The simplest application is the fluid level in an open tank. The figure shows a typical open tank level measurement installation using a pressure capsule transmitter.
If the tank is open to atmosphere, the high pressure side of the level transmitter will be connected to the base of the tank while the low-pressure side will be vented to atmosphere.
In this manner, the level transmitter acts as a simple pressure transmitter.
Phigh = Patm + S.H
Plow=Patm
Differential pressure, ΔP = Phigh – Plow = S.H