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Why change the SI?

The revision of the SI will ensure that the SI continues to meet the needs of science, technology, and commerce in the 21st century.

Of the seven base units of the SI, only the kilogram is still defined in terms of a material artefact, namely the international prototype kept at the BIPM. The major disadvantage of the present definition of the kilogram is that it refers to the mass of the artefact which, by its very nature, we know cannot be absolutely stable.

click for more details about the periodic verifications

The results of comparisons between the official copies and the international prototype show some divergence with time. The graph opposite shows changes of about 5 parts in 108, equivalent to 50 µg, in the mass of the standards since their first calibration more than 100 years ago. Note that this graph shows only the relative changes from the mass of the international prototype (corresponding to the zero value of the y-axis). The drift in the mass of the international prototype itself since 1889 cannot be shown but it must certainly be present. The rate of change of its mass can be determined only by absolute experiments which up to now are of insufficiently high precision.

Unknown changes in the mass unit also influence the electrical units, because the definition of the ampere is related to the kilogram. Similarly, the definitions of the mole and candela also depend on the kilogram.

At its 21st meeting (1999) the CGPM therefore recommended in its Resolution 7 that efforts continue to refine experiments linking the unit of mass to fundamental constants with a view to a future "quantum-based" redefinition of the kilogram. Any new definition would need to be consistent within some parts in 108 with the present definition to ensure continuity of mass values.

When the definition of the kilogram is based on an invariant of nature instead of a material artefact, it will be possible to realize the SI unit of mass at any place, at any time and by anyone (see also What is a mise en pratique?). Resolution 1 (2011) also highlights the following advantages:

  • The uncertainties of all SI electrical units realized directly or indirectly by means of the Josephson and quantum Hall effects together with the SI values of the Josephson and von Klitzing constants KJ and RK would be significantly reduced if the kilogram were redefined so as to be linked to an exact numerical value of h, and if the ampere were to be redefined so as to be linked to an exact numerical value of the elementary charge e.

  • The kelvin is currently defined in terms of an intrinsic property of water that, while being an invariant of nature, in practice depends on the purity and isotopic composition of the water used. The kelvin would be better defined if it were linked to an exact numerical value of the Boltzmann constant kB.

  • Redefining the mole so that it is linked to an exact numerical value of the Avogadro constant NA would have the consequence that it is no longer dependent on the definition of the kilogram even when the kilogram is defined so that it is linked to an exact numerical value of h. This would thereby emphasize the distinction between the quantities "amount of substance" and "mass".

  • The uncertainties of the values of many other important fundamental constants and energy conversion factors would be eliminated or greatly reduced if h, e, kB and NA had exact numerical values when expressed in SI units.