Metrology is of fundamental importance in industry and trade – not only from the point of view of the consumer but also for those involved in manufacturing. Both groups must have confidence in the accuracy and reliability of the measurements upon which they depend. Within the manufacturing process, to ensure the accuracy of measuring instruments, it is essential that they should be periodically calibrated against more accurate standards, which in turn should have their calibration traceable to even more accurate national measurement standards at the national level and, eventually, the international level. When these various levels of calibration have been documented, a chain of traceable calibrations is created.

Traceability means that the result of a measurement, no matter where it is made, can be related to a national or international measurement standard, and that this relationship is documented. In addition, the measuring instrument must be calibrated by a measurement standard that is itself traceable. Traceability is thus 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, through an unbroken chain of comparisons all having stated uncertainties. The concept of traceability is important because it makes possible the comparison of the accuracy of measurements worldwide according to a standardized procedure for estimating measurement uncertainty.

Within a chain of traceability, the units of measurement with the highest accuracy are realized by international measurement standards. The value of the international standard is usually determined by comparison of national standards of the highest quality, or in the case of the kilogram by the mass of the International Prototype. National measurement standards, maintained in a national metrology institute or NMI (for example, the NPL in the UK, the NIST in the USA or the NMIJ in Japan) must be compared with these international standards. The result of such comparisons, together with the precision and uncertainty of the national standard will be stated and available on, for example, the internet (see the BIPM key comparison database, www.bipm.org/kcdb/). Then the national measurement standard serves as a reference for calibration of standards of lower precision. Reference standards are kept in a national metrology institute or in an accredited calibration laboratory for calibrations not requiring the highest accuracy. Again, the result and the uncertainty will be stated.

At each stage in such a chain of traceability, one loses a certain degree of precision (see Figure). Thus the highest level standards are the international standards, known with the greatest level of precision, and the lower level standards will have been determined to a lower level of precision. This lower level of precision will be one which is acceptable or appropriate for the use of that particular standard.