What is CMC in calibration?

A laboratory's accredited scope lists, for each quantity and range, a Calibration and Measurement Capability (CMC). It is the smallest measurement uncertainty the laboratory can achieve during a routine calibration of a near-ideal device under normal conditions. It is a statement about the laboratory, not about any particular customer instrument.

CMC is a floor, not a promise

The uncertainty reported on your certificate can never be smaller than the CMC, and will usually be larger because the instrument under test contributes its own resolution, repeatability, drift and — for many devices — the inability to reach a truly stable reading. A laboratory with a CMC of 20 mK will not give a 20 mK uncertainty for a 0.1 °C-resolution digital thermometer.

How CMC differs from accuracy

• Accuracy is a qualitative closeness-to-true-value concept; manufacturers often use it for a specified maximum error.
• CMC is a quantified expanded uncertainty (typically k = 2) tied to an accredited scope.
• Accuracy describes the instrument; CMC describes the laboratory's capability.

Using CMC when choosing a laboratory

Compare the required uncertainty for your application with the laboratory's CMC for the right quantity, range and, where relevant, medium and method. Ask how the reported uncertainty will be built up for your device. The CMC alone does not tell you the laboratory can calibrate your specific instrument to the uncertainty you need.