Air rings and air snap gages fit over the outside of a part, thereby giving an external measurement of diameter.  The air ring gage is made with a ground hardened steel body, lapped polished with built in air gage nozzles. When the ring gage is slipped over the workpiece, the resulting back pressure is sensed by the air gage readout and displayed as the diameter of the part. Air Rings come in 2 and 3 jet configurations. The three jet air ring check for three lobe out of round conditions prevalent in centerless ground parts that cannot be detected with a two jet air ring gage. For both air rings and air probes, the best wear life is obtained by using thru-hole or center-jet styles when the application permits.