Introduction to ITS-90

On the 1st January 1990 a new temperature scale introduced: The International Temperatures Scale of 1990 (ITS-90), abbreviated to T₉₀. The new scale resulted in changes of 1.5°C at 3000°C although less than 0.025°C change over the -100°C to +100°C range. For those interested in precision temperature measurement, these changes may be significant, but for most temperature watchers, the impact is insignificant. A plot of the correction (T₉₀- T₆₈) is presented below:

The shape of this curve is curious, especially between 630°C and 1060°C and is due to a number of changes in reference points and interpolation methods. See the full text of ITS-90 (115k).

Interestingly, at the beginning of the same year the ITS-90 was introduced, the definition of the volt changed a little with the 1990 SI volt. ITS-90 is defined in terms of the 1990 SI volt.

Defining Points of ITS-90

   A table of the defining fixed points of the ITS-90 scale is given below. These are the fundamental reference points on which the temperature scale is based. They have been selected because of their repeatability, ease of handling and availability in pure forms.

The melting and freezing point measurements are conducted at a pressure of 101.325 kPa. The triple point is the temperature (and pressure) when solid, liquid and gaseous phases of the material are present simultaneously. A number of companies specialise in the manufacture of Defining Point Cells for use by researchers and standards laboratories.