5 Common Reasons for Incorrect Air Gage Readings

When an air gage reads wrong, the cause is almost always one of five things: the air supply, the masters, temperature, the tooling, or the part itself. Here's how to spot each one and fix it.

1. The air supply

An air gage measures back-pressure, so anything that changes the air before it reaches the jets changes the reading.

• Unregulated or low pressure. The gage needs a steady, regulated supply. If shop pressure drops below what the regulator is set to, readings drift. Check the gauge on the filter-regulator.
• Water, oil or dirt. Contamination partly blocks restrictors and jets and makes readings erratic. Drain the filter bowl and replace elements on schedule. See air filter-regulators.

2. The masters

An air gage is a comparator: it's only as accurate as the master it's set with.

• Dirty masters. A film of oil or a fingerprint in a ring master shifts zero. Wipe masters clean before every mastering.
• Damaged or worn masters. A nick or wear changes the size. Recertify masters on the schedule your quality system sets.
• The wrong master. Check the size on the master against the setting. For Dual Master tooling, make sure MIN and MAX aren't swapped.
• Mastering too rarely. Master at least at the start of each shift, and again after any change in temperature, air supply or tooling.

3. Temperature

Parts and masters grow with heat. Steel expands about 6.4 millionths of an inch per inch per °F, so a 1″ steel part that's 10°F warmer than the master reads about .000064″ large. That's enough to reject a good part on a tight tolerance.

• Let parts coming off the machine stabilize, or measure them at a consistent point in the cycle.
• Keep masters at the same temperature as the parts, and don't hold them in your hand longer than you need to.

4. The tooling

• Worn or damaged gaging surfaces. Jets are recessed below the body, so normal wear doesn't shift the reading, but a nicked jet or a badly worn body will. Inspect plugs and rings regularly.
• Leaks. A loose fitting, damaged O-ring or cut hose lets air escape before the jets and upsets the reading. If the gage won't master or the span won't set, check for leaks.
• Mismatched tooling and readout. Single Master tooling only works with a readout of the same amplification. Dual Master tooling works with any two-master readout.

5. The part and where you measure it

• The jet isn't fully covered. The land must cover the whole jet. Grooves, cross holes and chamfers near the jet give false readings.
• Surface finish. An air gage reads the average of the surface under the jet. A rough or porous surface can read differently from a contact gage or CMM.
• Tilted or partly inserted gage. Insert plugs fully and keep rings square to the part.
• Chips or heavy coolant. Air clears a light film of coolant, but chips and heavy buildup should be blown off first.

More troubleshooting in mastering and troubleshooting air gages, and upkeep in air gage care and maintenance.