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Thermometry: SI base unit (kelvin)

Definition agreed by the 26th CGPM (November 2018), implemented 20 May 2019:


    The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 x 10–23 when expressed in the unit J K–1, which is equal to kg m2 s–2 K–1, where the kilogram, metre and second are defined in terms of h, c and DeltanuCs.

    This definition implies the exact relation k = 1.380 649 x 10–23 kg m2 s–2 K–1. Inverting this relation gives an exact expression for the kelvin in terms of the defining constants k, h and DeltanuCs:

    which is equal to

    The effect of this definition is that one kelvin is equal to the change of thermodynamic temperature that results in a change of thermal energy k T by 1.380 649 x 10–23 J.

    The definition of the unit of thermodynamic temperature was given by the 10th CGPM which selected the triple point of water, TTPW, as a fundamental fixed point and assigned to it the temperature 273.16 K, thereby defining the kelvin. The 13th CGPM adopted the name kelvin, symbol K, instead of "degree kelvin", symbol °K, for the unit defined in this way. However, the practical difficulties in realizing this definition, requiring a sample of pure water of well-defined isotopic composition and the development of new primary methods of thermometry, led to the adoption of a new definition of the kelvin based on a fixed numerical value of the Boltzmann constant k. The present definition, which removed both of these constraints, was adopted in Resolution 1 of the 26th CGPM (2018).