Redefining the International System of Units, the SI

Redefinition of SI Units

NG Wee Hoe

National Metrology Centre, Agency for Science, Technology and Research

Measurement scientists and representatives of the member states of the Metre Convention (an international treaty on measurement), approved the revisions to the SI Units in 2018, which defined the SI based on the fixed numerical values of a set of seven defining constants from which the definitions of the seven base units of the SI are to be deduced. The redefinition became effective on 20 May 2019, which is a World Metrology Day celebrated annually by measurement scientists and engineers worldwide to commemorate the founding of the Metre Convention on 20 May 1875. In this presentation, we provide the background information on the SI, the international framework for their realisations and dissemination to harmonise measurement for scientific advancement, technological innovation, and fair trade. We also discuss the reasons for the redefinitions, and the implications to the world of measurement.