A016 Traceability of measurement results compared to national and international measurement standards

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1 Version 08 Page 1 de 9 A016 Traceability of measurement results compared to national and international measurement standards Modifications: p 2 South Lane Tower I 1, avenue du Swing L-4367 Belvaux Tél.: (+352) Fax: (+352) olas@ilnas.etat.lu Checked by Monique Jacoby Approved by Dominique Ferrand

2 Version 08 Page 2 de 9 1. Introduction This document is addressed to all organizations (testing or calibration laboratories, laboratories for biomedical analysis, inspection bodies) accredited or candidates for OLAS accreditation that use analysis, testing or calibration equipment in the frame of their activities of conformity evaluation. The assurance of traceability of the measurement results to the national or international measurement standards is an essential condition for the exactness of the result of an analysis, a testing or a calibration. The criteria to which the testing and calibration laboratories must comply are included in the chapters and of the standard ISO/IEC 17025:2005. The criteria to which the laboratories for biomedical analysis must comply are included in the chapter of the standard ISO 15189: The criteria to which the inspection bodies must comply are included in the chapters 6.2.6, 6.2.7, and of the standard ISO/IEC 17020: This document is based on the document ILAC-P10:01/ Definitions: See appendix at the end of this document. 3. Policy: 1. For a calibration laboratory: To guarantee the traceability of the measurement results, the connection of the analysis, testing and calibration equipment to the national or must be assured by: the national calibration institute (NCI) or, the accredited calibration organizations. The proof of connection must be formed: by a calibration certificate issued by a national calibration institute of the area, if the NMI services are covered by the mutual recognition agreement MRA of the CIPM, as can be viewed in appendix C of the BIPM KCDB, or, by a calibration certificate wearing the logo of an accreditation body signatory of the mutual recognition agreement of EA or ILAC. The calibration certificates issued by these laboratories shall contain the measurement uncertainties associated to the calibrated equipment except if it s a requirement of the customer. Then, this requirement must be written in the contract binding the customer to the calibration laboratory and defining the service. In this case, the calibration certificates should be issued without uncertainty of measurement. 2. For a testing, calibration laboratory or an inspection body: To guarantee the traceability of measurement results, the connection of the analysis, testing or calibration equipment must be assured by: a national calibration institute (NCI) or, the accredited calibration organizations or, the internal calibration laboratories of the firms. If the traceability of the measurement results by the connection of the analysis, testing or calibration equipment to the national or is difficult to realize (technical or cost reasons) it must be assured by: the use of certified reference material (or non certified), or,

3 Version 08 Page 3 de 9 the inter laboratory comparisons. The proof of the connection must be constituted: by a calibration certificate issued by a national calibration institute of the area, if the NMI services are covered by the recognition agreement MRA of the CIPM, as can be viewed in appendix C of the BIPM KCDB, or, by a calibration certificate wearing the logo of an accreditation body signatory of the mutual recognition agreement of EA or ILAC or, by a calibration certificate issued internally by the analysis, testing or calibration laboratory for its own account. OLAS reminds that an accredited analysis, testing or calibration laboratory or an inspection body is not accredited to realize calibrations. So they cannot propose a calibration service for third parties because they do not constitute a proof of connection to the International System of Units (SI). For a non-critical equipment of measurement (cf. definition in appendix) entering into the execution process of an analysis, a testing or a calibration, the policy defined above cannot be applied provided that the laboratory proofs that its incidence is not significant, particularly concerning the uncertainty calculation provided with the result.

4 Version 08 Page 4 de 9 Appendix: Definitions 1. Definitions of basic and general terms in metrology: The following definitions are taken out of the International vocabulary of basic and general terms in metrology JCGM200:2012 (VIM) (the numbers between parentheses are making reference to the chapters of the VIM of 1993). Metrological traceability, VIM 2.41 (6.10): Property of a measurement result whereby the result can be related to a stated reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty. For this definition, a «stated reference» can be a definition of a measurement unit through its practical realization, or a measurement procedure including the measurement unit for a nonordinal quantity, or a measurement standard. Metrological traceability requires an established calibration hierarchy. Specification of the stated reference must include the time at which this reference was used, along with any other relevant metrological information about the reference, such as when the first calibration in the calibration hierarchy was performed. For measurements with more than one input quantity in the measurement model each of the input quantities should itself be metrologically traceable and the calibration hierarchy involved may form a branched structure or a network. The effort involved in establishing metrological traceability for each input quantity should be commensurate with its relative contribution to the measurement result. Metrological traceability by itself does not ensure adequate measurement uncertainty or absence of mistakes. A comparison between two measurement standards may be viewed as a calibration if the comparison is used to check and, if necessary, correct the quantity value and measurement uncertainty attributed to one of the measurement standards. The abbreviated term «traceability» is sometimes used for «metrological traceability» as well for other concepts, such as «sample traceability» or «document traceability» or «instrument traceability», where the history («trace») of an item is meant. Therefore, the full term is preferred. Calibration, VIM 2.39 (6.11): Operation that, under specified conditions, in a first step establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication. A calibration may be expressed by a statement, calibration function, calibration diagram, calibration curve, or calibration table. In some cases it may consist of an additive or multiplicative correction of the indication with associated uncertainty. Calibration should not be confused with adjustment of measuring system, often mistakenly called «self-calibration», nor with verification of calibration. Sometimes the first step alone in the above definition is perceived as being calibration.

5 Version 08 Page 5 de 9 Measurement standard, etalon, VIM 5.1 (6.1): Realization of the definition of a given quantity, with stated quantity value and measurement uncertainty, used as a reference. The «realization of the definition of a given quantity» can be provided by a measuring system, a material measure, or a certified reference material. A measurement standard is frequently used as a reference to assign measurement results to other quantities of the same kind, thereby establishing metrological traceability through calibration of other measuring standards, measuring instruments, or measuring systems. In many cases, measurement standards are realizations of the definition of a measurement unit. The standard measurement uncertainty associated with a measurement standard is always a component of the combined standard measurement uncertainty in a measurement result obtained using the measurement standard. Frequently, this component is small compared with other components of the combined standard uncertainty. Several quantities of the same kind or of different kinds may be realized in one device which is commonly also called a measurement standard. The word «embodiment» is sometimes used in the English language instead of «realization». In science and technology, the English word «standard» is used with two different meanings: as a widely adopted written standard, specification, technical recommendation or similar document (in French «norme») and as a measurement standard (in French «etalon»). This vocabulary is concerned solely with the second meaning. The quantity value and measurement uncertainty must be ensured at the time when the measurement standard is used. International measurement standard, VIM 5.2 (6.2): Measurement standard recognized by signatories to an international agreement and intended to serve worldwide. National measurement standard, VIM 5.3 (6.3): Measurement standard recognized by national authority to serve in the country. Primary measurement standard, VIM 5.4 (6.4): Measurement standard whose quantity value and measurement uncertainty are established using a primary measurement procedure. Reference measurement standard, VIM 5.6 (6.6): Measurement standard designated for the calibration of working measurement standards for a quantity of the same kind. Working measurement standard, VIM 5.7 (6.7): Measurement standard that is used routinely to calibrate or verify measuring instruments or measuring systems. Transfer device, VIM 5.9 (6.8): Device used as an intermediary to compare measurement standards. Note : Sometimes, measurement standards are used to compare measurement standards.

6 Version 08 Page 6 de 9 Reference material, VIM 5.13 (6.13): Material, sufficiently homogeneous and stable regarding one or more properties, used in calibration, in assignment of a value to another material, or in quality assurance: «Reference material» comprises materials embodying quantities as well as nominal properties. In this definition, «value» covers both quantity value and «nominal property value». Reference materials with or without assigned quantity values can be used for precision control whereas only reference materials with assigned quantity values can be used for calibration or measurement trueness control. Some reference materials have quantities that are metrologically traceable to a measurement unit outside a system of units. Such materials include vaccines to which International Units (IU) have been assigned by the World Health Organization. The specifications of a reference material should include its material traceability, indicating its origin and processing. In a given measurement, a reference material can only be used for either calibration or quality assurance. A reference material is sometimes incorporated into a specially fabricated device. Certified reference material, VIM 5.14 (6.14): Reference material, accompanied by documentation issued by an authority s body and referring to valid procedures used to obtain a specified property value with uncertainty and traceability. The «documentation» is given in the form of a «certificate», see ISO Guide 30:2015. Valid procedures for the production and certification of certified reference materials are given, e.g., in ISO Guides 34 and 35. In this definition, «uncertainty» covers both measurement uncertainty and uncertainty of a nominal property value, such as for identify and sequence, expressed as probabilities. «Traceability» covers both «metrological traceability» of a quantity value and «traceability of a nominal property value». «Certified reference material» is a specific concept under «reference material». Specified quantity values in certified reference materials require metrological traceability with associated measurement uncertainty. Measurement, VIM 2.1 (2.1): Process of experimentally obtaining one or more quantity values that can reasonably be attributed to a quantity. Measurement implies comparison of quantities or counting of entities. Measurement presupposes description of the quantity commensurate with the intended use of the measurement result, a measurement procedure, and a calibrated measuring system operating according to a specified measurement procedure. Measuring instrument, VIM 3.1 (4.1): Device used for making measurements, alone or in conjunction with supplementary device(s). A measuring instrument alone may be considered to be a measuring system.

7 Version 08 Page 7 de 9 A measuring instrument may be an indicating measuring instrument or a material measure. Measurement accuracy, VIM 2.13 (3.5): «Classical approach» closeness of agreement between a measured quantity value and a true quantity value of the measurand. The concept «measurement accuracy» is not given a numerical value, but a measurement is said to be more accurate when it offers a smaller measurement uncertainty. Measures of measurement accuracy are found in ISO The term «measurement accuracy» should not be used for «measurement trueness» and the term «measurement precision» should not be used for «measurement accuracy». Measurement uncertainty, VIM 2.26 (3.9): Parameter characterizing the dispersion of the quantity values being attributed to a measurand, based on the information used. Measurement uncertainty includes components arising from systematic effects, such as components associated with corrections and the assigned quantity values of measurement standards, as well as the definitional uncertainty. Sometimes known systematic effects are not corrected for but are instead treated as uncertainty components. The parameter may be, for example, a standard deviation called standard measurement uncertainty (or a specified multiple of it), or the half-width of an interval, having a stated coverage probability. Measurement uncertainty comprises, in general, many components. Some of these may be evaluated by Type A evaluation of measurement uncertainty from the statistical distribution of the quantity values from series of measurements and can be characterized by experimental standard deviations. The other components, which may evaluated by type B evaluation of measurement uncertainty, can also be characterized by standard deviations, and evaluated from probability density functions based on experience or other information. 2. Notion of critical measurement equipment: Critical measurement equipment is defined as being equipment that is necessary to the execution of an analysis, a testing or a calibration being part of the accreditation scope and having a significant incidence on the exactness of the result of an analysis, a testing or a calibration. It belongs to the organizations to define the list of critical measurement equipment and to justify this list. The organizations (testing or calibration laboratories, laboratories for biomedical analysis, inspection bodies) accredited or candidates of accreditation must be able to proof that a critical measurement equipment calibration and, consequently, the results of generated measurements by this equipment, corresponding to its area of accreditation, are traceable to the International System of Units. Each time that traceability is not technically realizable, or at an unrealistic cost, the organization, according with the client and the other concerned third parties, can appeal to one or more of the following alternative methods to assure the traceability of its results: Utilization of reference materials or certified reference materials comparable to the measurement standards associated to the physical measure; Application of specific methods (e.g. inter laboratory equipment) and/or measurement standards by consensual agreement clearly defined and agreed by all parties concerned.

8 Version 08 Page 8 de 9 3. Institutions assuring the traceability of measuring results: 1. The National Calibration Institutes: The national calibration institutes are the highest authorities in the area of metrology. They are represented in almost all countries. In most of the cases they are responsible of the realization and the conservation of measurement standards that assure the traceability to the measured physical measures. The exactness of the realization of the measurement standards is assured by the inter laboratory comparisons. If the NCI has the equipment at its disposal to establish the connection to the International System of Units (the term units of the IS covers all derived units), then the measurement standard is identical or directly traceable with regard to the primary measurement standard representing the unit. If the NCI does not have this equipment at its disposal, then it must assure that the measures are traceable with regard to primary measurement standard preserved in another country. The NCI s assure themselves also that the measurement standards are themselves comparable on the international level. They are responsible of the putting at the disposal of the measurement units to the users within the scientific community, the public authorities, the laboratories or the industrial firms. So they represent the superior hierarchic level of the metrology in a country. The NCI s of Western Europe are cooperating in EURAMET. 2. The accredited calibration organizations: The accredited laboratories are often at the summit of the hierarchy of the internal metrology of a firm. Their task is to compare, at appropriate intervals, the internal working measurement standards to the reference measurement standards calibrated by an NCI or by an accredited laboratory disposing of a better capacity measure. Many accredited laboratories are doing calibrations for third parties upon request, for example for the societies that do not possess calibration and appropriate measurement equipment or for the private testing laboratories working in the area of product certification. The results of certification are documented on a calibration certificate. The accreditation bodies (in charge of the accreditation of the calibration laboratories) are grouped together inside of the European co-operation for Accreditation (EA) at the European level in order to guarantee that the calibration certificates, delivered by each accredited laboratory, are accepted by all member states. This implicates that the metrology laboratories and the accreditation bodies are operating in the same manner. To make certain, EA has installed an evaluation system by peers of the accreditation bodies associated to inter laboratory comparisons that, if the final result is positive, conducts to the signature of a Multilateral Agreement (MLA) and to a mutual recognition of the calibration certificates. All official calibration certificates (presenting the accreditation program logo) delivered by an accredited laboratory by a signatory organization of the MLA is equivalent to all other delivered certificate by an accredited laboratory by any other organization signatory of the same MLA. 3. The internal calibration laboratories of the firms: To give proof of the technical competences, the internal calibration laboratories must reply to the following criteria and dispose of: adequacy of equipment a procedure adapted to the expected calibration the connection of its measurement standards through the reference measurement standards to the whenever this is possible a correct estimation of the measurement uncertainty a measurement uncertainty adequate for the expected calibration a sufficiently trained and competent staff

9 Version 08 Page 9 de 9 a quality system conforming to the requirements of the standard ISO/IEC 17025:2005, guaranteeing the permanent quality of its service delivery, that can be considered equivalent to a quality system conforming to the standard ISO/IEC 9001:2000. The evaluation of technical competences will be realized in the frame of internal audits. The entire results coming from internal and external audit must be evaluated by the direction during the management review. This ultimate assumes the entire responsibility of the calibration quality of the measuring and testing equipment. 4. The reference material: The reference materials certified or not, contribute in a considerable manner to the quality of the results in areas as diverse as analytical chemistry, alimentary, environment, biology or also biomedicine. These reference materials and certified reference materials are comparable to the measurement standards (like weights, thermometer, condenser ) associated to the physical measures. One distinguishes several types of reference materials: pure substances: characterized by its purity and/or the traces of impurity that it contains. standard solutions and gas mixtures: characterized by the composition of its components (present in the form of trace or major ingredient). Physicochemical reference material: characterized by properties like viscosity, optical density or the thawing point. Reference objects or artifacts: characterized by its functional properties like smell, taste, flash point or hardness. Two classes of material are formally recognized by ISO. These are «Certified Reference Materials» (CRM) and the «Reference Material» (RM). By definition, the certified reference material (CRM) must be traceable to national and international measurement standards. Each certified value must be accompanied by a certificate with indication of the measurement uncertainty to an indicated level of confidence. The reference materials are materials which property values are sufficiently homogenous to be used for calibration of equipment or the verification of an analysis method. 5. Inter laboratory comparisons: The appeal to inter laboratory comparisons (cf. Appendix A015) is only done when it is impossible to establish the traceability of a unit of the IS in a different manner. The traceability can be realized with the help of scheduled comparisons between laboratories in an appropriate manner. This method requires the designation of one of the laboratories as reference laboratory as well as the analysis and the record of testing results of all the participants (see ISO/IEC 17043).