Precision, Performance, Confidence. ™

CMC Uncertainties and MET/CAL

As far as the not reporting anything less than your CMC goes –


A2LA guidelines state “2.3.5 Contributions to the uncertainty stated on the calibration certificate shall include relevant short-term contributions during calibration and contributions that can reasonably be attributed to the customer’s device. Where applicable the uncertainty shall cover the same contributions to uncertainty that were included in evaluation of the CMC uncertainty component, except that uncertainty components evaluated for the best existing device shall be replaced with those of the customer’s device. Therefore, reported uncertainties tend to be larger than the uncertainty covered by the CMC. Random contributions that cannot be known by the laboratory, such as transport uncertainties, should normally be excluded in the uncertainty statement. If, however, a laboratory anticipates that such contributions will have significant impact on the uncertainties attributed by the laboratory, the customer shall be notified according to the general clauses regarding tenders and reviews of contracts in ISO/IEC 17025.”



Based on the outlined section above two things are derived from it.

  • The contributions must be documented in the budget based on the ““Calibration and Measurement Capability” (imagine a perfect Device Under Test with no standard deviation of infinite number of readings and a DUT with infinite digits of resolution).
  • When real uncertainties are calculated with the customer unit (DUT), then the real standard deviation and the real DUT resolution should be used.  NOT the best case one that we used in our budget.


So if you calculate your CMC with Standard Deviation of 0 (zero) and UUT resolution of 1x10-15 but still show them (as placeholders) in your uncertainty budget, then…when MET/CAL calculates it will always be larger, when it takes into account the “real” standard deviation and “real” DUT resolution.  Which fully meets and supports the 2.3.5 paragraph!


Let me know if you wish to discuss this, but this has been reviewed by our Chief Corporate Metrologist (Jeff Gust) and our former one (David Deaver) along with several customers during Audits.


The biggest issue is knowing what to state to an Auditor when they ask “why did you not put in a value for those uncertainty components in your budget?”.

As long as you explain the ““Calibration and Measurement Capability” as above then you can do it quite easily with the MET/CAL software.



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