Traceability 


What is Traceability and Why is it Important?

In practice you probably have seen statements like “Directly Traceable to NIST”. But what does it mean? First let’s look at the definition for metrological traceability: Property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty. ​

Traceability is not only desirable but a requirement per ISO/QS 9000 quality systems. Calibration and traceability requirements are often interpreted as only requiring a calibration sticker on the measuring equipment and the reference to a NIST test number on a calibration certificate. Of importance to note is that traceability is the property of the result of the measurement, not of a standard, calibration report or laboratory. Only the measurement result for a standard can be traceable, and only if the standard was appropriately maintained, used, and documented.


The BIPM, OIML, ILAC, and ISO endorse the following recommendations on metrological traceability:​

  • In order to be able to rely on their international acceptability, calibrations should be performed in National Metrology Institutes which should normally be signatories to the CIPM MRA and have CMCs published in the relevant areas of the KCDB or in laboratories accredited to ISO/IEC 17025 by accreditation bodies that are signatories to the ILAC Arrangement.
  • Measurement uncertainty should follow the principles established in the GUM (Guide to the Expression of Uncertainty in Measurement)​ AC Wattmeters - At any combination of above voltage and current points.
  • The results of the measurements made in accredited laboratories should be traceable to the SI.
  • NMIs providing metrological traceability for accredited laboratories should normally be signatories to the CIPM MRA and have CMCs published in the relevant areas of the KCDB.​
  • Within the OIML-CS, accreditation should be provided by bodies which are signatories to the ILAC Arrangement and the above policies on metrological traceability to the SI should be followed.​

When can you claim that your measurement is traceable?

For a measurement in your organization to be considered traceable, the organization must:

Schedule Regular Calibrations – The calibration of each measurement standard in an organization must be calibrated at regular intervals. If the calibration expires for any standard in the traceability chain, the traceability is broken.

Use Competent Laboratories – The calibration of measurement standards needs to be conducted by an accredited calibration laboratory or by a National Metrology Institute. This ensures that the laboratory is competent to perform the calibrations and properly calculate the measurement uncertainty values.

It is critical that the calibration certificates for equipment that are sent to an external calibration laboratory are reviewed to ensure that the proper information is included on the certificate. The following items must be included on the calibration certificate to state that the measurement value is traceable: 

Reference Standards Used

The calibration certificate must list the reference standards that the external calibration laboratory used in the calibration. Those standards provide the link to the chain of comparisons that establishes a connection to the national or international reference standards. 

Documented Measurement Uncertainty

Every quantitative measurement needs to include the measurement uncertainty value. If the uncertainty information is missing from the calibration certificate, you cannot claim that the measurement is traceable. 

Documented Measurement Procedure

The calibration certificate must list the procedure that was used for the calibration. The calibration must be completed according to a written procedure that is a part of the external calibration laboratory’s quality system. 

Traceability

Radian is ISO/IEC-17025:2005 accredited and offers calibration/verification services through its global network of CSP’s (Certified Service Partner). ISO/IEC 17025 is the General requirements for the competence of testing and calibration laboratories and is the standard for which most labs must hold accreditation in order to be deemed technically competent. In many cases, suppliers and regulatory authorities will not accept test or calibration results from a lab that is not accredited. Please see diagram below on how Radian maintains IEC17025 accreditation and traceability to NIST/SI measurements.

Radian Research Chain of Traceability

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