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