Air gaging is often referred to as a non-contact form of measurement. This is accurate, to the extent that there's no metal-to-metal contact between a sensitive gage component and the workpiece. Nevertheless, air gage tooling—including air plugs for inside diameter measurements—does generally come in contact with the workpiece and may show wear after several thousand measurements or years of use. (The comments here are equally applicable to electronic plug gages.)
When, due to wear, the clearance between the gage and the workpiece exceeds the design clearance, centralization error results. The air jets then measure a chord rather than the true diameter of the part. As the distance between the chord and the bore centerline increases, we begin to see measurement inaccuracy. Another form of error occurs when the jet centerline is not on the plug centerline. In this case, the plug will always measure a chord of the part.
How much centralization error is allowable depends upon both the diameter of the workpiece and the dimensional tolerance specification. Obviously, looser tolerances can "tolerate" more measurement error. But equal amounts of misalignment will cause greater centralization error in a small bore than in a large one.
For a gage to function properly, all the jets in an air plug (or ring) must have a common recess depth and orifice diameter. But recesses and orifices may become clogged with contaminants, damaged through accident, or worn unevenly through very heavy usage. This causes another form of measurement error, called "balance error."
It is easy to inspect for these various forms of error. Certain types of gage usage will demonstrate characteristic wear patterns. In hand-held gaging applications, wear usually occurs fairly evenly around the circumference of the plug. If, however, the plug is horizontally mounted on the front of an air gage display, then the top surface will probably experience the most wear.
If wear is expected to be fairly regular around the circumference, start by securing the gage horizontally, with the jets also horizontal. Place a master on the plug, release it, and note the reading (Figure 1A). Then carefully raise the master until it contacts the lower surface of the plug. If the plug is worn, the readout will change as the measurement moves from a chord, through the maximum diameter (Figure 1B), to another chord. Wear may be considered excessive if the reading changes by an amount equaling 10 percent or more of the part tolerance.