Level DetectorsLEVEL DETECTORSDifferentialPressureLevelDetectorsThe differential pressure (DP) detector method of liquid level measurement uses a DP detectorconnected to the bottom of the tank being monitored. The higher pressure, caused by the fluidin the tank, is compared to a lower reference pressure (usually atmospheric). This comparisontakes place in the DP detector. Figure 9 illustrates a typical differential pressure detector attachedto an open tank.Figure 9 Open Tank Differential Pressure DetectorThe 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 maximumheight of liquid above the transmitter. The minimum level that can be measured is determinedby 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 vaporsor steam from escaping, or to allow pressurizing the contents of the tank. When measuring thelevel in a tank that is pressurized, or the level that can become pressurized by vapor pressurefrom the liquid, both the high pressure and low pressure sides of the DP transmitter must beconnected (Figure 10).Rev. 0 Page 7 IC-03