Introduction to Dimensional Air Gauging

July 27, 2020   |   In Measurement


[Updated 27 July 2021]


What is air gauging?

Air gauging is a non-contact comparative measuring method. It has been used in the industry for more than 80 years. The concept of air gauging is based on the law of physics that states flow and pressure are directly proportionate to clearance and react inversely to each other. Clearance in this case refers to the distance between the nozzle of the air gauge probe and the workpiece. As clearance increases, air flow also increases, and air pressure decreases proportionately. As clearance decreases, air flow also decreases, and air pressure increases.

Flow & Pressure Chart

This is made possible by having a regulated air flow through the nozzle in the air jets of the air gauges. The nozzle acts as a restrictor. As the measured product is brought closer to the nozzle, air flow is reduced and the back pressure is increased. When the nozzle is completely obstructed, the flow is zero, and the back pressure is equal to the regulated air. Conversely, when the nozzle is open to the atmosphere, air flow is at a maximum, and the back pressure is at a minimum. The pressure differences is then converted electronically to get an accurate dimensional value.

Who Uses Air Gauging?

Air gauging is highly suited for the measurement of soft, highly polished, thin wallet, delicate components that requires high accuracy. As a result, air gauging is used in many industries, they include the following:

  • Automotive
  • Aeropsace
  • Bearings
  • Medical
  • Molds
  • Machinery components
  • Packaging
  • and many others...

What are the benefits of using air gauging for measurement?

  • Ease of use, the operator will not be required to be specially trained to use the equipment
  • Operators will not be able to influence the results of the measurement
  • Air gauging can be used to measure complex geometric tolerance
  • High accuracy and repeatability
  • Possible to measure parts without cleaning them first
  • Technology particularly well suited to automation


While air gauging as a measurement tool offers many advantages, good results also depend on crucial external parameters where extra attention is required for it to be successful:

  • Air gauge design and quality
  • Accuracy of the setting masters
  • Air supply control

What can be measured with air gauges?

Recommended range: up to 0.15 mm (150 µm), short range better
Minimum diameter: 0.5 mm
Maximum diameter: up to 500 mm (but very rarely since air gauge will be heavy)
Display resolution: up to 0.01 µm (usually 0.1 µm)
Max. error: ˂0.1 µm
Theoretical linearity: Measuring range divided by 80-100 (depending of air gauge)
Repeatability: up to 0.02 µm
Display refresh rate: up to 150 times per second (depending on air jets’ diameter, length of tubes and air gage design/quality)
Air pressure: Precisely regulated at 3 Bar

Air Gauges & Contact Measurement

 Air Gauges vs Contact Measurement

Air gauging is sometimes considered as a "non-contact" type of measurement. With air gauging, the measurement point is actually the average area of the surface that the air from the air jets cover. If we were to consider the finish or roughness of the workpiece's surface, the air gauges will measure the average of the peaks and valleys that the air jet is exposed to. This is different from the contact type of measuring instrument, like the bore gauge in this example. It measures on the peaks of the surface only.

1. External diameters

External Diameter Air Gauges External Diameters Air Gauge

One way external diameters are measured with air gauges is using the ring type air gauge. This is done by inserting the part in an air jet ring gauge. Normally, a guide is used to facilitate the measurement of the components. This is done to prevent the air jet ring gauge from wearing out easily. Air gauges in snap gauges form are also used for manual gauging. It is used to mount into fixtures and also for automatic type of measurement machines.

Ojiyas Air Jet VC Type Ojiyas Air Jet LV Type Ojiyas Air Jet OR Type

It is also possible to have multiple air jets at different positions for multi-gauging applications. Here are some examples of external diameters types that can be meausre with air gauges:

  • Ring gauge
  • Snap gauge
  • Taper
  • Multi gauging
  • Average diameter

2. Internal diameters

Using air gauges to measure internal diameters is one of the more popular applications. In this instance, the air gauge is usually made with a set of opposed air jets. One of the benefits of using an air gauge to measure internal diameter is that this is independent of how the operator positions the air gauge radially within the components. Thus, using air gauges to measure internal diameters ensure a higher level of repeatibility in the measuring process.

Air Gauge Internal Diameter Diagram

The air gauges can be used to measure diameters as small as 1 mm and as big as 87 mm. They can be fabricated with multiple air jets at different levels for a multi gauging application. Average diameter can also be measured with air gauges. This is done by having multiple air jets equally located around the circumference of the air gauge to allow for average size measurement. 

Air Gauges Small Diameter Air Gauges Big Diameter

Below is a list of internal diameters air gauges can help to measure:

  • Plug gauge
  • Taper
  • Multi gauging
  • Average diameter
  • Roundness
  • Flatness
  • Straightness
  • Squareness
  • Parallelism
  • Concentricity

Ojiyas Air Jet IA Type

Ojiyas Air Jet IB Type

Ojiyas Air Jet IC Type

Ojiyas Air Jet LA LB Type 

3. Combination of measurements

The air gauge can also be used for measuring parts at multiple points and dimensions. This can be achieved either by using displays with multiple channels, for instance the Acquest Air Micrometer. Another way we can do this, is by using a modular display system. The Sylvac D400S can be combined with multiple M-Bus units to make measurement at different points of a workpiece.

Air Gauges Multi Gauging

The air gauge combines the measurement of several elements - ie diameters, depths, angles, etc.

Air Gauges D400S

Ojiyas Air Jet for Special Application

You can click here to find out how we used air gauges to measure connecting rods, which are commonly used in the automotive industry.

Air Gauge Lifetime

Air gauges can be manufactured with different materials, with or without coating, depending on application and number of cycles required. Even if air measurement is a non-contact measuring method, air gages are subject to wear due to:

  • Air abrasion
  • Introduction of the parts into the air gauge

Worn air gauges will affect repeatability of the measurement. The most sensitive air gauges are those with air jets close to the edge, because the air jets are in the contact zone.

Air Units & Modules



Sylvac D70A

Sylvac MB-AG + D400S

Acquest CAG-3000

How to use air gauges?

1. Air Supply

Before you start to make any measurements, it is important to ensure that the air supply is highly stable. This is done by using a precision regulator. Using poor air regulators might lead to unstable measurements.

2. Setting masters

Setting masters are used in the beginning of the measuring process with air gauges. They help to set the minimum and maximum measuring range on the air units. It is recommended to use setting masters which are between 10-15 times more accurate than the tolerance interval. High quality masters are important to achieve consistent measuring results. Ojiyas setting master ring and plugs below are good examples of good setting masters.

Ojiyas Master Ring Gauge

Ojiyas Master Plug Gauge

3. Setting Air Units

After setting the air units, the air gauges are ready to make measurements on the workpiece.

LFC is the authorized distributor of Ojiyas air gauges, Acquest air micrometer and Sylvac measuring equipment in Singapore and Indonesia. Contact us if you need help with your air gauging set up.