Open Tank Differential Pressure Detector:

The tank is open to the atmosphere; thus, it is necessary to use only the high pressure (HP) connection on the P transmitter. The LP (low pressure) side is vented to the atmosphere; thus, the pressure differential is the hydrostatic head, or weight, of the liquid in the tank. A maximum level which could be measured through the ?P transmitter is determined through the maximum height of liquid above the transmitter. The minimum level which could be measured is determined through the point where the transmitter is linked to the tank.

Figure: Open Tank Differential Pressure Detector