Resolution 10 of the 23rd CGPM (2007)

Clarification of the definition of the kelvin, unit of thermodynamic temperature

The 23rd General Conference,

considering

• that the kelvin, unit of thermodynamic temperature, is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water,
• that the temperature of the triple point depends on the relative amount of isotopes of hydrogen and oxygen present in the sample of water used,
• that this effect is now one of the major sources of the observed variability between different realizations of the water triple point,

notes and welcomes the decision by the International Committee for Weights and Measures in October 2005, on the advice of the Consultative Committee for Thermometry, that

• the definition of the kelvin refers to water of a specified isotopic composition,
• this composition be:

which is the composition of the International Atomic Energy Agency reference material Vienna Standard Mean Ocean Water (VSMOW), as recommended by the International Union of Pure and Applied Chemistry in "Atomic Weights of the Elements: Review 2000",

• this composition be stated in a note attached to the definition of the kelvin in the SI Brochure as follows:

"This definition refers to water having the isotopic composition defined by the following amount-of-substance ratios: 0.000 155 76 mole of ²H per mole of ¹H, 0.000 379 9 mole of ¹⁷O per mole of ¹⁶O and 0.002 005 2 mole of ¹⁸O per mole of ¹⁶O".

DOI : 10.59161/CGPM2007RES10E