The air probe (also referred to as an air plug gage, air spindle or mandrel) consists of a precision ground hardened steel body incorporating two or more air gage nozzles. Air is passed through the probe body to the nozzles where a back pressure is produced by the surface of the workpiece. An air gage readout (air comparator) senses the resulting back pressure and displays the size of the workpiece.

As shown in the drawing, the nozzle tips are recessed a small amount below the body of the probe. This feature makes the air gage measurement essentially non-contact, consequently wear does not directly affect the accuracy of the gage measurement. Moreover, the flow of air purges the gaging surface of contaminants making the measurements highly reliable.

Through the use of the opposed set of nozzles, the measurement is made independent of how the operator positions the probe radially within the test bore. This “differential” type of measurement makes it possible to obtain highly accurate measurements with limited operator skill.

Air probe styles vary depending on how close the air jets are to the leading edge of the probe body. “Thru-hole” style probes have gaging nozzles located near the center of the probe body. “Blind” or “Super-blind” probe styles have nozzles located near the leading edge.

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