Diaphragm box method is an open tank level measurement technique. The diaphragm is a hydrostatic instrument.
The hydrostatic theory says, when a pressure greater than atmospheric is imposed on the surface of a liquid in a closed vessel, this pressure adds to the pressure due to the hydrostatic head and must be compensated for by a pressure measuring device which records liquid level in terms of pressure.
The diaphragm box shown above is submerged in the process liquid and connected to a pressure gage by a gage line. The hydrostatic head produced by the level of the liquid in the tank exerts pressure on the bottom of the diaphragm causing it to flex upward. This action compresses the gas in the box and the gage line. The pressure is applied to a gage or other pressure element that is part of an indicator assembly calibrated to indicate liquid level units.
As the level in the container increases, the pressure exerted by the hydrostatic head on the diaphragm increases in direct proportion. The diaphragm will continue to flex until the gas pressure in the box and the meter line is equal to the pressure exerted by the liquid level in the lower part of the diaphragm.
The diaphragm box must be free of leaks. If the gas was filtered into the tube, there would be less increase in gas pressure with an increase in the level of the liquid, and the indications would be inaccurate.
Air trapped diaphragm box:
This the variation diaphragm box level meter:
This uses an air- trap sensor, or inverted bell, instead of a diaphragm box. As the liquid level rises, the hydrostatic head forces liquid up into the bell. As the level of the liquid rises, it compresses the air trapped in the bell and the gauge line until an equilibrium between the air pressure and the pressure exerted by the hydrostatic head is reached. The pressure of the compressed air can be used to determine level.
This design is useful where extreme operating temperatures or corrosive fluid applications might damage a diaphragm