The principles of air gaging are simple: When you blow compressed air through opposing jets of air across the diameter of a bore, the upstream back-pressure of that air drops as the size of the bore increases. In a tightly configured system, this generates a pressure distance curve. Air gaging is one of the first forms of precision measurement, with systems employed in
industry since the 1930s. Air gaging was the earliest form of extremely close tolerance measurement available, and many of the air plug configurations developed during the early days have not changed much since then.