Is the kelvin still defined using the triple point of water at 273.16 K?
Is the kelvin still defined using the triple point of water at 273.16 K?
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No. Since 20 May 2019, the kelvin is no longer defined by the triple point of water. Under the 2019 revision of the SI (adopted at the 26th CGPM in November 2018), the kelvin is defined by fixing the numerical value of the Boltzmann constant.
Current SI definition (BIPM): "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 1.380649 x 10^-23 when expressed in the unit J K^-1, which is equal to kg m^2 s^-2 K^-1, where the kilogram, metre and second are defined in terms of h, c and DeltavCs."
What changed:
- Old definition (1967-2019): the kelvin was 1/273.16 of the thermodynamic temperature of the triple point of water. This made the triple point exactly 273.16 K by definition, and the Boltzmann constant was an experimentally measured quantity.
- New definition: the Boltzmann constant is now exactly 1.380649 x 10^-23 J/K (an exact, defined value), and this fixes the size of the kelvin. The definition references no material property of water.
Status of 273.16 K and the triple point of water:
- The triple point of water is now an experimentally determined quantity, not a definition. Its value is still very close to 273.16 K but now carries a small uncertainty (NIST notes well-made triple-point cells sit at "almost exactly 273.16 K," with roughly 50 microkelvin spread among laboratory-grade cells).
- The triple point of water remains widely used in practice as a convenient, reproducible fixed point for calibrating thermometers (e.g., standard platinum resistance thermometers), and it is one of the defining fixed points of the ITS-90 temperature scale. So 273.16 K lives on as a practical reference, just no longer as the definition of the unit.
In short: the number 273.16 K is no longer where the kelvin gets its definition; the Boltzmann constant is. The triple point of water has been demoted from "defines the kelvin" to "a high-quality calibration reference whose temperature is now measured rather than fixed."
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