How much does the SI, namely the proposed “New SI”, conform to principles of the Metre Treaty?

Abstract

The International System of Units (SI) was first adopted in 1960, as the more recent implementation of the Metre Treaty signed in 1875. Basic features of the original SI are that (a) seven units are chosen as “base units”, all the others being “derived units”, and (b) the definitions of the base units should not create interdependence. This way, the SI conforms to the basic principle of the Metre Treaty that each signatory country can realise its choice of primary national standards of the very definitions of the units without needing to resort to calibrations obtained from another country, and without obligation to have them realised for all the units. A mismatch already occurs to some extent with respect to the above features in the present definitions of SI base units. This contribution, strictly based on metrological considerations, illustrates how the present proposal concerning new definitions for the base units, called “New SI”, would extend the mismatch. In this frame, also the meaning is discussed of the concepts of hierarchy and traceability in metrology. By outlining some of the consequences, a discussion is stimulated related to the status of base unit, to the meaning of calibration at the level of the standards of the unit definitions, and to the interdependence of countries’ standards.