Air Gaging vs. Contact Gaging

Contact gaging measures the dimension from surface roughness peaks of the part being measured (i.e. at plane A - A).

Open nozzle air gaging measures the mean surface of the part, which is approximately the average of surface finish peaks and valleys (i.e. at plane B - B). Technically, the mean surface would be an imaginary plane established by using the material from the peaks to fill the valleys until a level or zero line is formed.

The result is that there is a difference in measurement between air gaging and contact gaging. The amount of this difference is a function of surface roughness. The inside diameter of a hole will read larger with an air gage than it will if measure with a contact gage. Conversely, the outside diameter of a shaft will read slightly smaller with an air gage.

The following chart shows the diametral difference between air gaging and contact gaging:

Air Gaging vs. Contact Gaging
2 .000005 in. 50 .000140 in.
5 .000013 in. 60 .000165 in.
10 .000025 in. 70 .000200 in.
20 .000040 in. 80 .000225 in.
30 .000080 in. 90 .000255 in.
40 .000110 in. 100 .000280 in.

Edmunds- air gaging vs. contact gaging. N.p., n.d. Web. 20 June 2017