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).