In 1990 the international practical temperature scale was revised with a very small shift in the definition of the scale reference point from the freezing point of water, as used in the previous 1968 scale, to the triple point of water.
A digital thermometer is an essential tool for measuring temperature.
The triple point of water is the single combination of pressure and temperature at which pure water, pure ice, and pure water vapour can coexist in a stable equilibrium. This occurs at exactly 273.1598 K (0.0098 °C) and a pressure of 611.73 Pascals (ca. 6.1173 millibars,
0.0060373057 atm ) a difference of 0.01°C in the 1990 scale ITS90.
With the introduction of the ITS90 scales the previously called Degrees Kelvin” became Kelvin. The ITS90 scale has been adopted
internationally and supersedes the previous International Practical Temperature Scale of 1968.