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