This document provides information on different methods for liquid level measurement. It discusses direct methods like hook gauges, dipsticks, sight glasses, and floats. It also covers indirect methods using hydrostatic pressure like pressure gauges, air bellows, and air/liquid purge systems. Electrical methods like capacitance and radiation are also summarized. Other technologies discussed include laser, microwave, optical, ultrasonic, eddy current, and vibrating fork level sensors. The document provides details on the operation, advantages, and limitations of each level measurement technique.

1. LEVEL
MEASUREMENT
PRESNTED BY
MR. R. H. UKEY SIR
MAHARASHTRA INSTITUTE OF POLYTECHNIC, BETALA
rahulukey815@gmail.com

2. Contents:
1. Introduction
2. Selection of Level Measurement Devices
3. Methods of Liquid Level Measurement
▶ Direct Method
▶ Indirect Method
4. Direct Methods
5. Indirect Methods
▶ Hydrostatic Pressure Type
▶ Electrical Methods
6. Other Methods
▶ Laser Level Sensor
▶ Microwave Level Switch
▶ Optical Level Detector
▶ Ultrasonic Level Detector
▶ Eddy Current Level Measurement Sensor
▶ Vibrating Fork
2

3. Introduction
▶ It influences both flow and pressure rates in a running process.
▶ Incorrect or inappropriate measurements can cause levels in vessels to be excessively
higher or lower than their measured values. Low levels can cause problems and
damage equipment, while high levels can cause overflow and potentially create safety
and environmental problems.
▶ Upgraded level measurement precision makes it possible to reduce chemical-process
variability, resulting in higher product value, reduced cost, and less leftover.
3

4. Selection of Level Measurement Devices
▶ Which substances need to be measured, for example, liquid, slurry, solid, interface, granular, or
powder.
▶ Specify whether the level sensor can be inserted into the tank or kept outside.
▶ Decide upon the suitability of continuous level or point sensor for the given application.
▶ Specify whether the substance being measured is electrically conductive or not.
▶ State the type of output required i.e. analog, relay or digital display.
▶ Indicate the required temperature and pressure ranges depending upon the application.
▶ Specify the required level measurement range.
▶ Identify whether turbulence, foam or vapour arises at the surface of the liquid or not.
▶ Make out whether material under consideration coats or builds up on surfaces
4

5. Methods of Liquid Level Measurement
▶ Generally, it is measured by two distinctive methods
1) Direct and 2) Indirect
▶ Under direct measurement method comes few more subheads viz.
a) Hook – type Level indicator
b) Dipstick
c) Sight Glass technique
d) Float – type technique and
e) Displacer level indicator.
▶ Indirect method comprises
a) Hydrostatic pressure devices and
b) Electrical methods.
5

6. Direct Method: Hook – Type Level Indicator:
They are used in open tank by using a direct scale with the limitation of parallax error.
This device is constructed from corrosion resistant alloy (stainless steel) having 1/4th of an
inch diameter, it is casted into a U – shape with one longer arm than the other.
The longer arm is attached to a slider with a vernier scale which moves along the main
scale showing the reading and the shorter arm is pointed with a 60 ° taper.
The device is pushed below the liquid surface and then raised up gradually till it is about
to break through the level. The rod is them clamped and level is read out from the scale.
6

7. Direct Method: Dipstick
▶ Dipsticks are widely used for quick and easy level measurement for liquids in hard to
reach places.
▶ The design is simple. They consist of a metal strip connected to a handle. The handles
are usually plastic or metal. The metal strip has markings on it to indicate liquid level.
▶ The dipstick is inserted into the container in which the level is to be measured. Then
after a couple minutes, or enough time to allow the liquid to settle, the dipstick is
removed and the level can be read off the markings on the metal strip. If they are used
properly, they can give very accurate readings.
▶ Its main advantage is its simple design and easy use while the limiting factors are
operator’s error and time consuming process.
7

8. Direct Method: Sight glass
▶ It is a continuous level indicating device attached to a tank or container whose fluid
level needs measurement.
▶ It has a graduated toughened glass in connection with the container. As the level in the
container increases or decreases, the sight glass simply reads out the data on the scale.
▶ Principally it is used for open tanks, but in case of closed tanks or under differential
pressure situations, the upper end of the sight glass is connected to the top of the tank
to maintain the pressure difference and thereby avoid wrong reading.
▶ It is normally used within a height of 900 mm with a maximum pressure of 1000 psi.
▶ Its main advantages are direct read out and easy operation whereas the limitations are it
cannot handle extreme temperature fluctuations and pressure ranges, viscous fluids
may clog to the glass surface, overlapping gauges are needed for long span, glass body
makes it fragile.
8

9. Direct Method: Float type
▶ Here, a float rests on the fluid and follows the changes in the level.
▶ Float movement is transmitted to a suitable read out meter. In its simplest design, a
float is balanced by a counter weight which moves a pointer as it moves up and down
in accordance with the level.
▶The cable that links the counter weight and float is normally stainless steel or phosphor-
bronze.
▶ The float itself is made up of a non-corrosive material like steel. With a float of this
design, a level between ½ foot and 60 feet can be measured.
▶ The float material can be attached to magnetic or hydraulic transmission system where
a bellow arrangement monitors the pressure difference as the float moves and display it
on a pointer scale. Range here goes up to 250 feet.
▶ Main advantages of float are low cost, reliable design, high temperature range and long
life because of choice of corrosion resistant element.
▶ Limiting factors are pressure dependency and tank geometry and only be used in non-
freezing environment.
9

10. Direct Method: Displacer Indicator
▶ It works on Archimedes’ principle where the body immersed in fluid partially or
wholly is pushed up by a force equal to the weight of the fluid displaced.
▶ With the cross sectional area of displacer and density of fluid remaining constant, unit
changes in level will cause unit changes in displacer apparent weight, detecting which
gives a measurement of the level.
▶ The displacer here is always heavier than the fluid under measurement and it is
suspended from a spring scale. The scale reads full weight when the fluid is below the
displacer level.
▶ Weight of displacer decreases because of buoyant force from the fluid which maintains
a linear proportionality between spring tension and level.
▶ Factors those need to be considered are corrosion coefficient, pressure and temperature
ranges and frictionless environment. Main drawback of this set up is it is limited to
only open tanks.
10

11. Direct Method: Displacer Indicator
▶ Operation is based upon simple buoyancy, whereby a spring is loaded with weighted displacers,
which are heavier than the liquid. Immersion of the displacers in the liquid results in buoyancy
force changing to net force acting on the spring. The spring compresses as the buoyancy force
increases.
▶ Based on output scales and designs, displacer detectors come in the following viz. (i)
Magnetically coupled switch type; (ii) Torque tube type; (iii) Diaphragm and force bar type (iv)
Spring balanced type (v) Flexible disc type and (vi) Flexible shaft type.
▶ Their merits are high accuracy, reliability in clean fluids and demerits are limitation of range
with maximum 3 metres, higher cost and external heat supply to avoid freezing.
11

12. Indirect Measurement Methods
▶ The indirect methods are further classified into the following groups.
(i) Hydrostatic and (ii) Electrical
The hydrostatic methods are categorized into
a. Pressure gauge method
b. Air bellow
c. Air purge system
d. Liquid Purge system
The electrical methods have two sub classes
a. Capacitance method
b. Radiation method
12

13. Indirect – Hydrostatic – Pressure Gauge
▶ Hydrostatic pressure of any liquid in open tank is given by this equation
𝑃 = 𝜌 ∗ ℎ ∗ 𝑆𝑔 → ℎ =
𝑃
𝜌 ∗ 𝑆𝑔
▶ For closed tanks, it reads as
𝑃 = 𝜌 ∗ ℎ ∗ 𝑆𝑔 + 𝑒𝑥𝑡𝑒𝑟𝑛𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑜𝑛 𝑙𝑖𝑞𝑢𝑖𝑑
▶ Apressure gauge is attached to the lowest level of tank.
▶ Another pressure gauge is fitted at a different level, known as reference level and the static pressure
measured is the measure of the height.
▶ The location of the pressure gauge should be chosen carefully as difference in elevation will affect the read
out.
▶ Demerit: The instrument has to be mounted at minimum level in the tank rendering it inconvenient as tank
may at times be placed at an elevated height than the control room.
13

14. Indirect – Hydrostatic – Air Bellow
▶ Where positioning the indicator at the datum point is not convenient, air bellow method is
employed.
▶ It has a bellow element connected to the pressure indicator. Air is sealed in the cavity in the
bellow and pressure indicator arrangement. With empty tank, air is uncompressed citing zero
reading.
▶ As level rises, the liquid head flexes the bellow compressing the air above bellow creating a
pressure marked on the pressure indicator.
14

15. Indirect – Hydrostatic – Air Purge
▶ Air purge or bubbler tube is suitable for all liquids.
▶ It compromises a hollow tube inserted into the tank. There are two connections, one to the air regulator and
second to the pressure gauge.Abubbler is connected to the air supply for visual check.
▶ When there is no liquid in the tank, the air passes out without any pressure into the atmosphere and meter
reads zero, but as level increases, the air suffers back pressure proportional to the depth and this is recorded
into recorder.
▶ Normally fluid for purge chosen is air, but if it reacts then other fluids like carbon or nitrogen are also
chosen.
▶ Its main advantages are that it is suitable even for corrosive liquids and the pressure gauge may be placed at
a far distance from the tank.
15

16. Indirect – Hydrostatic – Liquid Purge
▶ When air is not suitable for purging as it may interfere with the liquid crystallization,
liquids like water or light mineral oil may be used.
▶ The design and principle remain the same but purging nature should be such that its
introduction should not hamper the process fluid, it should be free flowing ad non-
vaporizing at process temperature.
▶ It may be soluble or insoluble in the process fluid.
▶ Its figure of merits are firstly it is suitable for measurement of corrosive or abrasive
liquids and secondly the device positioning may be above or below the tank level and
can also be placed at a maximum distance of 500 feet.
16

17. Indirect – Electrical - Capacitance
▶ The device comprises an insulated capacitance probe firmly attached parallel to the
metal wall of the container.
▶ With liquid being non – inductive, the probe and the wall act as two parallel plates with
the liquid acting as the dielectric.
▶ With conductive liquid, the probe and the liquid form the capacitance plates and the
insulation acts as the dielectric.
▶ By measuring the capacitance value calibrated in terms of level, the required
measurement can be performed.
▶ With increase of liquid inside the container, the capacitance value increases and vice
versa.
▶ Capacitance level indicators find applications in measuring granular solid levels e.g.
power or grain levels in hoppers or silos.
17

18. Indirect – Electrical - Capacitance
𝐶 = 𝐾
▶ Capacitance is developed between no parallel plate capacitor given by the equation
𝐴
𝐷
▶ C = capacitance in Farad; K = Dielectric constant; A = Area of plate; D = Distance
between two plates in metres.
▶ With A and D constant, C is directly proportional to the dielectric constant and this
principle is utilized in measuring the level.
18

19. Indirect – Electrical - Capacitance
Major merits
▶ High sensitivity
▶ No moving parts
▶ Compatible with vast range of liquids, solids and granular materials.
▶ Relatively low cost
▶ Continuous read out
▶ Works good in slurry medium and corrosion resistant probe materials are available.
Figures of demerit are
▶ Result may get affected by dirt or contamination
▶ It is sensitive to temperature
▶ Selection of correct electrode is important.
▶ Fluid needs to have proper dielectric qualities.
▶ Rely on uniform contact being made.
19

20. Indirect – Electrical - Radiation
▶ It is a no contact method.
▶ The method indulges gamma rays source and detector on either side of the tank whose
level is under measurement.
▶ A thin band gamma ray is released which after penetrating the tank wall loses its
intensity hugely.
▶ At the detector end, the reception is inversely proportional to the tank wall thickness
and intermediate medium.
▶ Tank wall thickness remaining constant all along, the dip in received signal strength is
due to the presence of liquid medium intermediately and the amount of received
radiation is therefore inversely proportional to the amount of fluid within the tank.
▶ The radiation signal calibrated in terms of level provides the required measurement
results.
20

21. Indirect – Electrical - Radiation
The merits of this method are
▶ No physical contact with the measured item.
▶ Suitable for molten metals as well as fluids (corrosive, viscous, adherent and
abrasive)
▶ Can operate even at very high temperature
▶ With no moving parts, provides good accuracy and response.
The demerits of this method are
▶ Change of density has effects in the output.
▶ Relatively heavy radiation source holders
▶ Relatively high cost.
21

22. Other Methods of Level Measurement
▶ There exists other methods of level measurement viz.
1. Laser Based Method
2. Microwave Based Method
3. Optical Based Method
4. Ultrasonic Based Method
5. Eddy Current Based Method
6. Vibrating Fork Method
22

23. Laser Based Method
▶ They are dependent on accurate detection of time as taken by a laser source for transmission and
reflection back from the material surface.
▶ The velocity of light for any medium is given by 𝐶 = 𝐶0
𝜇
▶ The refractive index is actually a function of temperature and pressure and it varies as shown
below
0
𝜇 = 𝜇 − 1
𝑃 𝑇0
𝑃0 𝑇
+ 1
▶ This is non-contact method suitable for both continuous and point measurement and for most
kind of liquids.
23

24. Laser Based Method
▶ A laser beam is transmitted towards the material whose level is to be detected and the
time required for the time for to and from motion is measured.
▶ The entire signal does not return as a part of it is absorbed by the material.
▶ There are two types available in laser based method, 1) Triangular measurement
technique and 2) time of reflection measurement technique.
▶ In the first technique, a laser source is emitted towards the material and the reflection is
captured by a CCD (charged coupled device) placed at an angle to the laser source.
The two reflections received at the CCD are one from the top level of the material
under test and second from the base of the tank provided the material under test is a
liquid. The drawback of this approach is that with increasing distance the angle
diminishes thereby reducing the accuracy of measurement which is why this technique
is rarely used.
▶ The second technique is a simpler mechanism where a strong and highly focused laser
source is emitted towards the material and a return signal is captured by a receiver
situated just by the laser source. As this technique generates no angle of reflection, it is
suitable for long distance measurement.
24

25. Laser Based Method
Merits of Laser based method:
▶ Non-contact type
▶ Suitable for even vacuum services unlike ultrasonic detector.
▶ Less interference effects from welding joints and other parts as laser beam runs
parallel.
Demerits of Laser based method:
▶ Unperturbed transparent liquid poses challenge as bottom of tank gets detected at
times other than liquid surface.
▶ Shiny surfaces do not offer good results.
▶ Dusty, smoggy environments are not suitable for this method.
25

26. Microwave Based Method
▶ In this method of level detection, microwaves in the X – band (around 10 KHz) or in
the K – band (24 KHz) are used.
▶ Calculation of the wavelength is done by division of wavelength in vacuum with the
square root of window material’s dielectric constant.
▶ Though impervious to metal walls, microwave can pass through ceramic and glass
windows.
▶ Attenuation is kept to minimal with low dielectric window materials and their
thickness being close to even multiple of half wavelengths.
▶ Top mounted level detectors find application in continuous measurement whereas side
mounted detectors are used for point measurements with hard to handle solid, liquid –
solid, liquid – liquid interfaces.
▶ There exists two types of microwave level detectors viz. 1) Beam – breaker switches
and 2) Reflection level switches.
26

27. Microwave Based Method
▶ Beam – breaker switches: Similar to laser based method, a strong microwave signal is sent
towards te tank and with air or vapour, received signal strength is high whereas with
material intervention, the signal loses its strength due to reflection and absorption by the
material.
▶ This kind of detectors is useful for large and abrasive materials like coal, wood chips and
very light materials like sawdust and powdered materials.
▶ Separation between the two windows can be up to 30 metres which is more than ultrasonic
method. In this method, very small antennas do the job with wide angle ranges of around 25
° and 50 ° for K – band and X – band respectively.
▶ Reflection level switch: Changes in amplitude and/or phase of reflected signal is the
determining factor for material presence, reflection being proportional to the dielectric
constant of material next to process window.
▶ Similar to the previous method, microwave signal is launched into the material through the
window and received at the other window where detectors are placed. In a bridge circuit,
the received beam is compared against a reference beam.
▶ This method is more suited for liquid – liquid interface and liquid – solid interface. It is
useful for viscous, toxic or hazardous liquids. Higher the dielectric constant more is the
reflection percentage of the signal.
27

28. Microwave Based Method
Merits of Microwave detectors:
▶ With very low intensity microwave radiation, health, safety and product contamination risks are minimal.
▶ Dust, mist and foam have negligible effect on measurement accuracy.
▶ With thick windows, abrasion of detectors can be easily avoided.
Demerits of Microwave detectors;
▶ Very expensive
▶ Requirement of multiple device installation as detectors along the windows.
28

29. Optical Based Method
▶ Here the reflection property of light is exploited for measurement of level. In case of optical
fibre level detectors, refraction property of light comes into play. Viscous liquids are not
measured by this method neither powdered solid as dirt and dust may rest on the indicators.
▶ This type of device can be used for point measurement as well as continuous measurement.
▶ The temperature ranges between – 40 ° C and 70 ° C.
▶ Visible or infrared light sources are used to detect the level of solids or liquids. Light
sensors are set in the same holder of the source.
▶ A light signal is set towards the liquid or material whose level is to be measured and the
reflection is receiving the light sensors which generally is a quartz crystal or prism. This
measurement is calibrated in terms of level measurement units. In case of fibre optics, a
light pulse is made to travel through the fibre.
▶ With no liquid in the cable, the intensity of light remains the same and in case of liquid
presence, the intensity diminishes with the liquid level as refractive index due to the liquid
presence.
29

30. Optical Based Method
Merits of optical level detectors:
▶ Non-contact method effective for corrosive and abrasive liquids.
▶ Provides high precision and accuracy.
▶ Low cost instrument
▶ Small device set-up.
Demerits of optical level detectors:
▶ Process is adversely affected by changes in reflectivity of medium.
▶ May encounter problems with highly opaque or reflective materials.
30

31. Ultrasonic Based Method
▶ Ultrasonic level measurement method utilizes the propagation time of sound waves in
the ultrasonic region.
▶ The operational frequency lies somewhere between 30 and 40 KHz.
▶ This method is used for both continuous as well as point measurement.
▶ It operates either by attenuation of a vibrating diaphragm face or by absorbing acoustic
energy while traveling towards receiver from the source.
▶ An ultrasonic wave is generated and the time required for the echo produced to return
is measured.
31

32. Ultrasonic Based Method
▶ There are two orientation of the ultrasonic transmitter, one in which it is placed at the
top of the tank where the speed of sound is somewhere around 330 meter/second and
here in this case, the depth of the vapour space above the liquid is measured.
▶ With the transmitter set the bottom of the tank in the second orientation, the depth of
the liquid in the container is measured.
▶ Temperature compensation is provided as sound velocity changes proportionally with
the square root of temperature.
▶ A piezoelectric crystal is used to generate the required mechanical wave from applied
electrical signal. The sound pulse is released towards the material and is received back
by the transducer and the time is recorded for this transition.
▶ Loose dirt particles are poor reflectors as they tend to absorb the sonic pulse whereas
larger particles are better reflectors.
▶ Smooth surfaces are preferred over rough surface as they offer better reflection
compared to diffused reflection produced by the latter surface.
32

33. Ultrasonic Based Method
Merits of ultrasonic method:
▶ Non-contact type with no moving parts
▶ Reliability is not dependent on the composition, density, moisture content, dielectric constant or
electrical conductivity.
▶ Self-cleaning
▶ Low maintenance
▶ Relatively expensive.
▶ Pressure should not exceed 115 psi.
Demerits of ultrasonic method:
▶ Subject to multiple interferences.
▶ Temperature compensation is required.
▶ Dirt and rough reflecting surface may affect the accuracy.
33

34. Eddy Current Based Method
▶ It employs eddy current principle for level measurement where the set up comprises
three coils – one primary and two secondary coils with secondary coils being placed on
either side of the primary.
▶ With the supply of a huge frequency to the primary coil, a high – frequency magnetic
field is generated which results in generation of eddy current in the molten liquid metal
inducing voltages in the secondary coils.
▶ Sensors attached to the secondary coils detect the voltage difference corresponding to
the changing distance between liquid surface and the sensors which are calibrated in
level measurement scale.
▶ The use of two secondary coils is to reduce the effect of the tank walls. Air cooling
arrangement is provide to withstand extreme temperature conditions.
▶ The sensor output is connected to microprocessor based amplifiers which provide
current output in the 4 – 20 mA range with linear proportionality with the level
measurement.
34

35. Eddy Current Based Method
▶ The relationship between voltage output and level is expressed as given below
𝑉1− 𝑉2 = 𝐸𝑜𝑢𝑡 ∗ 𝑓ℎ
−𝐸𝑖𝑛 + 𝐸𝑓 ∗ 𝐺1 = 𝐸𝑜𝑢𝑡
𝐸𝑓 = 𝐾 ∗ 𝐸𝑜𝑢𝑡 − 𝐺2 ∗ (𝑉1− 𝑉2)
▶ From the above three equations, output voltage is calculated as beneath
𝐸𝑜𝑢𝑡 =
−𝐺1 ∗ 𝐸𝑖𝑛
1 − 𝐺1(𝐾 + 𝐺2 ∗ 𝑓ℎ)
Eout = feedback amplifier output voltage; Ein = feedback amplifier input voltage
G1 = feedback amplifier open loop gain; G2 = differential amplifier gain
K = positive feedback rate; V1 = induced voltage in secondary one coil
V2 = induced voltage in secondary two coil; fh = differential voltage output function; h =
height of molten metal in the container.
35

36. Eddy Current Based Method
Merits of the method:
▶ Non-contact type with no moving parts
▶ Light and small sensors with wide range
▶ Differential method minimizing external noise influences.
▶ Good stability with nominal thermal drift.
▶ Best suited for molten metal level measurement.
36

37. Vibrating Fork Method
▶ Vibrating fork method is used for both level indication and control.
▶ Here in this method, a tuning fork-like rod vibrates.
▶ With material coming in contact with the fork, the vibrations are damped.
▶ The stopping of vibrations specifies that material has reached the level of the gauge.
▶ Normally, this operates an alarm or switch that will halt material flow. When the fork is
relinquished of material, it will throb again.
37

38. Vibrating Fork Method
▶ These are generally used for measuring levels of powdery or granular materials.
▶ Merits of Vibrating Method:
I. Useful in slurries and particles laden solutions.
II. Corrosion resistant
III. Can be used in any direction.
▶ Demerit: Requirement of constant power supply
38

39. References:
▶ Chapter 10: Level Measurement, “Industrial Instrumentation and
Control” by S K Singh. Tata McGraw Hill, 3rd Edition. 2009, New
Delhi. ISBN-13: 978-0-07-026222-5.
▶ Chapter 6: Level Measurement, “Principles of Industrial
Instrumentation”, 2nd Edition. D. Patranabis Tata McGaw-Hill, New
Delhi, 2004. ISBN: 0-07-462334-6.
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THANK
YOU….!