Metrology traceability

The key difference between a CRM and an RM is the traceability. In order to play any role at aU in metrology, traceability is a key property. Traceability refers to a property value of the CRM, and thus to the underlying measurements. Insufficient traceability of these measurement results will eventually lead to a RM that cannot be certified, as the property value cannot be related to other standards. In the ideal case, traceability is realized up to the International System of Units, SI, but this is only feasible for a very small number of CRMs. 

Associated with method validation, but not part of it, are two properties of results that have been previously mentioned. These parameters are measurement uncertainty and metrological traceability. Measurement uncertainty is covered in Chapter 6 and metrological traceability in Chapter 5. If considered at the planning stage of method validation, the information obtained during validation is a valuable input into measurement uncertainty evaluation. Traceability depends on the method s operating procedures and the materials being used. 

Section 5.2 introduced the subject of metrological traceability and calibration and the use of pure chemical substances and reference materials in achieving trace-ability. Reference materials are used as transfer standards. Transfer standards are used when it is not possible to have access to national or international standards or primary methods. Transfer standards carry measurement values and can be... 

A reference material characterized by a metrologically valid procedure for one or more specified properties, accompanied by a certificate that states the value of the specified property, its associated uncertainty and a statement of metrological traceability. 

Traceable equipment, e.g. balances, thermometers, glassware, timepieces, pipettes, etc. are appropriately calibrated, and the corresponding certificates or other records demonstrating metrological traceability to National Standards are available. 

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. 

Metrological traceability Property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty... 

EURACHEM / CITAC (2008) Metrological Traceability of Analytical Results. Leaflet available from www.eurachem.org... 

Details of the metrological traceability and measurement uncertainty of any assigned value... 

Each of these principles will arise in some guise or other in this book. For example, principle 5 relates to metrological traceability (chapter 7) and measurement uncertainty (chapter 6). These principles will be revisited in the final chapter. 

The certified value is usually taken as the grand mean of the valid results. The organizer uses standard deviation as the basis for calculating the measurement uncertainty. Results from the laboratories will include their own estimates of measurement uncertainty and statements of the metrological traceability of the results. There is still discussion about the best way to incorporate different measurement uncertainties because there is not an obvious statistical model for the results. One approach is to combine the estimates of measurement uncertainty as a direct geometric average and then use this to calculate an uncertainty of the grand mean. Type A estimates will be divided by /n n is the number of laboratories), but other contributions to the uncertainty are unlikely to be so treated. 

In this chapter I use the term metrological traceability to refer to the property of a measurement result that relates the result to a metrological reference. The word metrological is used to distinguish the concept from other kinds of traceability, such as the paper trail of documentation, or the physical trail of the chain of custody of a forensic sample. When the term traceable standard is used to refer to a calibration material, for example, the provenance of the material is not at issue, but the quantity value embodied in the standard. 

Metrological traceability is a property of a measurement result, not a method or measurement procedure, but the result itself. 

Measurement uncertainty of the value of the calibrators increases down the chain, and although, if properly chosen, the measurement uncertainty of the value of the calibrator might be small compared with other uncertainties of the measurement, the measurement uncertainty of the value of the calibrator is an essential ingredient of the metrological traceability of the result. 

Perhaps this sounds unnecessarily complicated, but an understanding of basic concepts and terms in metrology help us appreciate the importance of metrological traceability, measurement uncertainty, and the like. 

A direct consequence of metrological traceability is that if two measurement results are metrologically traceable to the same metrological reference, then they must be comparable (i.e., can be compared) within their measurement uncertainties. This is sometimes explained in terms of the futility of... 

The foregoing discussion identifies the need for comparability and its achievement by metrological traceability. Comparability of measurement results is conveniently defined in terms of metrological traceability. If two results are traceable to the same stated metrological reference, then they must be comparable. Please note that in common speech comparable often means about the same magnitude, but this is not the case here. In the laboratory comparability of measurement results means simply that the results can be compared. The outcome of the comparison, whether the results are considered near enough to be equivalent, is not a factor in comparability here. 

The first criterion is that the measurements must be made in the same units—that is, traceable to the same metrological reference. This sounds obvious, but without attention to metrological traceability of a measurement... 

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. 

Figure 7.5. Metrological traceability of mass measured by bathroom scales.

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