National Metrology Institute of Japan (NMIJ)

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.

Defining fixed points of the International Temperature Scale of 1990 (ITS-90) and interpolating thermometers

The Realization of the triple point of water

Phase diagram 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".

Water triple-point cell

Water triple-point cell: 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