The differential pressure (DP) sensor/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 illustrates a typical differential pressure detector attached to an open tank.
Open Tank Differential Pressure Level Measurement
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…