The accuracy of the mass measurements made by, for example, the weighing scales in a local supermarket are ultimately calibrated through the national traceability systems against standard weights kept at a national metrology laboratory and these national weights are calibrated against an international standard. One thus has a chain of calibrations, which allows the measurements in the supermarket to be traced to an internationally accepted set of standards. The consumer can therefore have confidence in the accuracy of local systems of weights and measures. In addition, we can ensure the consistency of measurements worldwide, because all measurements can either be traced to a single, highly accurate international reference because national standards may be compared with each other.

While the primary focus of legal metrology concerns measurements that directly affect consumers, metrology as a whole is of importance to all those engaged in the various chains of measurement and calibration. So while the physicists who maintain the International Prototype of the kilogram (the last physical artifact which defines a base unit and against which all other measuring systems in the world are calibrated) at the BIPM, Sèvres, can be thought of as scientific metrologists, they are not directly connected with the concerns of legal metrology in their daily work. On the other hand, the technicians who calibrate weighing scales in shops and markets are directly concerned with legal metrology, and are of course also metrologists.

Legal metrology, as represented by the work of the International Organization of Legal Metrology (OIML) is concerned with the chain of measurement traceability that directly affects consumers, and has the backing of national laws which protect the consumer from, for example, shopkeepers whose weighing or other measuring devices may not calibrated correctly. This means that regulations issued by the OIML are often incorporated directly into national and international laws and regulations concerned with consumer protection. Such OIML regulations are therefore used directly by regulators and law makers and may have the force of law.

Metrologists in NMIs are, however, concerned with metrology in a somewhat broader context. At the highest or most scientifically-oriented level, they ensure the consistency of the International System of Units (SI). In most cases, this implies research into the base definitions of the units and on the fundamental constants of science; for example, the speed of light and properties of atoms.

Each NMI may construct equipment to realize the definitions of the base units of the SI, and so maintain national representation of units such as the metre, second or the volt, which are used as reference measurements in that country. NMI metrologists also compare their national references so as to ensure that they are equivalent and that there is a worldwide consistency in SI measurements at all levels of accuracy.

Consider length measurement. Physicists working at the forefront of metrology are concerned that the base unit of length (the metre) is defined so as to meet the up-to-date needs of science, commerce and society. When combined with SI traceability this means that length measurements are nationally and internationally consistent. Legal metrology steps in when there are length measurements used in regulation; and the OIML is concerned that, for example, measuring tapes used by builders and surveyors are accurate and that the construction industry is giving the customer what it is being contracted to give them. The difference between metrology and legal metrology is therefore often one of scale of precision, although traceability and the accreditation of technical competence of the scientists and technicians are common to both.

The metre is currently defined with respect to laser radiation and may be measured by metrologists to one part in 10¹², whereas in consumer protection terms (i.e. legal metrology) one is more concerned that a building which is supposed to be several metres in length is as close as possible to this in terms of the precision of building materials – very far from one part in 10¹². However, because accuracy is lost during the different stages of calibration in the chain of traceability between one set of standards and another set, measurements at the highest level of precision must be made to give interested parties confidence in precision at those levels where the consumer is directly concerned by the measurements. Thus, although there is extreme precision in the experiments that realize the metre, this degree of precision is lost to some extent as one moves down the chain of traceability to routine industrial measurements (see Figure). However, these losses are inevitable, and to have the appropriate level of precision required by industry, one needs the extreme precision of the measurements made by scientific metrologists.

However much metrology and legal metrology are concerned with very different levels of precision, both deal with related problems. Both are essential in ensuring that as wide a constituency as possible is involved in and concerned with measurements and measurement science. Both metrology and legal metrology are essential in ensuring consistent national measurement systems, traceable to international standards; thereby establishing that there are no significant differences in measurements and tests made in different countries. Regulators and legislators who need to have confidence in various systems of national measurements, can therefore have confidence that measurements made in one country will be accepted in other countries, which helps reduce or eliminate the possibility that lack of acceptance of calibration and tests could be used as a technical barrier to trade.

The BIPM, established in 1875 by the Metre Convention (a diplomatic treaty between 51 nations), ensures worldwide uniformity of measurements and their traceability to the International System of Units (SI).		The OIML, established in 1955, is an intergovernmental organization whose principal aim is to harmonize the regulations and metrological controls applied by the national metrology services of its national members.
Bureau International des Poids et Mesures, Pavillon du Breteuil, 92312 Sèvres, France www.bipm.org		Organisation Internationale de Métrologie Légale, 11 rue Turgot, 75009 Paris, France www.oiml.org