LEVEL DETECTORSLevel DetectorsFigure 10 Closed Tank, Dry Reference LegThe high pressure connection is connected to the tank at or below the lower range value to bemeasured. The low pressure side is connected to a "reference leg" that is connected at or abovethe upper range value to be measured. The reference leg is pressurized by the gas or vaporpressure, but no liquid is permitted to remain in the reference leg. The reference leg must bemaintained dry so that there is no liquid head pressure on the low pressure side of the transmitter.The high pressure side is exposed to the hydrostatic head of the liquid plus the gas or vaporpressure exerted on the liquid’s surface. The gas or vapor pressure is equally applied to the lowand high pressure sides. Therefore, the output of the DP transmitter is directly proportional tothe hydrostatic head pressure, that is, the level in the tank.Where the tank contains a condensible fluid, such as steam, a slightly different arrangement isused. In applications with condensible fluids, condensation is greatly increased in the referenceleg. To compensate for this effect, the reference leg is filled with the same fluid as the tank.The liquid in the reference leg applies a hydrostatic head to the high pressure side of thetransmitter, and the value of this level is constant as long as the reference leg is maintained full.If this pressure remains constant, any change in DP is due to a change on the low pressure sideof the transmitter (Figure 11).IC-03 Page 8 Rev. 0
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