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Standards for the SI Base Units - Temperature
The Definition of the Kelvin and the International Temperature Scale
The kelvin (K), unit of thermodynamic temperature, is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The International Temperature Scale (ITS) is a temperature scale which is designed to be consistent with thermodynamic temperatures measured on the basis of this definition. ITS is calibrated by using temperature fixed-points (defining fixed-points), which are obtained when specific substances are in phase equilibrium, and several stable thermometers. The first International Temperature Scale was established in 1927. After a series of subsequent extensions of temperature range and precision improvements, the International Temperature Scale of 1990 (ITS-1990) was adopted and is still in use today. Numerical values in the unit of Celsius temperature, symbol °C (degree Celsius), are produced by those in kelvins minus 273.15.

The Realization of the triple point of water

The state in which the three phases of water, i.e. ice (solid), water (liquid) and vapor (gas), coexist is called the "triple point of water".


This is a sealed cell made of glass to realize the triple point of water. The temperature where the three phases coexist is 273.16K.
Platinum resistance thermometer


Radiation thermometer

3He vapor pressure thermometer
