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

RMS

2

5

10

20

30

40

DIFFERENCE

.000005 in.

.000013 in.

.000025 in.

.000040 in.

.000080 in.

.000110 in.

RMS-VALUES

50

60

70

80

90

100

DIFFERENCE

.000140 in.

.000165 in.

.000200 in.

.000225 in.

.000255 in.

.000280 in.