International Temperature Scale of 1990

However, the alteration was very slight compared to the ITS-90 uncertainties, and so the ITS-90 remains the recommended practical temperature scale without any significant changes.

It is anticipated that the redefinition, combined with improvements in primary thermometry methods, will phase out reliance on the ITS-90 and the PLTS-2000 in the future.

Accordingly, ITS-90 uses numerous defined points, all of which are based on various thermodynamic equilibrium states of fourteen pure chemical elements and one compound (water).

The defining fixed points of the ITS-90 refer to pure chemical samples with specific isotopic compositions.

The standard also compensates for the pressure effect due to how deeply the temperature probe is immersed into the sample.

For instance, precise measurements show that the boiling point of VSMOW water under one standard atmosphere of pressure is actually 373.1339 K (99.9839 °C) when adhering strictly to the two-point definition of thermodynamic temperature.

ITS-90 does not address the highly specialized equipment and procedures used for measuring temperatures extremely close to absolute zero.

It had become apparent that ITS-90 deviated considerably from PLTS-2000 in the overlapping range of 0.65 K to 2 K. To address this, a new 3He vapor pressure scale was adopted, known as PTB-2006.