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Calculating Laboratory CMC

     I just took over a calibration laboratory as supervisor and we're planning to go for ISO 17025 accreditation. I haven't had training on uncertainty but I've been reading up on it and I find it isn't really very difficult. I am, however, having trouble calculating our laboratory CMC.

     I've read a number of articles about calculating CMC's and they're not much different from calculating calibration uncertainties.

     Simply speaking, CMC is the smallest uncertainty that a laboratory is capable of for a measurement quantity (voltage, for example). So, the smallest uncertainty would likely come from the laboratory's reference standard (a 5502A, for example).

     If I were to calculate the repeatability and reproducibility uncertainties of the 5502A, I would need another standard to provide the signals against which to obtain readings from the 5502A. Since the 5502A is already has the smallest uncertainty, I don't have that standard. How do I work this out?

     'Hope I'm making sense.

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