Air purge method for of level measurement:
Principle: When liquid is held in a tank, then it exerts equal pressure on the walls of the tank. Such a pressure is due to the weight of liquid present above a certain reference point or base and is called hydrostatic head or pressure. Air purge system works on measuring the pressure required to force a gas into a liquid at a point beneath the surface. This method uses a source of clean gas or air and is connected through a restriction to a bubble tube immersed at a fixed depth in to the vessel.
Explanation(construction/working):
Construction: The air purge system consists of a 1 inch bubbler pipe installed vertically having its open end 3 inch above the bottom of the vessel containing the liquid. The other end of bubbler pipe has two connections; out of which one is connected to regulated metered and filtered air or gas supply while the other is connected to pressure gauge.
Working: To make level measurement, the air supply is adjusted so that the pressure is slightly higher than the pressure due to the height of the liquid and bubbles can be seen slowly leaving the open end of the pipe. The bubble rate is adjusted as 1 bubble / minute. During bubbling, the back pressure in the bubbler pipe exactly equals the hydrostatic pressure. The gauge then measures the air pressure needed to overcome the pressure of the liquid.