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The International temperature scale (ITS) is a combination of established numerical values of reference points, interpolation dependencies and techniques which ensure the unity of temperature measurements. It is developed on the basis of generalizations made by the consulting committee on thermometry and is affirmed by the General Conference on Weights and Measures.

The first ITS-27 approved by the 7th session of the General Conference on Weights and Measures in 1927 covered the sphere of determination from the normal boiling point of oxygen (−182.97°C) up to the gold solidifying point (l,063°C) and contained six reference points. Suggested as interpolation instruments between the reference points were the platinum resistance thermometer and the platinum-rhodium thermocouple (660°C−l,063°C), For determining the temperature above the gold solidifying point, an optical thermometer and Wien's formula were recommended.

The international practical temperature scale (IPTS ) of 1948 was the result of refinements in the values of reference points of ITS-27. In IPTS-68, "Kelvin" (K) was first defined as a unit of temperature. This scale can be used for the low temperature range down to the triple point of hydrogen of 13.81 K (−253.34°C). The number of main reference points was increased to 13. The estimated error of values of reference points was 0.01 K for the low-temperature range and reached 0.2 K at the gold solidifying point.

In 1976, a Preliminary Temperature Scale (EPT-76) was accepted for use which ensured unity of measurements in the temperature range of 0.5−30 K. ITS-76 coincided with IPTS-68 at the neon boiling point (~27 K).

ITS-90 was placed in service beginning Jan. 1, 1990. The unit of thermodynamic temperature T90—kelvin (K)—was defined in ITS-90 as 1/273.16 part of the thermodynamic temperature of the triple point of water. The temperature expressed in degree Celsius (°C) is designated as t90 and is defined

The ITS-90 has the following fields of definition: T90 is defined between 0.65 and 5.0 K from the saturated vapor pressure of helium isotopes 3He and 4He; T90 between 3.0 K and the triple point of neon (24.5561 K) is defined by the helium interpolation gas thermometer, calibrated to three experimentally-realized temperatures which have assigned values; T90 between the triple point of equilibrium hydrogen (13.8033 K) and the silver solidifying point (961.78°C) is defined by a platinum resistance thermometer, calibrated to reference points by the interpolation dependence of platinum resistance on the temperature between these points; T90 above the silver point is determined from reference points and from Plank's radiation formula.

The ITS-90 is based on 17 reference points, the 9th of which corresponds to the triple point of water where temperature values T90 = 273.16 K and t90 = 0.01°C. Differences between ITS-90 and IPTS-68 are shown in Figure 1.

Temperature differences between ITS-90 and IPTS-68.

Figure 1. Temperature differences between ITS-90 and IPTS-68.

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