Figure 10 Closed Tank, Dry Reference Leg
The high pressure connection is connected to the tank at or below the lower range value to be
measured. The low pressure side is connected to a "reference leg" that is connected at or above
the upper range value to be measured. The reference leg is pressurized by the gas or vapor
pressure, but no liquid is permitted to remain in the reference leg. The reference leg must be
maintained 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 vapor
pressure exerted on the liquids surface. The gas or vapor pressure is equally applied to the low
and high pressure sides. Therefore, the output of the DP transmitter is directly proportional to
the hydrostatic head pressure, that is, the level in the tank.
Where the tank contains a condensible fluid, such as steam, a slightly different arrangement is
used. In applications with condensible fluids, condensation is greatly increased in the reference
leg. 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 the
transmitter, 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 side
of the transmitter (Figure 11).