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While quite durable and reliable compared to mechanical gages, air gaging is not carefree. Accurate air gaging requires proper tooling maintenance and air supply vigilance.

Let’s start with the foundation of air gaging: the air supply. Shop air is difficult to keep clean and dry. Air dryers are not entirely adequate. The very act of compressing air produces moisture, and a compressor’s need for lubrication inevitably generates some oil mist in the line. Oil and water mist can actually act as an abrasive and cause part wear over long periods of time. Air also can be costly, so don’t let it run unless needed. The goal is simply to prevent mist from entering the gage and fouling the jets. To do this, we must employ proper air-line design to intercept it before it enters the meter.

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Air main lines should be pitched down from the source, with a proper trap installed on the end. Feed lines should also be equipped with traps. Take air from the top rather than the bottom of the mains, so moisture doesn’t drain into the gage. Bleed air lines before connecting them to gages. Operating gages must always have a filter in place, which should be changed when it becomes saturated.

Basic air-tool maintenance simply means keeping the tool clean and dry inside and out. Contaminants such as chips, dirt, coolant and cutting fluid may be picked up from workpieces, while water and oil are likely to come from the air source itself. While air pressure may flush out most contamination, the gage must be inspected and cleaned when necessary. Repeated mastering that produces varying readings is a good indication of dirty jets.

With single master air gaging systems, the accuracy is built in to both the air tooling and display. This means that the air pressure, jet diameters and clearance between the jet and the part are manufactured to standards creating the same pressure distance curve for the tooling and the display. Just as gage blocks act as standards for mechanical or electronic gaging system, master restrictor kits act as standards for air gage systems, providing standards for the pneumatic zero and span of the reference system.

To check for contaminated air tooling, the zero-restrictor would be used to verify pneumatic zero on the display unit, and then the tooling and its master would be compared to this pneumatic zero setting. Large variation would indicate some form of contamination or damage to the tool’s jetting.

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 make contact with the workpiece, and like any tooling, may show wear after several thousand measurements or years of use.

When the clearance between the gage and the workpiece exceeds the design clearance due to wear, 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.

The amount of allowable centralization error depends on 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 a greater centralization error in a small bore than in a large one.

To inspect for wear, secure the gage with the jets oriented horizontally. Place a master on the plug, release it and note the reading. 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 to another chord. Wear may be considered excessive if the reading changes by an amount equaling 10% or more of the part tolerance.

To check for balance error, the test is performed as above but the tooling is rotated so that the jets are mounted vertically. Now the master is placed on the plug by resting it on the top jets and noting the reading. Lift the master so that it restricts the lower jet and note this reading. Normally, two-jet air plugs automatically balance themselves when one of the jets is closer to the workpiece than the other—as is the case here, where the master is allowed to rest on the upper jet. However, if one jet or orifice is damaged or worn, this test will demonstrate the gage's inability to maintain that balance.

Plug gages tend to be highly durable, because they contact the workpiece across a broad surface area. But that doesn't mean you can ignore the possibility of poor centralization or balance. Include these tests in the annual gage calibration program.