Differential pressure measurement as indirect method for level measurement:
Closed vessel:
Differential pressure (DP) transmitters used for liquid level application measure hydrostatic pressure head. In a closed vessel, pressure above the liquid is due to the gas column above the liquid surface. This pressure affects the pressure measured at the bottom of the vessel. Pressure at the bottom is the sum of the liquid pressure as well as the gas pressure above the liquid. A valve at the bottom side of the vessel is connected to the high pressure (HP) side of the DP and a valve at the top of the vessel to the low pressure side. Thus the vessel pressure will get applied to both the high and low sides of the transmitter. The resulting differential pressure is proportional to the liquid level.
(OR)
Open vessel
A differential pressure (DP) transmitter mounted near the bottom of the tank measures the pressure of the liquid inside it. A valve at the bottom side of the vessel is connected to the high pressure (HP) side of the DP and the low pressure side is vented to atmosphere.