At a turn of the 20th to the 21st centuries, the methods and systems used to ensure the measurement uniformity in the world radically changed. This was caused by the need to significantly reduce the influence of international trade barriers, which put obstacles in the way of its further development and hampered the progress of business, health protection, preservation of the environment, cooperation in industry, etc.
At a meeting held in Paris on October 14, 1999, the directors of the national metrol- ogy institutes (NMIs) of 38 states signed a Mutual Recognition Arrangement (CIPM MRA) for the mutual recognition of national measurement standards and for calibra- tion and measurement certificates issued by NMIs [119]. Later on a number of other institutes also signed this document. At present it is possible to get acquainted with information about all the MRA signatory countries at www.bipm.org. (The CIPM MRA has now been signed by the representatives of 90 organizations, from 51 member states, 35 associates of the General Conference on Weights and Measures (GCPM), and 4 international organizations, and covers a further 144 institutes designated by the signatory bodies.)
The MRA is a significant and important step towards further development of the International Measurement System (IMS). The IMS is based on the system of physical quantity units (SPQU) and decisions of the GCPM, which is the highest international body on issues of establishing the units of quantities, their definitions, and the methods of reproducing their size.
The Joint Committee of the Regional Metrology Organizations (JCRB) and the BIPM were created with the purpose of promoting the cooperation of metrological institutes and organizations on the basis of discussion on and solutions for mutual me- thodical problems connected with implementation of the MRA. The MRA is put into effect by the CIPM committees, five regional organizations (EURAMET, COOMET, APMP, SIM, AFRIMET), and national metrological institutes, signatories of the CIPM MRA.
The measurement uniformity assurance implies solving a complex of scientific- technical, legal, and organizational problems related to the development of measure- ment standards, enhancement of methods for transferring the size of physical quantities units, and the creation of modern advanced measuring instruments.
If the scientific-technical problems are first solved within the framework of national systems of measurement uniformity assurance, then the approaches to a solution of legal and organizational problems are developed, made to agree, and implemented within the framework of international organizations such as OIML, ISO, and others.
Among the fundamental documents one ought first mention Recommendations: OIML D 1 Elements for a Law on Metrology, OIML D 5 Principles for establishment of hierarchy schemes for measuring instruments, OIML D 8 Measurement standards:
choice, recognition, use, conservation, and documentation, and others.
Close cooperation in the field of metrology is impossible without a common termi- nology and unified methods applied to express the accuracy of measurements. The ap- pearance of a new version of the International Vocabulary of Metrology (VIM) [246]
has become an important step in the development of a “unified language”. This version reflects the present-day trends of metrology development, which are connected with the dissemination of measurements to new fields such as medicine and biology. This has resulted in the expansion of the concepts “quantity”, “measurement”, and changes of the terms linked with the concept “metrological traceability”. The appearance of the generalizing primary concept “reference”, which, depending on the context, may be used as a measurement unit or standard, or as a reference procedure, deserves special attention.
The mutual recognition of measurement results rests on the assurance of the compat- ibility of measurement results, i.e., on such a property of measurement results which does not allow the difference of values measured to overstep limits corresponding to measurement uncertainties.
The reliability of accuracy estimates is provided by certifying measurement pro- cedures and by using a hierarchy scheme for calibrating measuring instruments and standards which ensures the traceability of a measurement result to a reference mea- surement standard. Comparison of national standards leads to the establishment of their equivalence, which can be considered as the final stage of establishing the mea- surement result traceability to a measurement unit (Figure 1.1).
In 1993, under the auspices of the international organizations BIPM, IEC, ISO, OIML, the International Union of Pure and Applied Chemistry (IUPAC), the Interna- tional Union of Pure and Applied Physics (IUPAP), and the International Federation of Clinical Chemistry (IFCC), a document, called the Guide to the Expression of Un- certainty in Measurement (GUM) [243] was developed and since then has begun to be implemented into everyday practice.
At present the Guide is the main international framework document on the assess- ment of the accuracy of measurements. The supplements to the GUM are worked out as the methods of accuracy assessment are developed and enhanced. By this time Sup- plement 1 [244], devoted to the application of the Monte Carlo method for transfor- mation of probability density functions in evaluating the uncertainty of measurements, and Supplement 2, disseminating the evaluation of uncertainty to the models with an arbitrary number of output quantities, have been issued.
Section 1.1 Principles underlying the international measurement system 3
Metrological tracebillity Calibration hierarchy
Measured value x1 Uncertainty u1 Measured value x2 Uncertainty u2
Metrological tracebillity Calibration hierarchy
Reference
National standard I National standard II
Measurement result 1 Measurement result 2
Key comparison
Metrological compatibility of measurement results
2 2
1 2 1 2
x−x ≤k u +u
Figure 1.1.Assurance of the compatibility of measurement results.
This chapter deals with the methods of evaluating measurement results in key com- parisons of national measurement standards and calibration of measuring instruments.
The main purpose of the MRA is to create an objective base for the mutual recogni- tion of the results of measurements and measuring instrument calibrations obtained by different national laboratories. This main purpose is realized by solving the following problems:
fulfillment of international comparisons of national measurement standards, called key comparisons, following well-defined procedures, which lead to a quantitative expression of the degree of equivalence of national measurement standards – par- ticipants of the key comparisons;
successful participation of each NMI in corresponding supplementary comparisons of national measurement standards;
creation of quality management systems in each NMI;
demonstration of the competence of each NMI in the form of a declaration of cali- bration and measurement capabilities (CMC) of a given NMI, which is entered into a common database on key comparisons supported by the BIPM and open to general use through the internet (key comparison data base: www.bipm.org).
The key comparisons of national measurement standards are carried out by the Consultative Committees of the CIPM, BIPM, and regional metrology organizations (RMO). The key comparisons carried out by the Consultative Committees or BIPM are considered to be the CIPM key comparisons; the key comparisons performed by regional metrology organizations are considered as the RMO key comparisons. The RMO key comparisons have to be linked to the CIPM key comparisons through the results of the NMIs participating in both comparisons.
Participation in a CIPM key comparison is open to those laboratories which possess the highest technical competence and experience, i.e. for those that are members of a corresponding Consultative Committee. In the selection of participants by the Con- sultative Committees, the interests of regional representatives are necessarily taken into account. The number of laboratories participating in CIPM comparisons can be limited by technical reasons.
Participation in the key comparisons arranged by some RMO, and supplementary comparison is open to all members of this RMO and to other institutes which comply with the rules of this RMO and have the required technical competence with regard to each particular comparison.
The CIPM key comparison results in obtaining reference values (KCRV) of key comparisons. In most cases a reference value is understood to be the best estimate of a measurand obtained on the basis of results of all participants. The concept of a degree of equivalence of measurement standards means the degree of correspondence of the results obtained by the participants of a given comparison to a reference value of this key comparison.
The degree of equivalence of each national measurement standard is quantitatively expressed by two quantities: a deviation from a reference value of a key comparison, and the uncertainty of this deviation (at the level of confidence 95 %). The degree of equivalence between pairs of national measurement standards is expressed by a difference of their deviations from reference value and uncertainty of this difference (at the level of confidence 95 %).
The RMO key comparisons disseminate the metrology equivalence established in the process of CIPM key comparisons to a great number of the NMIs which for various reasons do not participate in the CIPM key comparisons. The results of the RMO key comparisons are related to the reference values established at the CIPM key compar- isons on the basis of results obtained by the participating institutes, both in the CIPM and RMO comparisons (linking institutes).
Regional organizations are responsible for
linking with CIPM key comparison through participation of a sufficient number of laboratories in both comparisons, in order to ensure that linking with the reference value of the key comparison is established with a very low level of uncertainty;
procedures used in the regional comparisons – evaluation of measurement data has to be compatible with the procedures applied in CIPM key comparisons;
Section 1.2 Classification of key comparisons of national measurement standards 5
coordination of time for carrying out RMO and CIPM key comparisons (at least their frequency);
evaluation of results of RMO key comparisons and supplementary comparisons, ap- plication of appropriate procedures, presentation of results for publication in Sup- plement B of the key comparison database;
postponing the linking of reference value of a RMO key comparison, carried out before the beginning of a corresponding CIPM key comparison to a reference value of this CIPM key comparison until both key comparisons are completed.
The CIPM and RMO key comparison results, reference values of key comparisons, deviations from reference values, and their uncertainties are published in the BIPM database (Appendix B).
The calibration and measurement capabilities of NMIs are represented in the form of quantities, ranges, and expanded uncertainty (usually at a level of confidence of 95%).
The CMC are listed for each NMI in Appendix C of the BIPM database. They must be confirmed by the results obtained in the key comparisons indicated in Appendix B.