Diaphragm and Bellows type instruments can be used as liquid tank level indicators. As shown in the image, The high pressure port connects to a tap at the bottom of a tank. The low port then connects to the atmosphere.
How To Measure The Liquid Tank Level
A differential pressure instrument can be used to measure liquid level as a function of head.
The user will specify the desired units (mm., inches of water, lbs., CO2, etc.). To measure the correct level in open tanks, the instrument should be mounted at the bottom of the tank.
If the tanks are closed or pressurized, the low pressure port can be connected to the top of the tank as shown in the figure. Regardless of the pressure, the liquid inside will give similar head as open tanks and the instrument will indicate the precise level.