This method is also known as bubbles method of level measurement. A pipe is installed vertically with its open end at the zero level.
Purge Level Sensor
The other end of the pipe is connected to a regulated air supply and to a pressure gauge or to ^P transmitter.
To make a level measurement the air supply is adjusted so that pressure is slightly higher than the pressure due to the height of the liquid.
This is accomplished by regulating the air pressure until bubbles can be seen slowly leaving the open end of the pipe. The gauge then measures the air pressure needed to over come the pressure of the liquid.
/\ P = H X D
USE : On for corrosive liquids where the transmitter cannot be directly connected to process eg… Acids, Some organic liquids.