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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.

RMS DIFFERENCE RMS VALUE DIFFERENCE
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.

air gaging vs contact gaging

See Also:

Air Gaging

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