to describe
the density of a material compared to the
density of water.
Level Terminologies
MASS
Mass, the amount of matter an object
contains, is often equivocated to weight.
Mass is typically expressed in terms of
kilograms, grams, tons, or pounds. Mass is
unaffected by temperature. Thus, 60 lb
(27.2 kg) of oil at 50 °F (10 °C) is still 60 lb
at 86 °F (30 °C)—however, the overall
volume of the oil may change due to
expansion.
Level Terminologies
VOLUME
Volume is the space occupied by a
quantity of material. Volume is typically
expressed in gallons, liters, cubic
centimeters, cubic feet, or barrels.
Volume is the measurement most
commonly derived from level.
Device Characteristics
A top-down measurement poses less

potential for leakage and enables level
measurement devices to be installed or
removed without emptying the tank
(e.g., dipstick).
Top-down measurements may or may not

contact the process fluid. A bottom-up
measurement typically contacts the
process fluid (e.g., weigh scale). Level
devices that use pressure transmitters are
bottom-up measurement systems.
Direct measurement indicates that level

is measured directly.
For example, when you use a dipstick to
check the oil level in your car, you are
making a direct measurement. A direct
measurement is independent of any
other process parameters.
Indirect measurement, also known

as inferred measurement,indicates
that a variable other than level is
first measured and then used to
determine a level measurement.
A continuous level-measurement system

monitors the height of product within a
range of all possible levels at all times.
Continuous measurement is used for
precise control, to maintain the level of a
material at a particular point, and to
ensure a consistent supply.
Single-point measurement indicates
whether a product is at least as high or
low as a certain point, usually the high- or
low-level limit.
In multipoint measurements, level
indication is observed at two or more
discrete points in the tank. Two single-
point measurement devices may sound
alarms or operate equipment at high and
low limits.
Single Point Level Measurement
The vibration/tuning fork is

piezoelectrically energised and
vibrates at a frequency of
approximately 1,200 Hz. When the
fork is covered in material, the
frequency shifts. The frequency
shift is detected by the internal
oscillator and converted into a
switching command.
Level measurement indicators enable an

on-site level check.
Indicators require the operator to interpret
the measurement and take the
appropriate action. Systems with level
measurement indicators are referred to
as open-loop control systems.
Automatic control systems, or closed-

loop systems, are able to electronically
control level in a vessel. A level-
measurement device, combined with a
transmitter, generates either a pneumatic
or electronic control signal that is
proportional to the level in the vessel.
In a contacting measurement, some part
of the measurement system is in direct
contact with the contents of the vessel.
Examples of contacting measurement
techniques include floats and dipsticks.
In a noncontacting measurement, no
part of the measurement system directly
contacts the contents of the vessel.
Classifying Level Technologies
Manual/mechanical
Electromechanical
Electronic contacting
Electronic noncontacting
MANUAL/MECHANICAL
Devices in the manual/mechanical category
have no electronic output. The operator
uses the device to get a visual indication
of the amount of material in the vessel.
Examples of level measurement devices in
this category are sight glasses or rod
gauging systems. These devices are low
cost but have no automation
ELECTROMECHANICAL
Devices in the electromechanical category
are mechanical assemblies with a number of
moving parts that produce an electronic
output for control. Unlike manual/mechanical
devices, electromechanical devices provide
an automated measurement that can be
read remotely.
ELECTRONIC CONTACTING
Devices in the electronic contacting category
have no moving parts. While they are not
immune to problems with coating or corrosion,
electronic contacting devices tend to be more
robust and thus require less maintenance than
electromechanical devices. Examples of level-
measurement devices in this category are
capacitance probes and pressure-based level
transmitters.
ELECTRONIC NONCONTACTING
Devices in the electronic noncontacting
category provide sophisticated level
measurement without ever touching the
product.
An example of a level-measurement device

in this category is a radar measurement
device
Level Technologies
• Pressure transmitters
• Bubbler systems
• HTG
• Radar
• Hybrid inventory systems
Level Technologies
Pressure transmitters are the most

commonly used level measurement
devices for liquids. In general, pressure
transmitters measure level by measuring
pressure at two taps, a high-pressure tap
(near the base of the tank) and a low-
pressure tap (near the top of the tank), and
manipulating the results to find level.
Level Technologies
There are two types of pressure transmitters:

Gauge pressure (GP) transmitters
Differential pressure (DP) transmitters
Level Technologies
OPEN-VESSEL LEVEL MEASUREMENT

Simple head measurement is used to
measure the level of liquids in open vessels.
Any column of liquid exerts a force at the
base of the column because of its own
weight. This force, called hydrostatic
pressure or head pressure, can be
measured in pressure units
Level Technologies
Level Technologies
CLOSED-TANK LEVEL MEASUREMENT

If a vessel is pressurized, the pressure
gauge measures not only hydrostatic
pressure resulting from the height of the
liquid column, but also the pressure above
the stored liquid (P). To solve this problem,
a DP transmitter is used.
Level Technologies
Depending on the atmosphere inside

the vessel, one of two types of
measurement methods is used:
•Dry leg system
•Wet leg system
Level Technologies
Dry Leg System

The dry leg system is typically used when the
gas above the liquid cannot condense (e.g.,
nitrogen). The dry leg system provides
compensation for the pressure in the vapor
space, whether positive (above atmospheric)
or negative (vacuum). The output of the DP
transmitter is proportional to liquid level only.
Level Technologies
Wet Leg System

In applications where the gas above the
liquid can condense (e.g., water into
steam), condensation will gradually fill
the low-pressure piping and cause
measurement error.
REMOTE SEALS
Remote seals are transmitter extensions
that enable the transmitter to avoid direct
contact with the process liquid
A bubbler system consists of three main
components:
• Dip tube (vertical pipe extending the
height of the liquid to be measured)
• Air supply and regulator
• Pressure transmitter
Hydrostatic tank gauging (HTG) uses a
multipoint system to measure mass,
volume, density, level, and temperature
for liquid inventory and process
applications
Radar (radio detection and ranging)
technologies transmit a continuous
microwave signal from a radar gauge
mounted on top of a holding vessel to the
surface of the material held inside. The
transmitted signal is reflected back to the
radar gauge, and the gauge measures the
distance (and thus level) to the surface of the
material by differentiating the received
signals from the transmitted signals.
Hybrid inventory systems combine the
advantages of an HTG system with those of
a radar system to provide a complete
system for precise measurements of level,
volume, density, and mass.
An average temperature measurement can
also be made.
Ultrasonic
 It consists of an ultrasonic
generator or oscillator
operating at a frequency of
approximately 20 KHz and
a receiver.
 The time required for the
sound waves to travel to
the liquid and back to the
receiver is carefully
measured. The time is a
measure of level.
Radiation
 A radiation level
measurement
generally consists
of a radioactive
source on one
side of the tank
and a suitable
detector on the
other side.
Capacitance
Application
 Used best to
nonconductive
liquids, since it
is best to avoid
the problems
generated by
conducting
materials like
acids.
I/P
(Current to Pressure)
Transducer

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LEVEL

By: Ramir M. Salenga

Precise control of the level of liquid or dry

 The following five sections are included:

Level measurements are typically

A top-down measurement poses less

Top-down measurements may or may not

Direct measurement indicates that level

Indirect measurement, also known

A continuous level-measurement system

The vibration/tuning fork is

Level measurement indicators enable an

Automatic control systems, or closed-

An example of a level-measurement device

Pressure transmitters are the most

There are two types of pressure transmitters:

OPEN-VESSEL LEVEL MEASUREMENT

CLOSED-TANK LEVEL MEASUREMENT

Depending on the atmosphere inside

Dry Leg System

Wet Leg System

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