Laboratory Accreditation, Traceability, and the Role of Mutual Recognition Arrangements
The following is a brief description of the roles of laboratory accreditation, the mutual recognition arrangements amongst laboratory accreditation bodies, and the mutual recognition arrangement amongst national metrology institutes as they relate to issues of traceability of measurement results.
Laboratory accreditation is recognition by an authoritative body that a laboratory is capable and competent to provide scientifically sound and valid calibration or testing services as documented on its scope of accreditation and on the calibration certificates and/or test reports it issues. To promote the use of laboratory accreditation worldwide, the National Voluntary Laboratory Accreditation Program (NVLAP) has entered into Mutual Recognition Arrangements (MRAs) or Multilateral Recognition Arrangements (MLAs) with the following organizations:
While laboratory accreditation does verify the capability of a laboratory to produce measurement results that are traceable to appropriate stated references (usually measurement standards maintained by the national metrology institute (NMI) of the laboratory's home country), it does not speak directly to the equivalence of traceability of measurement results to standards maintained by one NMI versus another. Traceability is, however, ultimately established to the SI (the International System of Units). Calibration and testing laboratories are required to disclose the path of traceability of their measurement results, where it is necessary and appropriate, in the calibration or test reports. It is through the dissemination of calibration services that the NMIs provide the necessary link to these SI units.
In response to the need for an open, transparent and comprehensive scheme to give users reliable quantitative information on the comparability of national metrology services and to provide the technical basis for wider agreements negotiated for international trade, commerce and regulatory affairs, the directors of the NMIs of a number of Member States of the Metre Convention and representatives of two international organizations have signed a Mutual Recognition Arrangement titled "Mutual recognition of national measurement standards and of calibration and measurement certificates issued by national metrology institutes," also referred to as the Comité International des Poids et Mesures (CIPM) MRA.
As the title states, this arrangement provides for the mutual recognition of national measurement standards and of calibration and measurement certificates issued by NMIs, and is founded on the efforts of each individual NMI to base its measurements and measurement uncertainties on SI units. A complete list of signatory NMIs is included as Appendix A of the CIPM MRA.
The technical basis of this arrangement is the set of results obtained in the course of time through key comparisons carried out by the Consultative Committees of the CIPM, the Bureau International des Poids et Mesures (BIPM) and the regional metrology organizations (RMOs), and published by the BIPM and maintained in the key comparison database as Appendix B of the CIPM MRA.
It is the equivalence of the calibration services offered by NMIs that is verified, or tested, by the key and supplementary comparison activities. The NMIs that have signed the MRA agree that each NMI is capable of providing the services listed in Appendix C, Calibration and Measurement Capabilities (CMC), which lists the quantities for which calibration and measurement certificates are recognized. Appendices B and C can therefore be used as indicators of the equivalence of measurement results between NMIs. For more details on the CIPM MRA, the key comparison database, and the CMCs, please see http://www.bipm.org/en/convention/mra/.
In summary, laboratory accreditation is formal recognition of the capability and competence of a laboratory to provide measurement results within its Scope of Accreditation. The calibration and test reports issued provide measurement results that are traceable to stated references (the SI, usually through standards maintained by the NMI of the laboratory's home country). The quoted uncertainties are based on an established path of traceability through an NMI to the SI units. The CIPM MRA provides for the mutual recognition of national measurement standards and of calibration and measurement certificates issued by NMIs. It logically follows that, metrologically speaking, it is possible for users of calibration and test results provided by accredited laboratories (accredited by MRA signatory ABs) to determine for themselves the fitness for purpose of measurement results, such as for traceability to an NMI not of the laboratory's home country, based primarily on the reported uncertainties.
NVLAP strongly encourages all to visit the referenced web sites to see how the recognized accreditation bodies and the signatory NMIs are working in concert, through their respective MRAs, to provide confidence in the results reported in the calibration and test reports issued by laboratories accredited by a signatory MRA accreditation body. Of particular interest are the following informational brochures, available on the ILAC web site under Publications and Resources/ILAC Documents/Brochures:
1. Why Use an Accredited Laboratory?
2. Why Become an Accredited Laboratory?
3. How Does Using an Accredited Laboratory Benefit Government and Regulators?
4. The Advantages of Being an Accredited Laboratory.