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

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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

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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

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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):

(5.51)

where,

However, for precise measurements we need to take into account that εair is a function of pressure, temperature and humidity:
(5.52)
and

(5.53)
where,

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

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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:

(5.48)

 for a closed tank:

     

(5.49)

where,          

ρliq  - density of the liquid in the tank under measurement, kg/m3;

gloc - local gravitational acceleration, m/s2.

Since  h1 =h2 , then

(5.50)

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

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