What is air gaging?
Air gages were first developed in France before World War II by a carburetor company that was looking for a more reliable method of measuring its carburetor jets.
Air gaging is based on the law of physics regarding flow and pressure. They are directly proportionate to the clearance and will react opposite to each other. In other words, if there is more clearance there will be more flow but less pressure and with less clearance there will be more pressure and less flow.
In order to measure this, you must have regulated air pressure that flows through a restriction, and then through the nozzle in the air tool. As the clearance due to the workpiece is smaller, air flow is reduced, and the back pressure is increased.
In order to calibrate the gage for the task at hand, a known artifact (MEAN) or artifacts (MIN and MAX) are used to set the display on scale and report accurate and highly repeatable diameter sizes. These artifacts are certified master rings and/or plugs and are typically set at the Nominal or the MIN and MAX ranges of your tolerance. Different manufacturers use different methods of calibration.