Recommendation 2 of the 94th CIPM (2005)
Clarification of the definition of the kelvin, unit of thermodynamic temperature
The International Committee for Weights and Measures (CIPM),
- 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,
- that the definition of the kelvin refer to water of a specified isotopic composition,
- that this composition be:
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,
which is the composition of the International Atomic Energy Agency reference material Vienna Standard Mean Ocean Water (VSMOW), as recommended by IUPAC in “Atomic Weights of the Elements: Review 2000”.
- that 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 exactly by the following amount of substance ratios:
0.000 155 76 moleof 2H per mole of 1H, 0.000 379 9 moleof 17O per mole of 16O and 0.002 005 2 moleof 18O per mole of 16O”.
The kelvin was redefined by the 26th CGPM in 2018 (Resolution 1).