Recommendation 1 of the 9th CCM (2005)

Conditions for a new definition of the kilogram

The Consultative Committee for Mass and Related Quantities (CCM),

recalling Resolution 7 of the 21st CGPM

“The 21st Conférence Générale des Poids et Mesures,

considering

• the need to assure the long-term stability of the SI,
• the intrinsic uncertainty in the long-term stability of the artefact defining the unit of mass, one of the base units of the SI,
• the consequent uncertainty in the long-term stability of the other three base units of the SI that depend on the kilogram, namely, the ampere, the mole and the candela,
• the progress already made in a number of different experiments designed to link the unit of mass to fundamental or atomic constants,
• the desirability of having more than one method of making such a link,

recommends that national laboratories continue their efforts to refine experiments that link the unit of mass to fundamental or atomic constants with a view to future redefinition of the kilogram.”

considering

• the recent proposal for a redefinition of the kilogram in 2007 that includes a “conventional value” for the international prototype,
• the presently unresolved discrepancy of up to 1 part in 10⁶ between the existing experiments linking the kilogram to fundamental or atomic constants,
• the best relative standard uncertainties of about 8 parts in 10⁸ of mass standards used in industry and legal metrology,
• the potential undesirable consequences for the SI that would result from the proposed “conventional value” for the international prototype,
• the views of many members of the CCM, of some of its working groups, as well as that of EUROMET,
• the CCM will review this recommendation at its future meetings in the light of progress,

recommends

• that the following conditions be met before the kilogram is redefined with respect to a fundamental constant:
  1. there are no significant unresolved discrepancies between results from independent experiments,
  2. the relative standard uncertainty of the best realization of the definition of the kilogram does not exceed two parts in 10⁸, at the level of one kilogram,
  3. the results of a sufficient number of independent experiments are available with the required uncertainty,
• that the CODATA recommended value be adopted for the relevant fundamental constant,
• that the BIPM and a sufficient number of NMIs continue to maintain, or invest in facilities for the practical realization of the new definition of the kilogram.
• that a mise en pratique for the realization of the new definition of the kilogram be drawn up that includes recommendations concerning the various linking experiments, as well as recommendation for the continuing use of the present artefact to maintain the present excellent worldwide uniformity of mass standards.

DOI : 10.59161/CCM2005REC1E