Metrology definition

The definitions given below are not always the official statistical or metrological definition. They are given in the context of chemical analysis, and are the authors best attempt at understandable descriptions of the terms. 

It may be realised by means of definite complex of intereonneeted and interrelated common rules and norms direeted to the assurance of traceability and uniformity of measuring equipment, NDT equipment including. In another words NDT equipment must be metrologically supported. 

Metrologia International Committee of Weights and Measures (CIPM) Pavilion de Breteuil Parc de St. Cloud, Prance Includes articles on scientific metrology worldwide, improvements in measuring techniques and standards, definitions of units, and the activities of various bodies created by the International Metric Convention. 

In general, a thermometer is called primary if a theoretical reliable relation exists between a measured quantity (e.g. p in constant volume gas thermometer) and the temperature T. The realization and use of a primary thermometer are extremely difficult tasks reserved to metrological institutes. These difficulties have led to the definition of a practical temperature scale, mainly based on reference fixed points, which mimics, as well as possible, the thermodynamic temperature scale, but is easier to realize and disseminate. The main characteristics of a practical temperature scale are both a good reproducibility and a deviation from the thermodynamic temperature T which can be represented by a smooth function of T. In fact, if the deviation function is not smooth, the use of the practical scale would produce steps in the measured quantities as function of T, using the practical scale. The latter is based on ... 

Starting with the ISO definition of metrology metrology is the science of measurement. 

Before continuing, let s see the official definition of traceability as given in the ISO international vocabulary of basic and general terms in metrology. Traceability is defined as the property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties. 

Traceability in Practice The practical establishment of traceability is based on a step-by-step implementation of the definition. The ISO definition of traceability, originating from metrology (see above), can be translated into three basic steps ... 

The International Vocabulary of Basic and General Terms in Metrology (VIM), second edition (ISO 1993a), defines uncertainty of measurement as the parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand (term 3.9). In the third edition (Joint Committee for Guides in Metrology 2007), the term measurement uncertainty is defined (term 2.27), and reasonably attributed is replaced by attributed to a measurand based on the information used. This definition emphasizes that the estimate of uncertainty is only as good as the information used to calculate it, so a report of an uncertainty should also state what information was used to calculate the uncertainty. 

From the definitions and examples above, it should be clear that metrological traceability is established by a series of comparisons back to a reference value of a quantity. In the mass example it is easy to picture a number of scientists with ever more sophisticated balances comparing one mass with another down the chain of masses from the international prototype of the kilogram to bathroom scales. This is known as a calibration hierarchy. A mass farther up the chain is used to calibrate the next mass down the chain, which in turn can be used to calibrate another mass, and so on, until the final measurement of the mass is made. 

At the top of the traceability chain is the stated metrological reference, which for our purposes is the definition of a unit. It might not be obvious how a piece of paper from Paris giving the interesting, but rather esoteric, definitions of the SI base units can be used as a reference. The metrological reference allows the creation of a primary calibrator that embodies the value of the unit (or some multiple or submultiple of it). The international prototype... 

Abstract By the definition of the mole as a base unit for amount-of-substance measures within the International System of Units (SI), chemists can make chemical measurements in full compliance with established metrological principles. Since the mole requires exact knowledge of the chemical entity, which is often neither available nor of practical relevance to the purpose of the measurement, the SI units of mass or length (for volume) are unavoidable in the expression of results of many chemical measurements. Science, technology, and trade depend upon a huge and ever increasing number and variety of chemical determinations to quantify material composition and quality. Thus, international harmonization in the assessments of processes, procedures, and results is highly desirable and clearly cost effective. The authors, with relevant experience and re-... 

Key words Traceability Mole Definition Measurements Chemical metrology Calibration Validation Reference materials... 

Reliable chemical measurements in future will depend on more RMs with direct links to the SI as well as on RMs of greater diversity than are available now. Chemical science will be assisted by clear consensus definitions of traceability, certification, and validation, as well as by a widely accepted system for describing RMs by material composition, degree of traceability, uncertainty, quality, and purpose. Ultimately, chemists, physicists, and engineers benefit from adherence to the well-grounded and well-established discipline of metrology under a coherent system of units. 

Metrological traceability, according to its definition, is a property of a measurement result or quantity value, and the trace is towards a stated reference through a metrological traceability chain. Its links and relations between them have to be established a priori in the opposite direction from the chosen reference towards the measurement result. This structure is termed a calibration hierarchy. 

The concepts of calibration and especially of metrological traceability were elaborated by physicists as mentioned above. The reference or top of the calibration hierarchy preferably is the definition of an SI unit, which is realized or embodied as a primary measurement standard. By direct comparison, the quantity value of a secondary measurement standard can be established. Subsequent comparisons may furnish quantity values of reference measurement standard, working measurement standard, and routine measurement standard with which the object carrying the measure and is compared to obtain its measurement result which then retrospectively is metrologically traceable to the SI unit. The primary measurement standard, as the definition of metrological traceability says, is preferably an international or national measurement standard. 

Fig. 1 Full calibration hierarchy providing metrological traceability from measurement results to SI unit definition as the stated reference. MP=measurement procedure, C=calibrator...

The definition of metrological traceability (see above) stipulates that each link in the chain has a known uncertainty. Nowadays, this concept and its application have been reformulated by the BIPM and recently detailed in the Guide to the expression of uncertainty in measurement (GUM) [26] parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand . Useful explanations are provided in several other guides [26-30] as well as commentaries [e.g. 31-33], The philosophy is to apply a bot-tom-up approach by formulating a function of all input quantities giving the measurand as output. An uncertainty budget of all sources of uncertainty is established. Important items to consider are ... 

According to definition [3], a primary standard is a standard that is designated or widely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of the same quantity . Thus, once defined, primary standards require no further reference. The essential quality of a primary standard is its intrinsic, long-term stability. Primary standards are mainly used to measure and determine the value of all other standards. 

The Consultative Committee for Amount of Substance (CCQM) has set up a definition of primary methods [1, 2] and has selected some methods with the potential of being primary , from the viewpoint of the end user. From the point of view of metrology, methods used for linking the chemical measurements with the SI system at the highest level should not refer to other amount of substance standards. This requirement excludes methods which are relative in their principle. Some other methods identified as having the potential to be primary yield information expressed as amount fraction. This is essential for evaluation of purity, but in order to convert it to a value useful for transfer of the unit, additional information on the identity (molar mass) and content of the impurities is required. This additional information is needed to convert the result into amount content or similar quantities. 

The 1993 edition of the International Vocabulary of Basic and General Terms in Metrology (VIM) defines traceability as a property of a measurement, not a process, and that traceability should involve quantified uncertainties. This definition is consistent with the measurement assurance approach and shows that the concept of quantified uncertainty as a requirement for good traceability is becoming accepted globally. 

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