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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 A1 illustrates a typical differential
pressure detector attached to an open tank.
Figure A1
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 A2).
Figure A2
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 liquid’s 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 A3).
Figure A3
The filled reference leg applies a
hydrostatic pressure to the high pressure side of the
transmitter, which is equal to the maximum level to be
measured. The DP
transmitter is exposed to equal pressure on the high
and low pressure sides when the liquid level is at its
maximum; therefore, the differential pressure is zero. As the
tank level goes down, the pressure applied to the low
pressure side goes down also, and the differential pressure
increases. As a result, the differential pressure and the
transmitter output are inversely proportional to the tank
level.
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