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.
![](http://images8.webydo.com/92/9263031/3958%2f1DFC724E-B5F7-3440-BB6B-5EF4D15C9BBC.png)
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A New Dimension In Precision Measurement
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