How to Establish the Calibration & Measurement Capability (CMC)?


Hello to all the members of the forum.
Our laboratory is in the process accreditation for the calibration of sound level meters. Procedures validation has already been carried out and uncertainties have been evaluated by calibrating instruments of ordinary use. However, to obtain the accreditation one of the requirements is to stablish the CMCs, understanding this as the uncertainty which "...shall include the contribution from a best existing device to be calibrated such that the CMC claimed is demonstrably realisable"... "The term BEST EXISTING DEVICE is understood as a device to be calibrated that is commercially or otherwise available for customers"... (ILAC P-14).
But, these statments from guide ILAC P-14 are precisely what we do not completly understand, because:
a) How to know which can be the "best instrument available"? and
b) Would it be possible to calculate theoretically the CMCs: introducing in the already defined uncertainty budget the specifications of a "best instrument", selected, for example, from manufacturers' catalogs".
Thank you in advance for your valuable comments.


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a) When calculating the CMC you will need to include all uncertainty contributors such as uncertainty of your calibration standards, environmental effects, repeatability, etc. These contributors must also include the accuracy and resolution of the device being calibrated. Based on the capabilities of your standards what is the "best" instrument that you can calibrate with your equipment while maintaining an adequate test uncertainty ratio? The uncertainty contributors of this "best existing device" must be included in your CMC calculations.
b) Yes, it is acceptable to use reliable published uncertainties in your calculations. If you can find the uncertainty of what would be the "best existing device" that you are capable of calibrating you can use that. Be sure to document your data source as part of your records.
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