DeFelsko PosiTector 100 v.4.0 - Manual (Page 44)

Page 44 100 Series
Appendix A - Making Your Own Check
Standards
Once you have calibrated your Gage you may wish to make some Check
Standards for use at the work site. These Check Standards should be made of the
particular material you are using on the job to best match the measurement
conditions. The linearity and accuracy of the Gage are verified during
calibration against a known thickness and coating material. Once the Gage is
calibrated it can be adjusted to a new sound velocity constant without effecting
its accuracy. Care must be taken during the adjustment process. Sloppy
technique may introduce systematic errors into your readings.
A major condition of making standards is the requirement to maintain
measurement traceability through proper technique and uncertainty analysis. It
appears quite simple to coat a substrate with your material to use as a standard.
But it is necessary to be able to measure the thickness of that coating to an
accuracy better than that of the Gage. According to various directives and
common usage, the measurement of the thickness standard has to be at least four
(4) times better than the accuracy of the Gage at that thickness reading. Lets
look at an example.
Say you are trying to prepare a Check Standard for use with the Model 100 with
a C probe at a thickness of 40 mils for an Imperial example or 1000 microns for
a metric example. The accuracy of the C probe at this thickness is calculated as:
Accuracy =
±
0.1 mil + 3% of reading
Accuracy =
±
[0.1 + (0.03)(40)] =
±
1.3 mils
Accuracy =
±
2 microns + 3% of reading
Accuracy =
±
[2 + (0.03)(1000)] =
±
32 microns
Therefore the Check Standard must be measured to better than one-fourth of this
accuracy or
±
0.325 mils or
±
8 microns or better. In this case we can use a
precision micrometer that has been calibrated to an accuracy of
±
0.1 mil (
±
2.54
microns) or better. If you have a micrometer that has been calibrated to this
accuracy, you can go ahead and follow the rest of this procedure and make your
Check Standards.