Regulation on estimating measurement uncertainy for calibration laboratories

Regulation on estimating measurement uncertainy for calibration laboratories
     With reference to documents such as ILAC-P14:01/2013: ILAC Policy for Uncertainty in Calibration, ISO/IEC Guide 98-3:2008: Guide to the expression of uncertainty in measurement (GUM:1995), EA-4/02 M:2013, Evaluation of the Uncertainty of Measurements in Calibration, AOSC issued Regulations on estimating measurement uncertainty for calibration laboratories to ensure the consistency in presenting measurement uncertainty in calibration certificate and the scope of accreditation for calibration laboratory. These requirements are relevant to the evaluation of measurement uncertainty in calibration, measurement, the calibration and measurement capability (CMC), and the report of measurement uncertainty on the calibration certificates. The regulation is for calibration laboratories and in-house calibration laboratories.
     As the “International Vocabulary of Metrology – Basic and General Concepts and Associated Terms” (VIM) defined, "calibration laboratory" means a laboratory that provides calibration and measurement services. A CMC (Calibration and Measurement Capability) is a calibration and measurement capability available to customers under normal conditions as described in the laboratory’s scope of accreditation, or as published in the BIPM KCDB. “Best existing device” is understood 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.
   AOSC accredited calibration laboratories shall estimate measurement uncertainty for all calibrations and measurements covered by the scope of accreditation.
       Accredited calibration laboratories shall estimate measurement uncertainty in conformity with:
       - “Guide to the Expression of Uncertainty in Measurement” (GUM);
       - ĐLVN 131: Guidance for Evaluating and Presenting Uncertainty in Calibration;
       - TCVN, international standards, regional standards;
       - Technical materials about calibration and testing.

Scopes of Accreditation for Calibration Laboratories
   Each calibration in the scope of accreditation for calibration laboratory shall include the calibration and measurement capability (CMC) for parameter and measuring range.
     The CMC on the scopes of accreditation is the smallest uncertainty of measurement that can be expected to be achieved by a laboratory during a calibration or a measurement with the best existing device.
     The uncertainty covered by the CMC shall be expressed as the expanded uncertainty having a specific coverage probability of approximately 95%. The unit of the uncertainty shall always be the same as that of the measurand or in a term relative to the measurand, e.g., percent. 
     Calibration laboratories shall provide evidence showing that they can provide calibrations to customers so that measurement uncertainties equal those covered by the CMC.

Statement of Measurement Uncertainty on Calibration Certificates 
    ISO/IEC 17025 requires calibration laboratories to give a statement of the measurement uncertainty in the calibration certificate.
    The measurement result shall normally include the measured quantity value y and the associated expanded uncertainty U.  In calibration certificates the measurement result should be reported as y ± U associated with the units of y and U. Tabular presentation of the measurement result may be used and the relative expanded uncertainty U / |y| may also be provided if appropriate.
      The coverage factor and the coverage probability shall be stated on the calibration certificate. For example: “The reported expanded uncertainty of measurement is stated as the standard uncertainty of measurement multiplied by the coverage factor k such that the coverage probability corresponds to approximately 95 %.”
       The numerical value of the expanded uncertainty shall be given to, at most, two significant figures.
       Note: For further details on rounding, see ISO 80000-1:2009
     As the definition of CMC implies, accredited calibration laboratories shall not report a smaller uncertainty of measurement than the uncertainty of the CMC for which the laboratory is accredited. 
Please refer to GR-12 in VLAT Document on AOSC website for more information.

Source: AOSC

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