Differential pressure transmitters measure level by inference from what?

Differential pressure transmitters are commonly used to measure the level of fluids in tanks or vessels by inferring the level from the hydrostatic pressure exerted by the liquid column above the measurement point. These transmitters work on the principle that the pressure difference between two points can be related to the height of the liquid column.

In a typical setup, one pressure port is located at the bottom of the tank, where it measures the pressure caused by the fluid's weight, while the other port is either open to the atmosphere (for open tanks) or measures the pressure in a gas above the liquid (for pressurized tanks). The transmitter calculates the level based on the hydrostatic head, which is determined by the pressure measurement and the specific gravity of the fluid. This is why hydrostatic head is the correct inference method for determining fluid levels using differential pressure transmitters.

The other options describe different measurement principles that are not applicable to the operation of differential pressure transmitters.