Triple point of water

Water has exactly one temperature and pressure at which solid, liquid and vapour coexist in equilibrium: 273.16 K and 611.657 Pa. A sealed triple-point cell holds that temperature by itself, reproducibly, without any external control, which made it the finest fixed point in thermometry for sixty-five years.

From 1954 to 2019 the kelvin was defined as 1/273.16 of the triple-point temperature, so 273.16 K was exact by fiat and all the uncertainty lived in the realisation. The 2019 redefinition fixed the Boltzmann constant instead, and the logic reversed: the kelvin is now defined from energy, and the triple point became an ordinary measured quantity, 273.1600 K with a standard uncertainty of about 0.1 mK. The value was chosen so that nothing observable changed at the changeover. One subtlety survives: the temperature depends on isotopic composition, so the definition specifies water of the VSMOW composition.