Towards a new SI: a review of progress made since 2011

Abstract

In 2011, the General Conference on Weights and Measures (CGPM) confirmed its intention to adopt new definitions for four of the base units of the SI based on fixed numerical values of selected constants. These will be the kilogram, the ampere, the kelvin and the mole. The CGPM was not able to adopt the new definitions at that time because certain experimental and coordination work was not complete. This paper reviews criteria proposed by the Consultative Committees of the CIPM for such a ‘new SI’ to be adopted and reports on recent progress with work to address them. We also report on work being undertaken to demonstrate that the most important technical aspects of realizing such a new system are practicable. The progress reported here confirms the consensus developing amongst the Consultative Committees and the National Metrology Institutes that it will be possible for the CGPM to adopt these new definitions in 2018.

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

“…Various experiments are now or will become available in the near future with higher accuracy than current ones, for example an optical clock based on Al + is more accurate than the Cs-clock [58]; these advances should be taken into account [57]. In contrast, other authors see all technical requirements set by CCU close to being met such that no delay is indicated [59] and the CODATA 2014 recommended values and uncertainties [3] give added support to the latter position. The SI being a practical system, the definition of the second will undoubtedly be updated to accommodate optical clocks when they become true clocks rather than spectacularly precise frequency standards, as they are today.…”

Approche du LNE pour la réalisation et la mise en pratique du kilogramme dans sa nouvelle définition

“…Ces vérifications « ordinaires » n'ont donc lieu que tous les cinquante ans environ depuis l'adoption de l'IPK pour réaliser l'unité de masse : distribution et certification des prototypes en 1889 [2], deuxième vérification périodique en 1946-1953 [9], troisième vérification périodique en 1988-1992 [10]. Une comparaison extraordinaire a eu lieu en 2014 en prévision de la refonte du SI [11].…”

“…[26]. En effet, le CIPM exige que trois expériences indépendantes (incluant des balances du watt [27] et des expériences de sphère de silicium [28]), permettent d'obtenir des valeurs cohérentes de la constante de Planck, avec des incertitudes types relatives qui ne soient pas supérieures à 5 × 10 −8 et dont l'une au moins possèderait une incertitude type relative qui ne soit pas supérieure à 2 × 10 −8 [11].…”

Determination of the Avogadro constant by the XRCD method using a28Si-enriched sphere

“…The new definition will be based on a fixed value of the Planck constant h and is anticipated to be accepted in 2018 [4]. Toward the change in the definition, national metrology institutes (NMIs) are now being requested to continue attempts to link the kilogram to h with a relative uncertainty of a few parts in 10 −8 [4].…”