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 Thermometry: SI base unit (kelvin)

The SI base unit kelvin (K) is defined as follows:

The degree Celsius (°C) is the special name for the kelvin used to express Celsius temperatures. The degree Celsius and the kelvin are equal in size, so that the numerical value of a temperature difference or temperature interval is the same when expressed in either degrees Celsius or in kelvins.

 SI base unit (kelvin) SI Brochure, Section 2.1.1.5

The definition of the unit of thermodynamic temperature was given in substance by the 10th CGPM (1954, Resolution 3) which selected the triple point of water as the fundamental fixed point and assigned to it the temperature 273.16 K, so defining the unit. The 13th CGPM (1967/68, Resolution 3) adopted the name kelvin, symbol K, instead of "degree Kelvin", symbol °K, and defined the unit of thermodynamic temperature as follows (1967/68, Resolution 4):

The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

It follows that the thermodynamic temperature of the triple point of water is exactly 273.16 kelvins, Ttpw = 273.16 K.

 The symbol, Ttpw, is used to denote the thermodynamic temperature of the triple point of water.

At its 2005 meeting the CIPM affirmed that:

This definition refers to water having the isotopic composition defined exactly 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.

Because of the manner in which temperature scales used to be defined, it remains common practice to express a thermodynamic temperature, symbol T, in terms of its difference from the reference temperature T0 = 273.15 K, the ice point. This difference is called the Celsius temperature, symbol t, which is defined by the quantity equation:

t = TT0.

The unit of Celsius temperature is the degree Celsius, symbol °C, which is by definition equal in magnitude to the kelvin. A difference or interval of temperature may be expressed in kelvins or in degrees Celsius (13th CGPM, 1967/68, Resolution 3, mentioned above), the numerical value of the temperature difference being the same. However, the numerical value of a Celsius temperature expressed in degrees Celsius is related to the numerical value of the thermodynamic temperature expressed in kelvins by the relation

t/°C = T/K – 273.15.

The kelvin and the degree Celsius are also units of the International Temperature Scale of 1990 (ITS-90) adopted by the CIPM in 1989 in its Recommendation 5 (CI-1989).

 Decisions relating to the base units of the SI SI Brochure, from Appendix 1

Thermodynamic temperature:

 9th CGPM, 1948 adopts the triple point of water as the thermodynamic reference point, adopts the zero of Celsius temperature to be 0.01 degree below the triple point CIPM, 1948 adopts the name degree Celsius for the Celsius temperature scale 10th CGPM, 1954 defines thermodynamic temperature such that the triple point of water is 273.16 degrees Kelvin exactly, defines standard atmosphere 13th CGPM, 1967/68 decides formal definition of the kelvin, symbol K CIPM, 1989 the International Temperature Scale of 1990, ITS-90 CIPM, 2005 note added to the definition of the kelvin concerning the isotopic composition of water 23rd CGPM, 2007 clarification of the definition of the kelvin, unit of thermodynamic temperature 23rd CGPM, 2007 on the possible redefinition of certain base units of the International System of Units (SI) 24th CGPM, 2011 on the possible future revision of the International System of Units, the SI 25th CGPM, 2014 on the future revision of the International System of Units, the SI

Documents related to the future redefinition of the kelvin:

 Thermometry Overview SI base unit (kelvin) Committee structure Strategic plan
 International equivalence Calibration and measurement capabilities (CMCs) International comparisons
 At the BIPM Recent publications in the scientific literature BIPM contact person