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Conductivity Probe Method
Figure 10    Closed Tank, Dry Reference Leg

Instrumentation and Control Volume 1 of 2
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Level Detectors LEVEL DETECTORS Differential Pressure Level Detectors The differential pressure (DP) detector method of liquid level measurement uses a  DP detector connected to the bottom of the tank being monitored.   The higher pressure, caused by the fluid in the tank, is compared to a lower reference pressure (usually atmospheric).   This comparison takes place in the DP detector.  Figure 9 illustrates a typical differential pressure detector attached to an open tank. Figure 9    Open Tank Differential Pressure Detector The tank is open to the atmosphere; therefore, it is necessary to use only the high pressure (HP) connection  on  the  DP  transmitter.    The  low  pressure  (LP)  side  is  vented  to  the  atmosphere; therefore, the pressure differential is the hydrostatic head, or weight, of the liquid in the tank. The maximum level that can be measured by the DP transmitter is determined by the maximum height of liquid above the transmitter.   The minimum level that can be measured is determined by the point where the transmitter is connected to the tank. Not all tanks or vessels are open to the atmosphere.  Many are totally enclosed to prevent vapors or steam from escaping, or to allow pressurizing the contents of the tank.   When measuring the level  in a  tank  that  is pressurized,  or  the  level that  can  become  pressurized by  vapor  pressure from  the  liquid,  both  the  high  pressure  and  low  pressure  sides  of  the  DP  transmitter  must  be connected (Figure 10). Rev. 0 Page 7 IC-03







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