National Physical Laboratory

Uncertainty of measurement and ascertaining conformance

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Uncertainty of measurement

You cannot ascertain conformance unless you know the uncertainty of measurement.

Uncertainty of measurement online module

Ascertaining conformance

Is it possible to measure the dimensions specified on the drawing to ascertain if the tolerance has been met?

Yes! If you check:

  • Appropriate equipment is available to ascertain conformance.(New measuring equipment should have been purchased and evaluated as necessary early in design process)
  • Designer has not specified tolerances for which the technology does not exist to prove conformance/non-conformance.
  • Early in the design stage a measurement process for each toleranced feature should have been agreed
  • If ascertaining compliance is going to be difficult, given the available equipment at your disposal ,that tolerances are reasonable.  – e.g. Check with the designer that the tolerance needs to be that tight for the part to function correctly.
  • Measurement of partial features on a drawing can be problematical. If partial features are wrongly specified on the drawing it is worth discussing with the designer if the features can be specified in a more appropriate way.
  • Datum features on the drawing are of the appropriate size.  Datum features are critical to all measurements and small measurement errors have larger effect when measuring a datum that is small in size compared to the feature being evaluated.
  • Problems due to access of features - Ideally such problems can be anticipated and solved at design time.

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