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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.8, #12)
Why do we need traceability and uncertainty evaluation of measurement and test results?
by Manfred Golze (pp. 539-540).
Role of measurement uncertainty in conformity assessment in legal metrology and trade by Håkan Källgren; Margreet Lauwaars; Bertil Magnusson; Leslie Pendrill; Phillip Taylor (pp. 541-547).
There is increasing interest in the role of measurement uncertainty in conformity assessment, particularly where measurements are in support of fair trade, health & safety and the environment. A lack of clarity, however, in treating measurement uncertainties and in setting specification limits for compliance may sometimes lead to drastic consequences. Examples concerning conformity assessment, particularly in the measurement of trace quantities (Cd in plastics and antibiotic trace residues in shrimps), are amongst cases identified in the ongoing EU project MetroTrade, which can be significant impediments to international trade. This paper considers the present status of the role of measurement uncertainty in conformity testing, covering both recent work in international standardisation and physical metrology (e.g. OIML recommendations on weighing). One conclusion is that, while much has been achieved, there still exists some work to be done in order to facilitate acceptance of conformity from both customer and supplier and avoid perceived barriers to trade and serious international trade disputes.
Keywords: Measurement; Uncertainty; Conformity; Assessment; Trade
Model-based measurement uncertainty evaluation, with applications in testing by Maurice G. Cox; Michèle Desenfant; Peter M. Harris; Bernd R. L. Siebert (pp. 548-554).
Basic concepts and implementations of the model-based approach to uncertainty evaluation are discussed. One implementation is that of the law of propagation of uncertainty with the use of the central limit theorem as recommended in the Guide to the expression of uncertainty in measurement (GUM). Another is the propagation of distributions, the subject of a supplemental guide to the GUM, which is at an advanced stage of development. It falls in the category of “other analytical and numerical methods” indicated in the GUM. Two testing applications are used to illustrate the principles: tennis-ball rebound and tensile strength.
Keywords: Models; Uncertainty evaluation; GUM; Propagation of distributions; Testing
Measurement uncertainty and the use of reference materials by Adriaan M. H. van der Veen (pp. 555-558).
The use of (certified) reference materials and quality control materials can form a suitable basis for evaluating measurement uncertainty of routine measurements. In particular when these materials are used for quality control purposes, it is not always evident how the quality control data can be used in the uncertainty budget of a routine measurement. Current guidance documents on the evaluation of measurement uncertainty and the use of reference materials treat this subject only in part, or in very generic terms. ISO/REMCO has established a new working group that will provide practical guidance and examples on how to use quality control data in the evaluation of measurement uncertainty. A short introduction to the subject is given.
Keywords: Reference material; Quality control; Measurement uncertainty
Measurement uncertainty of microbiological viable counts by Seppo Ilmari Niemelä (pp. 559-563).
Estimation of the uncertainty of microbiological viable counts by the statistical analysis of series of observations is impractical because the observed value, the number of colonies, usually determines a large share of the uncertainty. Every test result, therefore, has a unique uncertainty and no general method-specific values exist. The most practical ways to estimate the uncertainty are based on mathematical models and assumed statistical distributions where the observed value is a significant parameter. The article describes the traditional solution based on the Poisson distribution and more advanced models based on the negative binomial distribution. The latter model is capable of combining technical and procedural components of uncertainty with the inevitable random scatter of microbial suspensions. The additional uncertainty due to subsampling in the confirmation of presumptive positives is estimated with the help of a binomial model. Traceability issues are discussed.
Keywords: Uncertainty; Measurement; Microbiology; Viable count
Determination of uncertainty for hardness measurement: proposal of the standard, available software by T. Polzin (pp. 564-568).
After the next revision of the ISO-standards for hardness testing (ISO 6506–6508) an estimation of the uncertainty (ISO/IEC 17025, ISO 5725) is mandatory. The two proposed approaches of part one (ISO 6506–6508) of the ISO Standards for hardness are exemplified. The calculation approach implemented in an Excel-based file, which is available via www.mpanrw.de/crm , is announced.
Keywords: Uncertainty; Hardness measurement; Standardization; Excel sheet
Information on measurement uncertainty in test and product standards by Wilfried Hinrichs (pp. 569-575).
According to ISO/IEC 17025, accreditation bodies generally accept that test procedures in normative documents have been validated. This contribution investigates, with respect to measurement uncertainty, the methodical basis of this practice in general and examines specifically two very different groups of products: asphalt, with its major components mineral aggregates and bitumen, and metal wire cloth. The author concludes that both the general and the specific situations are incoherent. The practice of accepting standardised test procedures a priori as validated does not have a sound basis in all cases. Both the concept, and the practical application, of validation or fitness for purpose vary in different product groups. The impact of this situation on the practice of accreditation should be taken into account in order to reduce market distortions in single-product groups. Laboratories face fundamental consequences from this practice.
Keywords: Measurement uncertainty; Product standard; Validation; Fitness for purpose
Measurement uncertainty and trade: an example from the petroleum industry by Onno Janßen; H.-Thomas Feuerhelm (pp. 576-578).
The petroleum industry is and always has been one of the major players in global trade. The price of products depends largely on their characteristics, defined by specifications. Consequently the testing of these specifications and the acceptance of the results by all partners is a crucial pre-requisite for global trade. In the field of petroleum products this issue has been dealt with for decades by using internationally accepted standard test methods, which also define the required accuracy and measurement uncertainty, or the precision. Therefore the petroleum industry may well serve as an example of how measurement uncertainties can be treated and what impact they have on global trade.
Keywords: Measurement uncertainty; Precision; Global trade; Petroleum industry; Standardisation; FAM
News from CEN/BT/WG 122 uncertainty and EA expert group on uncertainty by Marc Priel (pp. 579-580).
Since the publication in 2000 of the standard ISO/IEC 17025 "General requirements for the competence of testing and calibration laboratories", the concept of uncertainty of measurement or test results seems to be a key issue for laboratories and their clients. Two working groups are trying to implement this concept in testing activities. The CEN/BT/WG122 has proposed to the technical bureau of CEN a series of recommendations on how to introduce this concept in European standards and the EA expert group on uncertainty has prepared a guidance document in order to help the testing community in uncertainty evaluation. The objective of this paper is to report the current situation of these two groups.
Keywords: Uncertainty evaluation; Standardization; Specification limits