How can differential pressure be used to measure level in a closed tank?

Differential pressure (DP) can be used to measure the level in a closed tank by measuring the pressure difference between the bottom of the tank and the vapor space above the liquid. This pressure difference is directly proportional to the height of the liquid column in the tank. A differential pressure transmitter is used to measure this pressure difference. The transmitter has two pressure ports: a high-pressure port (+) and a low-pressure port (-). The high-pressure port is connected to a tap at the bottom of the tank, which measures the total pressure at the bottom, including the pressure due to the liquid level and any pressure in the vapor space. The low-pressure port is connected to a tap at the top of the tank, which measures the pressure in the vapor space. The differential pressure transmitter measures the difference between these two pressures (Pbottom - Ptop). This pressure difference is equal to the hydrostatic pressure exerted by the liquid column, which is given by the formula: ΔP = ρ g h, where ΔP is the differential pressure, ρ is the density of the liquid, g is the acceleration due to gravity, and h is the height of the liquid column (level). Knowing the liquid density and the acceleration due to gravity, the level can be calculated from the measured differential pressure. In a closed tank, the pressure in the vapor space can vary, so it is important to compensate for this pressure when measuring the level. The differential pressure measurement automatically compensates for changes in the vapor space pressure, providing an accurate level measurement. For example, in a pressurized storage tank containing liquid propane, a differential pressure transmitter can be used to continuously monitor the liquid level. The transmitter's output signal, which is proportional to the differential pressure, can be used to display the level on a control panel or to control pumps that transfer the propane.