This articles briefs the basic level measurement instruments in Industry.

1. Electrical & Electronic Measurement
Part – VII
Level Measurement
ER. FARUK BIN POYEN
faruk.poyen@gmail.com
AEIE, UIT, BU

2. Contents:
 Level Measurement Techniques
 Displacer Indicator Method
 Pressure Gauge Method
 Air Purge Method
 Capacitance Method
 Radiation Method
 Laser Based Method
 Eddy Current Method
2

3. Level Measurement:
 It influences both flow and pressure rates in a running process.
3

4. Level Measurement Techniques:
 Generally, it is measured by two distinctive methods
1) Direct and 2) Indirect
 Direct Methods are
a) Hook-type; b) Dipstick; c) Sight Glass; d) Float-type; e) Displacer Indicator;
 Indirect Methods are
a) Hydrostatic Pressure devices b) Electrical Methods
4

5. 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.
5

6. Direct Method - Displacer Indicator:
 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.
 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.
 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.
6

7. Direct Method - Displacer Indicator:
 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.
7

8. Indirect Methods:
 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
8

9. Indirect – Hydrostatic – Pressure Gauge
 Hydrostatic pressure of any liquid in open tank is given by this equation
𝑃 = 𝜌 ∗ ℎ ∗ 𝑆 𝑔 → ℎ =
𝑃
𝜌 ∗ 𝑆 𝑔
 For closed tanks, it reads as
𝑃 = 𝜌 ∗ ℎ ∗ 𝑆 𝑔 + 𝑒𝑥𝑡𝑒𝑟𝑛𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑜𝑛 𝑙𝑖𝑞𝑢𝑖𝑑
 A pressure 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.
9

10. Indirect – Hydrostatic – Pressure Gauge
 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.
10

11. Indirect – Hydrostatic – Air Purge
 Air purge or bubbler tube is suitable for all liquids.
 It comprises a hollow tube inserted into the tank. There are two connections, one to the air
regulator and second to the pressure gauge. A bubbler 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 oxide or
nitrogen are also chosen.
11

12. Indirect – Hydrostatic – Air Purge
 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.
12

13. 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.
13

14. Indirect – Electrical - Capacitance
 Capacitance is developed between no parallel plate capacitor given by the equation
𝐶 = 𝐾
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.
14

15. Indirect – Electrical - Capacitance
 Major merits
1. High sensitivity
2. No moving parts
3. Compatible with vast range of liquids, solids and granular materials.
4. Relatively low cost
5. Continuous read out
6. Works good in slurry medium and corrosion resistant probe materials are available.
 Figures of demerit are
1. Result may get affected by dirt or contamination
2. It is sensitive to temperature
3. Fluid needs to have proper dielectric qualities.
4. Rely on uniform contact being made.
15

16. 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.
16

17. 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.
17

18. Other Methods of Level Indication:
 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
18

19. Laser Method:
 A laser source is used for transmission and reflection
back from the material surface, and ‘to and fro’ time is
measured.
 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
𝜇 = 𝜇0 − 1
𝑃
𝑃0
𝑇0
𝑇
+ 1
 This is non-contact method suitable for both
continuous and point measurement and for most kind
of liquids.
19

20. Laser Method:
 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.
20

21. Laser Method:
 Triangular measurement technique:
𝑑 = 𝑓
𝑥
𝑥′
; f= focal length.
 Time of reflection measurement Technique
𝑑 = 𝑐𝑡/2𝑛
21

22. Laser Method:
 Merits of Laser based method:
1. Non-contact type
2. Suitable for even vacuum services unlike ultrasonic detector.
3. Less interference effects from welding joints and other parts as laser beam runs
parallel.
 Demerits of Laser based method:
1. Unperturbed transparent liquid poses challenge as bottom of tank gets detected at
times other than liquid surface.
2. Shiny surfaces do not offer good results.
3. Dusty, smoggy environments are not suitable for this method.
22

23. Eddy Current Method:
 It employs eddy current principle 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 resulting in generation of eddy current in the molten liquid metal
inducing voltages in the secondary coils.
 Sensors calibrated in level measurement scale, are attached to the secondary coils to
detect the voltage difference corresponding to the changing distance between liquid
surfaces.
 The use of two secondary coils is to reduce the effect of the tank walls. Air cooling
arrangement is provided 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.
23

24. Eddy Current Method:
 The relationship between voltage output and level is expressed as given below
𝑉1 − 𝑉2 = 𝐸 𝑜𝑢𝑡 ∗ 𝑓ℎ
−𝐸𝑖𝑛 + 𝐸𝑓 ∗ 𝐺1 = 𝐸 𝑜𝑢𝑡
𝐸𝑓 = 𝐾 ∗ 𝐸 𝑜𝑢𝑡 − 𝐺2 ∗ (𝑉1 − 𝑉2)
 From the above three equations, output voltage is calculated as beneath
𝐸 𝑜𝑢𝑡 =
−𝐺1 ∗ 𝐸𝑖𝑛
1 − 𝐺1(𝐾 + 𝐺2 ∗ 𝑓ℎ)
𝐸 𝑜𝑢𝑡 = 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 coil 1;
𝑉2 = induced voltage in secondary coil 2; 𝑓ℎ = differential voltage output function;
h = height of molten metal in the container.
24

25. Eddy Current Method:
 Merits of the method:
1. Non-contact type with no moving parts
2. Light and small sensors with wide range
3. Good stability with nominal thermal drift.
4. Best suited for molten metal.
5. Minimum external noise influences.
25