Calibration

The CMC is normally stated numerically, but where the CMC is a function of the quantity to which it refers (or any other parameter), it should be given in the form of an equation. For example, the dominant uncertainty contributor for calipers is often resolution. In these cases, the best way to represent uncertainty is as a function of the resolution (e.g., U = 0.6R) where “R” is the resolution in microinches. Similarly, gage blocks often quantify uncertainty based on length (e.g., U = 4 + 3L) where “L” is the length of the block in inches.

The following is taken from ILAC-P14:11/2010 “ILAC Policy for Uncertainty in Calibration”:

In the context of the CIPM MRA and ILAC Arrangement, and in compliance with the CIPM-ILAC Common Statement, the following definition is agreed upon:

A CMC is a calibration and measurement capability available to customers under normal conditions:

• As described in the laboratory’s scope of accreditation granted by a signatory to the ILAC Arrangement; or
• As published in the BIPM key comparison database (KCDB) of the CIPM MRA.

The following is adapted from ILAC-P14:09/2020 ILAC Policy for Uncertainty in Calibration:

The term “best existing device” is defined as a device to be calibrated that is commercially or otherwise available for customers, even if it has a special performance (stability) or has a long history of calibration.

In exceptional instances, it is recognized that a “best existing device” does not exist and/or contributions to the uncertainty attributed to the device may significantly affect the uncertainty. If such contributions to uncertainty from the device can be separated from other contributions, then the contributions from the device may be excluded from the CMC statement. For such a case, however, the scope of accreditation shall clearly identify that the contributions to the uncertainty from the device are not included.

Example: “The contributions from the ‘best existing device’ are not included in the CMC claim.”

In general, no. CMCs are usually calculated based on optimistic (but realistic) estimates of the uncertainty contributors. Unless the conditions of an actual calibration match the assumptions used to estimate the CMC, the calibration uncertainty will generally be larger than the CMC.

The uncertainty of reported measurements has to be stated as the actual uncertainty of the measurement, not as the accredited CMC unless that CMC actually applies. An indiscriminate use of the CMC as the uncertainty of an actual calibration is not justified.

On the other hand, some calibration labs will calculate CMCs based on the worst conditions that they’re likely to see. In these cases, the CMC on the scope of accreditation will be large enough to cover all of those conditions. This is why some labs quote their accredited CMCs for most of their work. Although this method is acceptable, it has the disadvantage that it doesn’t allow the lab to quote a smaller uncertainty if a calibration is performed under conditions better than were assumed for the scope of accreditation.

The definition of Calibration and Measurement Capability (CMC) implies that, within its accreditation, a laboratory is not entitled to claim a smaller uncertainty of measurement than the CMC. This means that the laboratory shall be required to state a larger uncertainty than that corresponding to the CMC whenever it is established that the actual calibration process adds significantly to the uncertainty of measurement. Typically, the equipment under calibration may give a contribution. The actual uncertainty of measurement can never be smaller than the CMC.

No. Contributions to the uncertainty stated on the calibration certificate shall include contributions that can reasonably be attributed to the customer’s device. The exclusion option only applies for the uncertainty calculation that supports the calibration and measurement capabilities (CMCs) on the Scope of Accreditation.