Adapting the International System of Units to the twenty-first century

Abstract

We review the proposal of the International Committee for Weights and Measures (Comité International des Poids et Mesures, CIPM), currently being considered by the General Conference on Weights and Measures (Conférences Générales des Poids et Mesures, CGPM), to revise the International System of Units (Le Système International d'Unitès, SI). The proposal includes new definitions for four of the seven base units of the SI, and a new form of words to present the definitions of all the units. The objective of the proposed changes is to adopt definitions referenced to constants of nature, taken in the widest sense, so that the definitions may be based on what are believed to be true invariants. In particular, whereas in the current SI the kilogram, ampere, kelvin and mole are linked to exact numerical values of the mass of the international prototype of the kilogram, the magnetic constant (permeability of vacuum), the triple-point temperature of water and the molar mass of carbon-12, respectively, in the new SI these units are linked to exact numerical values of the Planck constant, the elementary charge, the Boltzmann constant and the Avogadro constant, respectively. The new wording used expresses the definitions in a simple and unambiguous manner without the need for the distinction between base and derived units. The importance of relations among the fundamental constants to the definitions, and the importance of establishing a mise en pratique for the realization of each definition, are also discussed.

A critical review of the proposed definitions of fundamental chemical quantities and their impact on chemical communities (IUPAC Technical Report)

“…Details of the proposed revision of the SI have been summarized [47]. The proposed explicit-constant formulations of the definitions is said to provide a simple and unambiguous approach applicable both for base and derived units.…”

The role of the international prototype of the kilogram after redefinition of the International System of Units

“…The speed of light and the molar mass of carbon-12 have exact numerical values in the present SI. The relative uncertainty of the product of the remaining three terms is dominated by u r (a 2 ) = 2u r (a) = 1.4 × 10 −9 [9,13]. Owing to the small uncertainty of N A h relative to either N A or h taken individually, any experiment described by equation (4.1a) as a determination of h can today be considered to be an experiment described by equation (4.2a) as a determination of N A ,…”

Plank’s Constant: Evaluation of Measurement Uncertainty

“…It also makes the calculations of the scatter of uncertainty much easier. In addition to the above types of uncertainties, in order to weight the approached uncertainty, the conventional value of the Planck constant is given by applying the conventional (i.e., adopted) values of the Josephson constant K J-90 and von Klitzing constant R K-90 [6,7]. Thus, the international standard of the Planck constant value was chosen to improve the uniformity of the comparison of subsequent measurements.…”