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Saturday, September 17, 2016

Nucleonic devices : Level

The phenomenon of reducing the intensity of gamma radiation when passing through liquids or solids is used in nucleonic devices for measurement of level of liquids and solids. Fig. 5.16 presents radiation-type level detector.

 Figure 5.16. Nucleonic level system.

A small quantity of radioactive substance (Cobalt 60, Radium 226, etc.) is placed in a source unit 1. This unit is attached to the wall of a tank 2 filled with a substance 3 which level is to be measured. A radiation detector 4, usually Geiger-Mueller tubes, is fixed to the wall of the tank on the opposite side of the tank. The intensity of gamma-radiation detected by the detector decreases with the increase of the level of a substance. The absorption of gamma-radiation by the wall of the tank is constant, whereas that of the gas space above the substance is negligible. A gamma-detector converts the gamma-radiation into the output signal (a series of small current pulses). This signal then is converted into a standard electrical signal, which can be measured by a secondary device or fed into a controller.

An accuracy of these devices can achieve +/- 1% of the span. The advantage of nucleonic level devices is that nothing comes in contact with the substance under measurement. Among disadvantages we can note the high cost and the difficulties with handling of radioactive materials.

Article Source:: Dr. Alexander Badalyan, University of South Australia


Ultrasonic devices : Level

The operational principle of these devices is based on the phenomenon of reflection of ultrasound waves from the phase boundary separating liquid and gas. In different media the speed of sound is different. Therefore, these devices may be used for interface level measurements, and in the case when the more traditional methods do not work well or do not work at all.

There are two modifications of ultrasound level measuring devices. In the first case ultrasound passes through gaseous phase; in the second case it passes through the liquid. Figure 5.15 shows a continuous ultrasound level-measuring device.

 Figure 5.15. Ultrasound level measuring device.

An electrical generator 1 generates electrical signals with a certain frequency. An acoustical transmitter 2 periodically sends ultrasound signals to the surface of the liquid 3. These ultrasound waves enter an acoustical receiver 4 after reflection from the surface of the liquid. After receiver the converted electrical signal is amplified in an amplifier 5 and enters a time interval counter 6 that measures the time between the transmission of a pulse and receipt of the corresponding pulse echo. Then a converter 7 converts thus measured time into a standard electrical signal 4-20 mA dc. Since an ultrasound permittivity depends on the properties of a gas, then a thermal compensation unit 8 is used to reduce the influence of temperature variation on the results of measurement. In real industrial environment pressure and chemical composition are additional factors which affect the velocity of acoustic propagation. These changes can severely affect the calibration of ultrasound devices. Therefore, additional electronic means are incorporated in these instruments to correct such changes.

Article Source:: Dr. Alexander Badalyan, University of South Australia


Conductance devices : Level

These instruments are used when there is a necessity of liquid control at one specific point or between maximum and minimum values. Their principle is based on the measurements of electrolytic conductivity of liquids. Two electrodes 1 and 2 are immersed in liquid 3, which fills a vessel 4 in Fig. 5.14. These electrodes through electric cables 5 are connected with an electric or electronic relay 6. Electrodes should be insulated from the vessel. Each of the electrodes forms an electrical circuit with the vessel through liquid. Therefore, the material of the vessel should be conductive. When liquid forms the circuit with the electrode 1, the electric relay starts to operate, and a signal is sent to a secondary device which detects that the lower limit of the level equal to hmin has been reached. When liquid is in contact with the electrode 2, this indicates that the upper limit of the level hmax has been reached.

Figure 5.14. Conductance-type level system.

These devices may be used for an interface-level control, where one liquid is conductive, whereas the second liquid is dielectric. The advantage of conductive-type level meters is that they can be used in vessels under atmospheric or manometric (or vacuumetric) pressures. When employing electric relays, the best results may be achieved for the most of aqueous solutions of electrolytes with electrolytic resistivities lower that 20000 Ohm*cm. If one deals with liquids with low electrolytic conductivity (water, alcohol), the sensitivity of an electric circuit becomes lower, so electronically operated relays are used to increase the sensitivity of the device.

Article Source:: Dr. Alexander Badalyan, University of South Australia


Capacitance devices : Level

Due to the difference in the dielectric constants of air and liquids, it is possible to measure level of liquids in tanks by measuring the change in the capacitance (measured between two coaxial cylinders partly immersed into the liquid) with liquid level. These devices can be used for level measurement of liquids at pressures up to 6 MPa. If liquid is conductive (specific resistance less than or equal to 105-106 Ohm*m), then cylinders (electrodes) are covered by an electrical insulation. Fig. 5.13 shows a schematic view of a capacitance device for level measurements of liquids.
Figure 5.13. Capacitance device for level measurement.

A tank 1 is filled with a liquid 2, which level is to be measured. Two electrodes (coaxial cylinders) 3 and 4 are immersed in this liquid. The value of capacitance for this device is determined by the two capacitances: that of the capacitor formed by the liquid and the electrodes, and the capacitor formed by air 5 and the electrodes. A measuring device 6 then measures the variation in the capacitance. In this system, the increase in the total capacitance is directly proportional to the increase of the level. This technique is best applied to nonconductive liquids, since it is necessary to avoid the problems generated by conducting materials like acids.

The following formula is taken from Bentley J. P. Principles of Measurement Systems, Longman, 1995, p. 145):



However, for precise measurements we need to take into account that εair is a function of pressure, temperature and humidity:


For polar dielectrics:
  • high dielectric permitivity, ε>12, F/m;
  • water, acetone, ethyl, methyl alcohol, etc.
For non-polar dielectrics:
  • dielectric permitivity, ε<3, F/m;
  • for condensed gases such as H2, O2 and N2 1.25<=ε<=1.5, F/m.
For weak-polar dielectrics:  
  • dielectric permittivity 3<=ε<=6, F/m.

For precision measurements an additional capacitance element is submerged in liquid to compensate for changes in the liquid characteristics.

Disadvantages of these devices are listed below:

  • these devices are not able to measure level of liquids, which have tendency to crystallise, and of very viscous liquids;

  • they are very sensitive to the variations of dielectric properties of liquids with process conditions and the variations of capacitances of connecting cables.

The range of level measurements varies from 1 to 20 m. The accuracy is equal to +/- 2.5%.

Article Source:: Dr. Alexander Badalyan, University of South Australia


Liquid head pressure devices : Level

The principle of this method is based on the measurement a hydrostatic pressure, caused by a liquid head, proportional to the level of liquid. There are several modifications of this method which are utilised in the following measuring systems:

    • hydrostatic differential-pressure meters;
    • the air-bubble tube or purging system;
    • the diaphragm-box system, etc.

Among them the hydrostatic differential-pressure method is the most popular for level measurements in open (at atmospheric pressure) or in closed (under gauge or vacuumetric pressures) tanks. Figures 5.10 and 5.11 give us examples of these two cases. A tank 1 at atmospheric or gauge (or vacuumetric) pressure is filled with liquid 2 which level is to be measured. A ‘positive’ chamber of the differential pressure transmitter 3 is connected to the tank 1 by tubing, whereas its ‘negative’ chamber is connected to a surge tank 4 which internal diameter is greater than that of tubing. Terms ‘positive’ and ‘negative’ indicate that pressure in the second chamber is lower compared with that in the first one. This doesn’t mean that the pressure is negative. A valve 5 is used to equate pressures in these two chambers of the differential pressure gauge, in order to check its zero point. This valve must be closed during level measurements. The liquid, which fills the surge tank, should be the same as that under measurement. Left and right tubes should be close to each other, because variations of an ambient temperature will cause the same changes in liquid density in both tubes. Since the diameter of the surge tank is greater than the diameter of tubing, therefore, the liquid displaced by the membrane in the differential pressure transmitter into the surge tank will not change the level in it. To eliminate the influence of variations of process pressure P in the big tank on the results of level measurement, the upper part of the big tank is connected with the upper part of the surge tank by tubing.

Figure 5.10. Level measurement in an open tank.  

Figure 5.11. Level measurement in a closed tank

The differential pressure measured by the differential pressure transmitter is equal to:

for an open tank:

 for a closed tank:


ρliq  - density of the liquid in the tank under measurement, kg/m3;
gloc - local gravitational acceleration, m/s2.

Since  h1 =h2 , then

Therefore, the output signal of the differential pressure transmitter is proportional to the
ΔP , and, finally, to the liquid level H  in the tank. In modern instrumentation surged tanks usually are not used. Instead, a counter-pressure P2= ρliqgloc h1 is created in the ‘negative’ chamber in the case of a pneumatic differential pressure transmitter, or a counter electrical signal corresponded to the value of  P2= ρliqgloc h1 is generated in an electrical circuit of an electronic differential pressure transmitter.

Let we use an electronic differential pressure transmitter in Figures 5.10 and 5.11. It is, therefore, appropriate to describe an operational principle of the electronic force-balance transmitter. In our case it converts the differential pressure into the standard electrical signal (4-20 mA dc) and transmits this signal by distance. This type of transmitter with some modifications in its design may be used for the conversion of any process variable into the standard electrical signal. Fig. 5.12 shows an operational principle of the electronic force-balance transmitter.

When the difference of pressures ΔP=P1-P2  increases, then a membrane with a disc in its centre 1 will move to the left, and through a bar 2 the force developed on this membrane will be transferred to a force bar 4. The force bar rotates clockwise around a cobalt-nickel alloy seal 3. As the result of these movements a bar 5 moves clockwise, and a ferrite disc 6 moves towards a differential transformer 7. The output signal (an electromotive force) of this differential transformer increases and is fed into an amplifier 8, which is powered by a power supply 9. This signal is amplified and rectified to a direct current, and results the standard electrical output signal of 4-20 mA dc. This rectified signal (greater than the signal corresponded to the previous

 Figure 5.12. Schematic of an electronic force balance transmitter.

balanced position of the lever system) enters a winding 10 which is placed between poles of a permanent magnet 11 and connected with a bar 12. As the result of the interaction of magnetic fields from the winding and the magnet, the former moves to the left under the force proportional to the signal from the differential transformer 7, and hence proportional to the measured differential pressure ΔP=P1-P2  . Thus, the lever system of the transmitter is rebalanced in a new position. The output signal of the transmitter is directly proportional to the ΔP.

Moving a mechanism 14 up and down can perform an adjustment of the span of the transmitter. Zero adjustment of the transmitter (for the case when  ΔP=0 , then output current should be equal to I = 4 mA dc) can be done by a mechanism 13.

Article Source:: Dr. Alexander Badalyan, University of South Australia


Friday, September 16, 2016

Displacer (buoyancy) devices : Level

This method is based on the application of the Archimedes' principle: every body immersed in the liquid or gas is exposed to the action of a buoyant force (sometimes called as the Archimedes' force), which acts upwards. This force is equal to the weight of the liquid, gas or vapour displaced by this body. The immersed body is called a buoy, thus giving the name to the method of measurement. Fig. 5.2 presents a schematic of this type of device.

Figure 5.2. Buoyancy-type level transmitter.

To measure the level of liquid 1 in a tank 2 a buoy 3 is partly immersed in the liquid. When the level varies so does the resultant force acting on the buoy as follows: 


: - gravitational force acted on the buoy,  N;

 : - the Archimedes’ force acted on the buoy, N ;

mb   : - mass of the buoy, kg ;
gloc   : - local gravitational acceleration, m/s2;

:- density of the liquid and the gas (or vapour) above it, respectively, kg/m3;

Sb  : -  a horizontal cross-section area of the buoy, m2 ;
 :-  parts of buoy length in the liquid and in the gas above it, respectively, m;


Since the Archimedes’ force acting from the gas on the buoy is negligible comparing with that from the liquid, and the gravitational force is constant, then the resultant force is proportional to 

and, hence, to the level of the liquid.

The displacer element, buoy, is a cylinder of a constant cross-section area, and its density is greater than that of the liquid. The buoy moves up or down, depending on the level variation. The resultant force through a lever 4 is converted by a force-balance or electronic transmitter 5 to a proportional pneumatic (20-100 kPa) or electrical (4-20 mA dc) signal, which is transmitted by the distance. It means that for each value of the level in the tank will correspond the certain value of an output signal. The length, diameter, material of the buoy and transmission ratio can be changed to suit various spans and various liquids. These instruments are used for measurements of liquid level and interface providing the level to vary within the length of the buoy, and for density measurements providing the buoy is fully immersed in liquid in the entire range of measured densities.

It is appropriate now to consider an operational principle of a pneumatic transmitter, which is used for converting level variations into the standard pneumatic signal. To be more precise, these transmitters can be used to convert a mechanical motion (which may be caused by the variation of any process variable) into the standard pneumatic signal. Fig. 5.3 shows a schematic view of the pneumatic transmitter.

When level of the liquid goes up, the Archimedes’ force moves the buoy 1 in the same direction. A membrane 2 separates the measuring part of the pneumatic transmitter from the part with a high process pressure in the tank where the level is to be measured. The motion of the buoy through levers 3 (rotates clockwise) and 4 (rotates clockwise) transmits to the force bar 5 (rotates clockwise). The force bar is connected with the flapper 6, which approaches to the nozzle 7. The pressure supplied to a pneumatic amplifier 8 is equal to 140 kPa. The pressure from this amplifier is fed to the nozzle, and then to atmosphere. When the flapper approaches to the nozzle, the pressure in the nozzle increases. This pressure enters the pneumatic relay in the pneumatic amplifier, where it is amplified, and so the value of the output pressure increases. The output pressure is transmitted to a measuring or controlling instrument, and is applied to the feedback bellows 9, thus increasing the counterclockwise moment of force acting from the lever 10 (rotates clockwise) on the force bar 5. This moment of force is sufficient to restore the force bar to the balance. When the balance has reached the output pressure is linearly related to the value of the measured liquid level. A gain adjustment holder 11 is used for the variation of the measuring range. An additional weight 12 is used for damping the vibration of levers. Zero adjustment can be achieved by the spring 13.

1.1 Flapper-nozzle system

Fig. 5.3 shows a flapper-nozzle system and Fig. 5.4 shows a relationship between the output pneumatic signal and the distance between the flapper and the nozzle.

The diameter of the supply restriction 1 is 0.2-0.3 mm, whereas that of the nozzle 2 is 0.8 mm. The distance between the flapper 3 and the nozzle determines the output pressure in the chamber between them. This pressure is measured by the pressure gauge 4. Small nozzle diameters increase gain, but also increase the danger of clogging. Large nozzle diameters increase the air consumption. The variation of the nozzle clearance by 0.04 mm gives the change in the output pressure from 20 to 100 kPa. Formulars below are taken from(from Bentley J. P. Principles of Measurement Systems, Longman, 1995, p. 315):


 Figure 5.3. A pneumatic transmitter.

 Figure 5.4. A flapper-nozzle system.

1 - restriction, Dor = 0.2 mm;    2 - nozzle, Dn = 0.8 mm;        3 - flapper.

Figure 5.5. Nozzle air pressure vas distance between the flapper and the nozzle.

For the steady state condition:

 and  (from Bentley J. P. Principles of Measurement Systems, Longman, 1995, p. 316):.


, and




 1.2 . Pneumatic relay amplifier




Steady-state sensitivity:

Figure 5.6. Pneumatic relay amplifier (from Bentley J. P. Principles of Measurement Systems, Longman, 1995, p. 317):.
1        - flapper;
2        - nozzle;
3        - orifice;
4        - diaphragm;
5        - double vent;
6        - transmission line.

1.3. Simplified model of pneumatic torque-balance transmitter

(from Bentley J. P. Principles of Measurement Systems, Longman, 1995, p. 319-320):
 Figure 5.7. Pneumatic torque-balance transmitter.

1    – beam;
2    - pivot;
3    - negative feedback bellows;
4    - nozzle;
5    - flapper;
6    -zero adjustment spring;
7    - pneumatic relay amplifier.
Anticlockwise moments:


Clockwise moment:


a). Condition of a perfect torque balance:



A simple model for a torque-balance transmitter:

         . (5.27)

The sensitivity of the transmitter

                 .  (5.28)


b). Condition of imperfect torque balance:

Anticlockwise moments:

       . (5.29)

Clockwise moment:



 .  (5.32)

An accurate model for torque-balance transmitter is as follows:



Figure 5.8. Block-diagram for a pneumatic torque-balance transmitter.

1.4. Simplified model of pneumatic differential pressure transmitter

(from Bentley J. P. Principles of Measurement Systems, Longman, 1995, p. 321-322):

According to Fig. 5.9 the resultant force on the diaphragm is as follows:


Clockwise moment on the force beam due to the action of  :

Anticlockwise moment on the force beam due to the action of the span nut:

For the condition of balance:
(5.37)        or


Anticlockwise moment produced by the span nut on the feedback beam:

Anticlockwise moment produced by the zero spring force on the feedback beam:

Clockwise moment on the feedback beam produced by
acting on the feedback bellows:

For the condition of balance:

 Figure 5.9. Simplified model of pneumatic differential pressure transmitter

1 - diaphragm capsule;    2 - force beam;    3 - flapper;    4 - nozzle;
5 - span nut;        6 -feedback bellows;    7 - feedback beam;
8 - zero adjustment spring.

 then we can get a simplified model for the differential pressure transmitter.



Adjusting the position of the span nut alters the ratio  e/d  , and the sensitivity.

Adjusting the zero spring force Fo   gives a zero pressure (when P1 = P2 ) of 20 kPa.

         Abel    - the effective area of the feedback bellows, m2;
         AD - effective area of the diaphragm, m2.

Article Source:: Dr. Alexander Badalyan, University of South Australia


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