BIPM - international equivalence: the CIPM MRA

The CIPM Mutual Recognition Arrangement (CIPM MRA) is the framework through which National Metrology Institutes demonstrate the international equivalence of their measurement standards and the calibration and measurement certificates they issue. The outcomes of the Arrangement are the internationally recognized (peer-reviewed and approved) Calibration and Measurement Capabilities (CMCs) of the participating institutes. Approved CMCs and supporting technical data are publicly available from the CIPM MRA database (the KCDB).

The CIPM MRA responds 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 CIPM MRA has been signed by the representatives of 102 institutes – from 57 Member States, 41 Associates of the CGPM, and 4 international organizations – and covers a further 156 institutes designated by the signatory bodies.

[ Click here for the full list of participants ]

The Regional Metrology Organizations (RMOs) play an important role in the CIPM MRA. The RMOs are responsible for carrying out comparisons and other actions within their regions to support mutual confidence in the validity of the calibration and measurement certificates of their member NMIs. Through the Joint Committee of the RMOs and the BIPM (JCRB), they carry out an inter-regional review of declared capabilities before approved CMCs are published in the KCDB, and they make policy suggestions to the CIPM on the operation of the CIPM MRA.

More information on the CIPM MRA is available by clicking on the tabs above, or following the links below:

The BIPM key comparison database (KCDB) is defined in the text of the CIPM MRA as "the database maintained by the BIPM (Bureau International des Poids et Mesures) which contains Appendices A, B, C and D of the Mutual Recognition Arrangement".

It comprises:

List of national metrology institutes that are signatories to the arrangement (Appendix A).
Key and supplementary comparisons (Appendix B)
For each key comparison the following are included:
- individual values for each institute, together with their declared uncertainties;
- the key comparison reference value with its associated uncertainty;
- for each institute, the deviation from the key comparison reference value and the uncertainty in that deviation (at a 95 % level of confidence), i.e. its degree of equivalence;
- the degrees of equivalence between the standards of each of the participating institutes.
Calibration and Measurement Capabilities – CMCs (Appendix C): Quantities for which calibration and measurement certificates are recognized by institutes participating in part two of the agreement. The quantities, ranges and calibration and measurement capabilities expressed as an uncertainty (normally at a 95 % level of confidence) are listed for each participating institutes.
List of key comparisons (Appendix D)

More information about the KCDB:

The outcomes of the CIPM MRA are the internationally recognized (peer-reviewed and approved) Calibration and Measurement Capabilities (CMCs) of the participating institutes. Approved CMCs are publicly available in the CIPM MRA database ().

The three fundamental elements leading to approval of an institute's CMCs are:

participation by the institute in reviewed and approved scientific comparisons;

operation by the institute of an appropriate and approved quality management system;

international peer-review (regional and inter-regional) of claimed calibration and measurement capabilities.

A generalized overview of the process is given below; for full details please refer to the text of the CIPM MRA .

Click here for the full list of participants, or
select a Member State or Associate using the dropdown list below:

The CIPM MRA is open to:

the NMIs of the Member States of the BIPM,
certain international organizations invited by the CIPM, and
the NMIs of Associate States and Economies of the General Conference.

This latter category results from the decision of the CGPM at its 21st meeting (1999) to create a category of Associate of the CGPM (see Resolution 3) with the specific purpose of providing a way of establishing links to the world's measurement system for those States not yet Members of the BIPM. The NMIs of Associates participate in the CIPM MRA through their local Regional Metrology Organization (RMO) as specified in the text of the CIPM MRA.

The rules about participation in the CIPM MRA, and the procedures for a State to become a Member State of the BIPM or for a State or Economy to become an Associate of the General Conference, can be found below:

The technical basis of the CIPM MRA is the set of results obtained over the course of time through scientific key comparisons carried out by the Consultative Committees of the CIPM, the BIPM and the Regional Metrology Organizations (RMOs), and published by the BIPM and maintained in the CIPM MRA database ().

When drawing up the CIPM MRA, the CIPM devised the following scheme – sometimes called the "Mickey's ears scheme" – for the organization of key comparisons:

	National metrology institute (NMI) participating in CIPM key comparisons
	NMI participating in CIPM key comparisons and in regional metrology organization (RMO) key comparisons
	NMI participating in RMO key comparisons
	NMI participating in ongoing BIPM key comparisons
	NMI participating in a bilateral key comparison
	International organization signatory to the MRA

The key comparisons are essentially of two types:

CIPM key comparisons, of international scope, are carried out by those participants having the highest level of skills in the measurement involved, and are restricted to laboratories of Member States. The CIPM key comparisons deliver "the reference value" for the chosen key quantity;

RMO key comparisons, of regional scope, are organized at the scale of a region (though they may include additional participants from other regions) and are open to laboratories of Associates as well as Member States. These key comparisons deliver complementary information without changing the reference value.

The CIPM delegates the task of organizing comparisons to its Consultative Committees (CCs). Each CC chooses the key quantities to be compared in its field of expertise, approves the protocols of the key comparisons, and approves their results before publication in the KCDB. Each CC is composed of the world's most competent laboratories in the field, and it is some or all of these CC members that participate in the CIPM key comparison, which is then generally referred to as a "CC key comparison" since the field of expertise is defined.

The RMOs organize corresponding RMO key comparisons with a number of common participants and with protocols allowing their results to be linked to those of the CC key comparison once these are treated in terms of equivalence (calculation of a key comparison reference value and of degrees of equivalence). The nomenclature used in the KCDB reflects this organization; for instance CCAUV.A-K1 is the CC comparison to which the regional equivalents APMP.AUV.A-K1, EURAMET.AUV.A-K1, etc. are linked.

CC and RMO key comparisons are carried out over a given period of time, and are then analysed and published. Each exercise may be repeated, in part or in full, according to need. However, a different organizational scheme is required in the case of the unique international facilities kept at the BIPM. For these, comparisons are organized as ongoing series of bilateral comparisons between the BIPM and outside laboratories, and continue as long as there is a need. When such a comparison involves a key quantity chosen by a CC, it is called a "BIPM key comparison".

Together, the set of CC key comparisons and BIPM key comparisons forms the ensemble of CIPM key comparisons conducted in the framework of the CIPM MRA.

A BIPM key comparison can (and often does) serve as the central comparison of the Mickey's ears scheme. The results of a BIPM key comparison are interpreted in terms of equivalence, with a key comparison reference value and degrees of equivalence. Since these comparisons are ongoing, there is a continuous accumulation of new data. According to the case (and the decision of the relevant CC), the key comparison reference value may be re-evaluated when new data arrive (as is the case for the International System of Reference for radionuclides) or may remain constant (as is the case for voltage comparisons, where the key comparison reference value is delivered by the BIPM Josephson standard). Beside the role of central key comparison, a BIPM key comparison may also serve as an on-demand exercise whenever waiting for the next full-scale international CC key comparison is impratical. This option is especially valuable when facilities in the laboratories are not simultaneously available but rather are introduced or modified over the years.

CIPM key comparisons is a generic term to designate the real exercises "CC key comparison" and "BIPM key comparison". Because it allows flexibility, the different nature of CC and BIPM key comparisons (full-scale exercise over a finite period of time versus a permanent ongoing series of bilateral comparisons) facilitates the implementation of the CIPM MRA. The CCs can use these tools at their convenience to best fulfil their needs.