Air gages measure length. Closing off of an open-air jet causes back pressure to be built up in the channel from the air supply. This backpressure can be graphed on a pressure distance curve and as the open-air jet becomes more restricted, the backpressure builds up. The pressure distance curve created by this effect is repeatable and usable for displacement measurement.
"The pressure distance curve is what makes up the air gage and is what ensures its practical use on the shop floor," says George Schuetz, director of precision gages at Mahr Federal Inc. (Providence, RI). "We have introductory information from the 1940s about air gaging, and the tooling and applications have not changed dramatically from those times, it's simply built upon the laws of physics.