National metrology institutes

What if instruments could be designed which were remoteiy self-calibrating with respect to calibration services of a National Metrology Institute (NMI) While this may sound very far-fetched , it is not difficult to speculate that future application of telemetry, transducers, intrinsic standards, detectors and computers will link some instruments directly to a NMI to obviate their need for RMs (Rasberry 1998). 

Detailed information relating to metrology in chemistry and traceability can be obtained from these web-sites. The addresses include international and regional organizations as well as some national metrology institutes. 

National Metrology Institute of Japan (NMIJ/AIST)... 

Euro-Asian Cooperation of National Metrological Institutions (COOMET) http //www.coomet.org/... 

At the metrological top, the CGPM defines SI units, and its executive, the CIPM, oversees their dissemination [12], The rather newly established CCQM organizes Key Comparisons of measurements on reference materials among the national metrology institutes to ensure adequate measurement capabilities at the highest metrological level. Several of these materials are relevant to calibration hierarchies in laboratory medicine. 

National standards are usually kept at the national institutes responsible for their development and maintenance and are used only for linking up secondary standards to them, which then are made available as transfer standards. In most countries this responsibility has been entrusted by law to the national metrology institutes (NMIs). In the field of chemical measurements for which traceability to the SI units and the development of national measurement standards have been the focal point of interest for only a few years, decentralized national responsibilities are now developing in such a way that high-level national chemistry institutes are entrusted in part with the task of maintaining national standards by agreements with NMIs. In this way the national reference level can be established on a broad basis, and this is underway now in many countries. 

Fig. 1 Traceability system for the determination of the most important diagnostic markers in human body fluids in Germany. The clinical reference laboratories at the intermediate level providing calibration means to the routine medical laboratories are accredited as calibration laboratories in the framework of the German Calibration Service (DKD) and are firmly linked to the national metrology institute, PTB, by comparison measurements carried out on actual laboratory samples. Accreditation is in part required by the Federal Physicians Council (BAK) or is voluntary. The traceability system is still under development...

BIPM (1999) Mutual recognition of national measurement standards and of calibration and measurement certificates issued by national metrology institutes, Bureau International des Poids et Mesures (BIPM), Pavilion de Breteuil, 92312 Sevres Cedex, France Internet www.bipm.fr... 

Abstract This paper reviews the current state of play of the Mutual Recognition Arrangement created by the International Committee for Weights and Measures in 1999. The aim of the MRA is to provide a framework within which National Metrology Institutes can demonstrate the equivalence of their realisations of the units and quantities of the SI system to which accredited laboratories are traceable. The article offers some views on the need for traceable... 

Abstract Current developments in Germany for establishing a traceability system for chemical measurements are reported. The focus is on a dissemination mechanism which employs chemical calibration laboratories accredited within the framework of the German Calibration Service (DKD) and acting as multipliers between the national standards level and the user level by providing the user with calibration means which are traceable to the SI via national standards. At the national standards level, a network of high-level chemistry institutes coordinated by the national metrology institute, PTB, provides the primary references for chemical measurements. 

The national metrology institute, PTB, coordinates the network on the basis of its legal mandate and its competence in those parts of chemical analysis which are relevant to the major demands for traceability. The contributions to the network as listed in the boxes are based on agreements between PTB and the three network part-... 

To have a network of laboratories at the top of a trace-ability system for chemical measurements instead of just the national metrology institute seems to be a requirement typical of metrology in chemistry and is under consideration in many industrialized countries, because the competence for chemical analysis in most countries (except U.S.A.) largely lies outside the domain of the metrology institutes. Another example that underpins this view is the development of metrology in chemistry in Switzerland, where the Swiss Federal Office of Metrology and Accreditation (METAS) and the Swiss Federal Laboratories for Materials Testing and Research (EMPA) jointly provide the national references for chemical measurements [2]. 

A tested example of a practical traceability structure has been described which has already proved useful in several fields of chemical analysis. It makes use of a national calibration service as a successful and efficient dissemination mechanism. An essential part of the structure is a network of high-level chemistry institutes at the national standards level providing the end points of traceability and coordinated by the national metrology institute. The need for a network of competence seems to be typical for metrology in chemistry for which in most countries the resources are largely to be found outside of the national metrology institute. 

Fig. 1 Metrological traceability and hierarchy of procedures and materials (according to ISO/IEC 17511). uc(y) Uncertainty BIPM International Bureau of Weights and Measures NMI National Metrology Institute ACL Accredited Calibration Laboratory MCL Manufacturer s Calibration Laboratory ML Manufacturer s Laboratory Mf Manufacturer...

One of the most important tasks of National Metrology Institutes is to assure the traceability of measurement... 

Another way of providing a link between chemical laboratories and the SI units can use RMs, reference methods and standard measuring devices which are made available by National Metrology Institutes. 

Abstract The primary method for pH is based on the measurement of the potential difference of an electrochemical cell containing a platinum hydrogen electrode and a silver/silver chloride reference electrode, often called a Harned cell. Assumptions must be made to relate the operation of this cell to the thermodynamic definition of pH. National metrology institutes use the primary method to assign pH values to a limited number of primary standards (PS). The required comparability of pH can be ensured only if the buffers used for the calibration of pH meter-electrode assemblies are traceable to... 

The primary method is applied by national metrology institutes to assign conventional pH values to a limited number of primary standard (PS) buffer solutions in dilute aqueous solutions. The experimental details are given in [8, 9] where national standard measurement devices for pH in Denmark and Germany are described. 

NPL, in line with other national standards laboratories, retains its primary standard gas mixtures (PSMs) in-house. These primary standards are disseminated, however, through different types of calibration gas mixtures. These disseminated standards are known at NPL as Primary Reference Gas Mixtures , Secondary Gas Standards and Certified Gas Mixtures . An NPL leaflet has been prepared which explains the differences between these types of traceable standards and which also explains the relationship of these with the different types of standards produced by other national metrology institutes (NMIs). The main type of gas standards disseminated by NPL, are however, secondary gas standards and the procedures used for preparing and certifying these are outlined below. 

Most of the primary measurement standards are the realization of the SI units, and are under custody of each country s National Metrology Institute (NMI). The link between the realization of the SI units and primary standards is established through primary methods of measurement. These are methods which do not require any reference of the same quantity. Additionally, through a series of comparisons between NMIs, comparability of measurements among traceable measurement systems at international level are recognized by each country. 

The reference materials have the form of pellets, globules, shot, wires or bars. They are intended for use as amount of substance standard (traceability to SI) and are applied for preparation of calibration solutions. They are available only for producer of calibration standards or for national metrology institutes. 

The mutual recognition of international standards can be achieved by agreements between National Metrology Institutes (NMI). Calibration as an important activity in establishing traceability should be done by competent personnel. Therefore the competence should be assessed by a third party. Consequently agreements are also needed for accreditation bodies. 

The demand for Certified Reference Materials is much higher than the international infrastructure of metrology institutes can provide. National metrological institutes are often not well enough equipped for efficient production of these materials. 

The used methods are proved by key comparisons between the national metrology institutes. For chemical measurements the Comit0 Consultative pour la Quantit0 de MatiLre (CCQM) has been established. The CMC database provides a reliable service for customers all over the world to establish traceability... 

Even if the concept of primary element standards has already proved a success it covers presently only a few elements. Only an international division of labor can build up this part of measurement infrastructure. In addition to the initiative of CIPM and the national metrology institutes the leading producers of reference materials should cooperate to establish a worldwide system... 

Mutual Recognition of national measurement standards and of calibration-and measurement certificates issued by National Metrology Institutes,... 

Communautaire. Accreditation aims at the reliability of measurement results and enables laboratories to demonstrate technical competence to their customers. Metrological concepts in food science have become a widely discussed topic over the last few years. Representing the top of the international measurement infrastructure, the National Metrology Institutes (NMIs) have the mandate to disseminate the best practice and measurement capabilities to the beld laboratories (FLs) in their countries. The European and international measurement infrastructure needs high-quality tools to assess the reliability and comparability of measurements results in view of the implementation of EU and international policies regarding internal market activities as well as health and consumer protection issues. 

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