Air Gaging FAQ

Straight answers to the questions we hear most about air gaging (air gauging): how it works, what it costs, how to choose, set up and troubleshoot a gage, and how to replace older dial and column readouts.

Basics: what air gaging is and how it works

What is an air gage (air gauge)?

An air gage is a precision comparator that measures size with air instead of a contact point. A plug, ring or snap gage blows regulated air through small jets, and a readout turns the change in back-pressure into a dimension, typically to 0.00005″ or finer. "Gage" and "gauge" mean the same thing. See how air gaging works.

How does an air gage work?

Clearance changes pressure. Regulated air flows through a fixed restriction to the jets; when the part surface is closer to the jets, air escapes less easily and back-pressure rises, and when it is farther away, pressure drops. Over a short working range that relationship is nearly linear, and the readout converts it to size. Full explanation: How Air Gaging Works.

What can an air gage measure?

Inside and outside diameters, plus form: out-of-round, taper, straightness, parallelism, perpendicularity (squareness), flatness and the clearance between mating parts. The jet layout of the tooling decides which features one gage checks. See measuring form with air gaging.

Who uses air gaging, and where?

Manufacturers of tight-tolerance parts in high volume, especially automotive, aerospace, hydraulics and medical. It is used at the bench for first-piece or 100% inspection, at the machine, in automated robot cells and in the gage lab. See why use air gaging.

Is air gaging really non-contact?

Yes. The plug or ring guides on the bore or shaft, but the measurement itself is made by air through jets recessed below the gage surface, so nothing measuring touches the part. Soft, polished, plated and thin-walled parts are not marked or deflected, and the escaping air blows coolant and chips off the measuring point.

Cost and buying

How much does an air gage cost?

A complete single-bore setup has four parts: tooling, a readout, setting masters and a filter-regulator. Our readouts start at $1,595 for a Metro M5 air display and $2,895 for a complete Air Gage Pro bench station; a 2-input station is $3,595. Our precision filter-regulator is $575. Air plugs, rings and masters are made to your size, so price them in minutes with the online air gage quote.

Is an air gage worth it compared with a dial bore gauge?

For production runs of the same bore held to a few tenths, usually yes. An air plug reads in a second or two, needs almost no operator skill and repeats far better between operators, which shows up in gage R&R. For one-off parts or wide tolerances, a dial bore gauge's flexibility can win. See air gage vs. dial bore gauge.

Can one readout run several plugs and rings?

Yes. You change tooling and master again. A dual-master readout works with any brand of two-master tooling; a single-master readout works with tooling of its own amplification group. A 2-input readout runs two tools at once. See air gage amplification explained.

What is your lead time?

Most products typically ship in 1–2 weeks, and items in stock often sooner. Custom tooling ships on the lead time stated in your quote. See our shipping policy.

Can I order on a purchase order or net terms?

Yes, by quote. First orders from a new customer are paid by credit card; customers with approved credit can buy on net 30. Contact us or request a quote, and see our terms and conditions of sale.

Do you ship outside the U.S.?

Online orders ship from Galena, Illinois to U.S. addresses. For international shipments, email info@airgaging.com and we will quote freight.

Choosing and comparing

Air gage vs. dial bore gauge: which is more accurate?

On production bores held to tenths, the air gage, because the result doesn't depend on the operator. A dial bore gauge must be rocked to find the true diameter and different people find it slightly differently; an air plug centers itself. The dial bore gauge wins on range, since one gauge covers many sizes. Details in this comparison.

Single master or dual master: which do I need?

Single master sets on one master at nominal and relies on factory-matched tooling and readout, so it is fast and needs fewer masters. Dual master sets on a low and a high master, calibrating the span itself, so it is most accurate at the tolerance limits and works with any two-master tooling. See single vs. dual master air gaging.

When should I use an air gage?

When tolerances are about .005″ or tighter, the same feature is measured repeatedly, results must not depend on the operator, or the surface must not be touched. It is also the fastest way to check taper and form at the machine. See why use air gaging.

When is air gaging not a good fit?

When tolerances are looser than about .005″, the surface is very rough (worse than roughly 60 rms), the part runs in many different sizes in small quantities, or you need a long measuring range. An air gage is built for one nominal size with a range of a few thousandths, so a dial bore gauge, CMM or contact gage may suit those jobs better.

Is air tooling interchangeable between brands?

Two-master tooling works on any two-master readout, because mastering on two sizes sets the span. Single-master tooling works on any readout of the same amplification group: low (Dimensionair D-2500 to D-8000, tool codes 20 and 50) or high (D-10000 to D-32000, tool codes 5 and 10). See how to identify your amplification.

Tooling and applications

What is the difference between an air plug and an air ring?

An air plug (probe) measures inside diameters, such as bores and holes. An air ring measures outside diameters, such as shafts and pins. Our standard plugs run from .059″ to 8.260″ and rings from .061″ to 7.500″. See air ring vs. air probe and browse air tooling.

How do you measure a blind hole with an air gage?

Use a blind or super-blind air plug, which places the jets close to the nose so it can read near the bottom of the bore. Super-blind reaches deepest but its nose wears faster; use a thru-hole plug whenever the bore goes through. Price one with the online quote.

How small a bore can an air gage measure?

Our standard air plugs start at .059″ (1.5 mm), and air gaging can measure bores well under 1 mm with special tooling. Small, deep, tight-tolerance bores are where air beats lasers and vision systems, which can't reach inside them. Ask us about your size.

Can an air gage check taper, roundness or straightness?

Yes. Roundness uses a 2-jet plug or ring rotated 180° while tracking max minus min. Taper uses two levels of jets, and the readout calculates the angle. Straightness, squareness and flatness use special jet layouts. See measuring form with air gaging.

Can air gaging run in-process or in an automated cell?

Yes. Readouts handshake with robots and PLCs over digital I/O, Ethernet or fieldbus, sort good and bad parts, and send offsets back to the machine. See automation and air gaging and in-process air gaging.

Setup, masters and calibration

How do you master (set up) an air gage?

Clean the gage and masters, then set it on certified masters. A dual-master gage is set on the MIN master, then the MAX master, and the span adjusts to match; a single-master gage is zeroed on one master at its certified size. Step by step: mastering and troubleshooting air gages.

How often should I master or calibrate an air gage?

Master at the start of every shift, and again after changing tooling, temperature or air supply, or whenever a master check drifts. Mastering is part of using the gage, not a periodic calibration. Recertify setting masters on the interval your quality system sets.

What are setting masters?

Certified rings, plugs or discs of known size used to set the gage. Bores are set with ring masters and shafts with setting discs. Every master we supply ships with a certificate traceable to NIST. See setting masters.

Why does my air gage read differently than my CMM or bore gauge?

Usually surface finish, temperature or form. An air gage reads the average surface under the jet while a contact probe touches the peaks, parts warmer than the master read large, and the two methods may be measuring at different points. Correlate the methods before comparing numbers. See troubleshooting by symptom.

Does temperature affect air gage readings?

Yes. Steel grows about 6.4 millionths of an inch per inch per °F, so a 1″ part 10°F warmer than its master reads about .000064″ large. Keep parts and masters at the same temperature, and don't hold masters in your hand longer than you need to.

Air supply, maintenance and troubleshooting

What air supply does an air gage need?

Clean, dry, regulated air. Shop air goes through a filter-regulator that removes water, oil and dirt and holds a steady pressure, and the supply must stay above the regulated setting or readings drift. Never feed an air gage unfiltered shop air. See our precision filter-regulator.

Why is my air gage drifting or unstable?

Almost always one of five things: the air supply (low pressure, water or dirt), the masters, temperature, the tooling (leaks, nicks, wrong readout) or the part itself. See 5 common reasons for incorrect air gage readings.

Why won't my air gage master or set its span?

Check for a leak at a hose, fitting or O-ring, a blocked restrictor or jet, a dirty or swapped master, or tooling that doesn't match the readout. See the troubleshooting table.

How long do air plugs last, and how do I know one is worn?

With clean air, years. Jets are recessed below the body so normal wear doesn't shift the reading, and the nose takes most of the wear. Replace a plug when it won't master, won't repeat, or shows nicks around the jets or a scored body. See air gage care and maintenance.

Accuracy and limitations

How accurate is an air gage?

Air gages repeat in the sub-micron range, around 0.0002 mm (0.000008″), and digital readouts resolve to 0.00005″ or finer. In practice, accuracy on the floor comes from repeatability: a gage should use no more than about 10% of the tolerance in a gage R&R study, which air gaging passes easily on tight bores. See accuracy compared.

What is the measuring range of an air gage?

A few thousandths of an inch, centered on the gage's nominal size. Higher amplification gives a smaller range with finer resolution; lower amplification gives a wider range. See amplification explained.

How does surface finish affect air gage readings?

An air gage reads the average of the surface under the jet, between the peaks and valleys, while a contact gage rides on the peaks. On smooth ground or honed bores the difference is negligible; on rough surfaces (worse than about 60 rms) it grows, so match the master finish to the part and correlate with your other methods. See incorrect readings.

Readouts, data and automation

How do I get air gage data into SPC software, Excel or a PLC?

Use a digital readout. The Metro M5 exports CSV to a USB drive, emulates a USB keyboard into Excel or SPC software, and outputs RS232 ASCII or Modbus RTU. Multi-gage systems on the Metro M400 or M-Bus add Ethernet (Modbus TCP) and digital I/O for PLCs and robots. See the Metro M5 and automation and air gaging.

What is an air-to-electronic converter?

A module that turns air gage back-pressure into an electronic signal, so air tooling can feed a digital readout, measuring computer or PLC. See air-to-electronic converters.

How many features can one readout display?

The Metro M5 shows up to 3 characteristics on one screen from 1 or 2 air inputs, up to 4 through M-Bus modules, with math between inputs such as taper. The Metro M400 runs multi-feature systems; our largest have measured more than 180 features on one part. See Metro displays.

Replacing older air gages

What replaces a Dimensionair?

The Air Gage Pro, built on the Metro M5. The M5-L replaces low-amplification readouts (D-2500, D-4000, D-5000, D-8000), the M5-H replaces high-amplification readouts (D-10000, D-16000, D-20000, D-32000), and the M5 Dual replaces two-master systems. You keep your existing plugs and rings. See the Dimensionair replacement.

Can I keep my existing air plugs with a new readout?

Yes. Two-master tooling works on any two-master readout, and single-master tooling works on any readout of its amplification group. Find yours from the D-number on the dial or the tool code on the plug: identify your Dimensionair model and tool code.

Is the Metro M1 or M3 discontinued?

Yes. The Metro M5 replaces both and works with the same air tooling, masters and footswitch. We still support existing M1 and M3 units and keep their manuals available. See replacing a Metro M1 or M3.

How do I upgrade a floor of many air gages?

Start with a station list: readout model, tool code and size, single or two master, and whether you need data output. We quote multi-unit upgrades case by case. See the Dimensionair fleet upgrade.