Level measurement theories of operation rev1

Defination
 Level is defined as the filling height of a liquid or bulk
material.
 The liquid level is expressed in terms of length of the liquid
column or in terms of the pressure the column exerts over
a datum level.

Methods of level Measurement
Two methods are generally used in industries for
measuring liquid level.
◦ Direct Method / Contact method
◦ Indirect Method / Non contact method

Direct Method
This is the simplest method of measuring liquid level where
the level is measured directly by means of the following
liquid level indicators
◦ Sight Glass Level Indicator
◦ Float Type Level Indicator

Indirect Measurement Methods
The indirect methods are further classified in to the following groups.
(i)Hydrostatic and (ii)Electrical
The hydrostatic methods are categorized in to
 Pressure gauge method
 Purge system
The electrical methods have two subclasses
 Capacitance method
 Radiation method

Sight Glass
A sight glass (also called as gauge glass) is used for the continuous indication of
liquid level within a tank or vessel
◦ Construction:- sight glass instrument consists of a graduated tube of toughened glass
which is connected to the interior of the tank at the bottom in which the liquid level is
required
◦ Working:- as the level of the liquid in the tank rises and falls, the level in the sight
glass also rises and falls accordingly. Thus, by measuring the level in the sight glass
the level of liquid in the tank is measured.

Main title
Write title here
Write Discussion here
Basics of Instrumentation and Control
Course Contents
Course Topics
Introduction to measurements and control concepts
Pressure measurement
Flow measurement
Level measurement
Temperature measurement
Control Valves
Process Control Loops
Control Systems (PLC, DCS, SCADA)
Level
Measurement
The only way to ensure proper two-part liquid interface level indication in a sightglass
is to keep both ports (nozzles) submerged
Interface level gauges
If a lighter (less dense) liquid exists above
a heavier (denser) liquid in the process vessel,
the level gauge may not show the proper interface

Float type gauge
 In this the float rests on the surface of liquid and follows the
changing level of liquid. The movement of the float is transmitted to
a pointer through a suitable mechanism which indicates the level on
a calibrated scale.
 Construction:- It consists of a float made of stainless steel or copper
or phosphor bronze with nickel plating to avoid rusting which rests
over the surface of the liquid. The float movement is transmitted to
the pointer by a stainless steel or phosphor bronze flexible cable
wound around a pulley, and the pointer indicates liquid level.
 Working:- When the liquid level rises or falls a buoyant force equal to
weight of the displaced liquid is available. It pushes the float up or
down. The float movement is transmitted to a pointer through a
suitable mechanism It indicates the level on a calibrated scale

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Level Measurement
Magnetic type Sight gauges have a float inside a nonmagnetic chamber. The float
contains a magnet, which rotates wafers over as the surface level increases or
decreases. The rotating wafers present the opposite face, which has a different
colour. It is more suitable for severe operating conditions where liquids are under
high pressure or contaminated.
Magnetic type

Hydrostatic Head Level Measurement
Hydrostatic head level measurement is based on
the principle that the pressure at the bottom of a
column of liquid is directly related to the height of
the liquid (h, in inches or millimeters) and
the relative density (RD) of that liquid. If the
pressure exerted by the head or height of liquid is
measured then the level can be
determined. Hydrostatic head is an indirect
method of level measurement as the level
is inferred by the pressure that is measured.
The relationship between the height of liquid or
level and pressure can be defined by the formula:
P=SGp x ρp xh
Introduction to Hydrostatic Head Level
Measurement

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Measuring Liquids Using Hydrostatic
Head There are two methods for
measuring liquids using hydrostatic
head, open tank and closed
tank. In both cases, the most common
instrument used to measure the
pressure is the differential pressure
cell or transmitter (D/P cell). A D/P cell
is used instead of a simple pressure
transmitter or pressure gauge to
eliminate any error due to changes in
atmospheric pressure. Any changes in
atmospheric pressure will be applied to
both sides of the D/P cell and will
cancel each other.
Measuring Liquids Using Hydrostatic Head

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Open Tank Measurement

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Elevation and Suppression Adjustments The
calculations in the examples are only valid if the
differential pressure transmitter (D/P cell) is
mounted at the datum point. In practice, the
D/P cell is quite often either mounted above or
below the datum point, in which case the
following adjustments must be made:
Suppression – is the adjustment made when the
D/P cell (or any measuring device) is mounted
below the datum line. Zero suppression
indicates you need to lower the nominal zero
point.
Elevation – is the adjustment made when the
D/P cell is mounted above the datum line. Zero
elevation indicates you need to raise the
nominal zero point.
Elevation and Suppression Adjustments

Transmitter Elevation Calculation
Example
Pressure@DP = (H × SpG)High Side – (H ×
SpG)Low Side
where H = Height
2. Calculate the LRV when the tank is
empty.
LRV = (120 x 1.0)High Side – (0 x 1.0)Low
Side
LRV = +120 inH2O
3. Calculate the URV.
URV = LRV + Span
URV = +120 + 100
URV = 220 inH2O
Therefore, transmitter range should be
120 to 220 inH2O

Transmitter Suppression Calculation Example
1. Calculate the differential pressure as
follows.
Pressure@DP = (H × SpG)High Side – (H ×
SpG)Low Side
where H = Height
2. Calculate the LRV when the tank is
empty.
LRV = (0 x 1.0)High Side – (100 x 1.0)Low Side
LRV = -100 inH2O
3. Calculate the URV.
URV = LRV + Span
URV = -100 + 100
URV = 0 inH2O
Therefore, transmitter range should be -
100 to 0 inH2O

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Closed Tank Measurement
Most level applications in industry involve closed tanks under some pressure. The
differential pressure transmitter (D/P cell) is also used for closed tank applications.
Note: Ps = Pstatic, the pressure inside the tank.
In order to accurately measure the head pressure of the liquid alone, closed tank
applications must compensate for the static pressure of the vapor above the liquid.
Both pressure input taps for the transmitter are connected to the tank. As in open tank
measurement, the high pressure (PH) side of the transmitter is connected to the base, or
0% datum of the tank. The low pressure (PL) side of the transmitter is connected to the
top of the tank through a pipe referred to as a "reference leg." The reference leg must be
either completely dry (empty) or completely filled with liquid.
Closed Tank Measurement

A “dry leg” on the low side refers to the low side piping
being completely void of liquids. This application only
works with non-condensing liquids in the tank. If used
with condensing liquids, some liquid could build up on
the low or dry side and would then create an error
since this height of liquid would induce an
uncompensated pressure on the low side.
The equation for calculating the pressure exerted by
the liquid in a closed tank with a dry leg is:
P = RDp x h but P = PH - PL and PH = P + PS and PL =PS
Dry Leg Closed Tank Measurement

Wet Leg Closed Tank Measurement If condensing
liquids or steam are to be measured, then the
installation must contain a “wet leg" on the low side.
It is called a wet leg since the low side piping is
always completely filled with the condensate liquid of
the tank or some other inert liquid such as glycol. In
this manner, a constant pressure is generated on the
low side and can be accounted for when calibrating
the instrument. Tocalculate the differential pressure
(PH – PL) at the D/P cell, two calculations are
required; one for the pressure resulting from the
process liquid in the tank minus that of the pressure
resulting from the liquid in the wet leg.
Wet Leg Closed Tank Measurement

Condensate PotTo ensure the wet leg is always
filled, a condensate pot is installed at the top of
the leg to provide adequate liquid. The condensate
pot is located at the top of the tank close to where
the wet leg is connected to the tank and is filled
with sufficient volume of liquid to completely fill
the wet leg piping. It can be filled with the process
fluid, but more often is filled with an inert liquid
such as glycol. In any case, it must be a liquid that
will not vaporize.
Condensate
Pot

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Used If the process liquid contains suspended solids or is chemically corrosive or
radioactive.
It is desirable to prevent it from coming into direct contact with the level transmitter.
A bubbler tube is immersed to the bottom of the vessel in which the liquid level is to
be measured.
A gas (called purge gas) is allowed to pass through the bubbler tube.
Consider that the tank is empty, so, the gas will escape freely at the end of the tube
and therefore the gas pressure inside the bubbler tube will be at atmospheric
pressure.
As the liquid level inside the tank increases, pressure exerted by the liquid at the base
of the tank (and at the opening of the bubbler tube) increases.
As a result, the gas pressure in the bubbler tube will continue to increase until it just
balances the pressure of the liquid & any excess supply pressure will escape as
bubbles through the liquid.
The bubbler tube is connected to the high-pressure side of the transmitter, while the
low pressure side is vented to atmosphere.
Bubbler System

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Bubbler system Considerations
The purge gas supply must be reliable,
if the flow stops for any reason, the
level measurement will cease to be
accurate.
The purge gas supply pressure must
exceed the hydrostatic pressure at all
times, or else the level measurement
range will fall below the actual liquid
level. (preferred to be better by 10 psi)
The purge gas must not adversely react
with the process.
The purge gas must not contaminate
the process.
The purge gas must be reasonable in
cost, since it will be continuously
consumed over time.
Bubbler System
Principle Video 1,
Video

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Displacer
It is based on Archimedes' principle of Buoyancy
Displacer level instruments exploit Archimedes’ Principle to detect liquid level by
continuously measuring the weight of the displacer immersed in the process liquid.
As liquid level increases, the displacer experiences a greater buoyant force, making it
appear lighter to the sensing instrument, which interprets the loss of weight as an
increase in level and transmits a proportional output signal.
Displacer

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Displacer
Used in clean liquids of constant density
Apparent weight causes an angular
displacement of the torque tube (a torsion
spring, a frictionless pressure seal).
This angular displacement is linearly
proportional to the displacer's weight
Standard displacer volume is 100 cubic
inches and the most commonly used
lengths are 14, 32, 48, and 60 in.
the buoyant force can also be detected by
other force sensors, including springs and
force-balance instruments
Torque
Tube

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Displacer
Dry Calibration

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Displacer
Displacement interface level measurement
 Suppose we have a displacer instrument measuring the interface level
between two liquids having specific gravities of 0.850 and 1.10, with a
displacer length of 30 inches and a displacer diameter of 2.75 inches (radius =
1.375 inches).
 Fbuoyant = γ1V1 + γ2V2
Fbuoyant = γ1πr2l1 +
γ2πr2l2 Fbuoyant (LRV) =
πr2γ2L Fbuoyant (URV) =
πr2 γ1 L

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Displacer
Calibration example

The operation is based upon the familiar equation of a parallel plate capacitor given by
C = K A/D
Where , C = Capacitance , in farad K = Dielectric Constant
A = Area of plate, in m²
D = Distance between two plates, in m
if A and D are constant, then the capacitance is directly proportional to the dielectric
constant, and this principle is used in the capacitance level indicators.
Principle of operation

Ultrasonic Level
• Ultrasonic waves detect an object in the same way as Radar does it. Ultrasonic uses the
sound waves, and Radar uses radio waves. When ultrasonic pulse signal is targeted
towards an object, it is reflected by the object and echo returns to the sender. The time
travelled by the ultrasonic pulse is calculated, and the distance of the object is found.
• Bats use well known method to measure the distance while travelling.
• Ultrasonic level measurement principle is also used to find out fish positions in ocean,
locate submarines below water level, also the position of a scuba diver in sea.
• An ultrasonic level transmitter is fixed at the top of a tank half filled with liquid.
• The reference level for all measurements is the bottom of the tank.
• Level to be detected is marked as “C”, and “B” is the distance of the ultrasonic sensor
from the liquid level.
• Ultrasonic pulse signals are transmitted from the transmitter, and it is reflected back to
the sensor.
• Travel time of the ultrasonic pulse from sensor to target and back is calculated. Level “C”
can be found by multiplying half of this time with the speed of sound in air.
• The measuring unit final result can be centimeters, feet, inches etc.
Level = Speed of sound in air x Time delay / 2

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Nuclear
Nuclear radiation systems have the ability to “see” through tank walls, and thus they
can be mounted on the outside of process equipment.
Suitable for liquid or solid material detection.
Composed of a radioactive source material and a radiation detector, the two are
mounted across the diameter of a storage vessel for either solid or liquid material.
The product to be measured is attenuating the radiation coming from the radioactive
source and according to the height of the product in the vessel, more or less of the
original radiation is reaching the detector.
This measuring signal is then transferred to an
output signal which directly correlates to the
actual Level of the product.
Nuclear level instrument

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Nucleur
Two typical nuclear level instruments:
Using a single low-level gamma-ray source on one side of the process vessel and a
radiation detector on the other side of the tank.
Using several gamma sources at different heights on the tank.
Nuclear level instrument

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Radar
The fundamental difference between a radar instrument and an ultrasonic
instrument is the type of wave used: radio waves instead of sound waves. Radio
waves are electromagnetic in nature (comprised of alternating electric and magnetic
fields), and very high frequency (in the microwave frequency range – GHz).
Sound waves are mechanical vibrations (transmitted from molecule to molecule in a
fluid or solid substance) and of much lower frequency than radio waves.
Radar level measurement

Main title
Write title here
Course Contents
Course Topics
Flow measurement
Level measurement
Control Valves
Radar
Example for radar level
transmitters

Electromechanical level Measurement,
Silo pilot
 A sensing weight is let down on a measuring tape via a counter wheel.
 Tensile force of the weight is reduced as it hits the product surface.
 This is recognized, the direction of rotation of the motor reversed and the tape rewound.
 As the sensing weight moves downwards, the revolutions of the wheel are counted using a
non-contact method.
 Every count pulse corresponds to a defined length. The level is obtained by subtracting this
length from the overall length (tank height).
 Old seafarers used a weight on a rope to test the depth to the bottom of the sea.
 In industrial level measurement, the basic idea of sounding is still utilized in the
electromechanical level system.
 Where other involving bulk measurements.
 Measurement methods are limited, applications solids predominantly use electromechanical
level

Level measurement theories of operation rev1

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Level measurement theories of operation rev1

Similar to Level measurement theories of operation rev1 (20)

More from Mostafa Ragab

More from Mostafa Ragab (6)

Recently uploaded

Recently uploaded (20)

Level measurement theories of operation rev1