Fundamental Metrological Standards

In metrology, a standard defines a quantity for a given value in a coherent system of units, with an associated measurement uncertainty. It can be a measuring system, a material measure or a reference material. The standard serves as a reference for obtaining measured values and measurement uncertainties. It is used to check the accuracy of the results given by a measuring device, or to calibrate the device. The accuracy of a measurement result is the closeness of agreement between the measured value and the true value of the measured quantity.

The value of a primary standard is obtained without reference to a standard of a quantity of the same nature, but it may refer to standards of other quantities. For example, a pressure balance, a primary standard for pressure, may be traceable to a metre, through surface measurements, but not to another pressure standard.

The origins of the International System of Units (SI) go back to the 18th century, with the creation of the decimal metric system, which provided the first basis for standardizing units of measurement. With scientific and technological developments, particularly at the end of the 19th century and throughout the 20th , a large number of researchers attempted to define units of measurement on the basis of physical constants in nature, which are inherently more universal than those derived from practical realizations (such as the triple point of water) or material artifacts (such as the international prototype of the standard kilogram).

However, even if the precision of units continued to improve, within the framework of the SI, certain unit definitions remained difficult or even impossible to implement (the ampere, for example).

All these factors led the metrology community to reflect on new definitions of units of measurement, based on physical constants. Between 1967 and 1983, three units (the second, the candela and the metre) were redefined in relation to a physical constant. Further research over several decades around the world led to the new "revolution" of 2018, where the kilogram, ampere, kelvin and mole are now also based on physical constants in nature. The seven basic units of the SI have thus been transformed, opening up new perspectives to support technological progress. Following the evolution of the SI and its latest edition in 2018, the article presents the main primary measurement standards for physical and chemical quantities, in the order of definition of the SI base units.