Temperature measurement

The base unit of temperature is the kelvin, abbreviated K. The kelvin is 273.16ths of the thermodynamic temperature of the triple point of pure water. The triple point (= 273.16 K) describes the state of water in which it exists simultaneously in the solid, liquid, and gaseous phases.

The triple point is 0.01 K higher than the melting point of water at an absolute pressure of 1013.25 mbar, and thus almost identical to it. In European countries, the Celsius scale is permitted as a legal temperature scale alongside the Kelvin scale. The unit is the degree Celsius, abbreviated °C. The Celsius scale is shifted by 273.15 K relative to the Kelvin scale, so that 0 °C corresponds to the melting point and 100 °C to the boiling point of pure water .