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2.3 Temperature and heat

2.3.1 The International Temperature Scale of 1990 (ITS-90)

The history of the ITS-90

The International Temperature Scale was adopted in 1927 to overcome the practical difficulties of the direct realization of thermodynamic temperatures by gas thermometry and to unify existing temperature scales. It was introduced by the Seventh General Conference on Weights and Measures with the intention of producing a practical scale of temperature which was easily and accurately reproducible and which gave as nearly as possible thermodynamic temperatures. The Scale was revised in 1948, amended in 1960 (the numerical values of temperature remaining the same as in 1948) and revised again in 1968 and 1990. The International Temperature Scale of 1990 was adopted by the International Committee for Weights and Measures at its meeting in 1989, in accordance with the request embodied in Resolution 7 of the 18th General Conference of Weights and Measures of 1987 (see Metrologia 27 1990). This Scale supersedes the International Practical Temperature Scale of 1968 (amended edition of 1975) and the 1976 Provisional 0.5 K to 30 K Temperature Scale. The 1990 revision reduced the lower limit of the Scale from 13.8 K to 0.65 K and the values of the defining fixed points of the new Scale were adjusted to conform as closely as possible to thermodynamic temperatures. The differences between temperatures measured on ITS-90 and corresponding temperatures on IPTS-68 are significant and are given in the table below.

Units of temperature

The unit of the fundamental physical quantity known as thermodynamic temperature, symbol T, is the kelvin, symbol K, defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water^†.

Because of the way earlier temperature scales were defined, it remains common practice to express a temperature in terms of its difference from 273.15 K, the ice point. A thermodynamic temperature, T, expressed in this way is known as a Celsius temperature symbol t, defined by:

   t/°C = T/K − 273.15

The unit of Celsius temperature is the degree Celsius, symbol °C, which is by definition equal in magnitude to the kelvin. A difference of temperature may be expressed in kelvins or degrees Celsius.

The International Temperature Scale of 1990 (ITS-90) defines both International Kelvin Temperatures, symbol T₉₀, and International Celsius Temperatures, symbol t₉₀. The relation between T₉₀ and t₉₀ is the same as that between T and t, i.e.:

   t₉₀/°C = T₉₀/K − 273.15

The unit of the physical quantity T₉₀ is the kelvin, symbol K, and the unit of the physical quantity t₉₀ is the degree Celsius, symbol °C, as is the case for the thermodynamic temperature T and the Celsius temperature t.

Principles of the ITS-90

The ITS-90 extends upwards from 0.65 K to the highest temperature practicably measurable in terms of the Planck radiation law using monochromatic radiation. The ITS-90 comprises a number of ranges and sub-ranges throughout each of which temperatures T₉₀ are defined. Several of these ranges or sub-ranges overlap, and where such overlapping occurs differing definitions of T₉₀ exist: these differing definitions have equal status. For measurements of the very highest precision there may be detectable numerical differences between measurements made at the same temperature but in accordance with differing definitions.

^† Comptes Rendus des Séances de la Treizième Conférence Générale des Poids et Mesures (1967–1968). Resolutions 3 and 4, p. 104.

Similarly, even using one definition, at a temperature between defining fixed points two acceptable interpolating instruments (e.g. resistance thermometers) may give detectably differing numerical values of T₉₀. In virtually all cases these differences are of negligible practical importance and are at the minimum level consistent with a scale of no more than reasonable complexity.

The differences between ITS-90 and EPT-76, and between ITS-90 and IPTS-68

    * A discontinuity in the first derivative of (t₉₀ − t₆₈) occurs at temperature of t₉₀ = 630.6°C when (t₉₀ − t₆₈) = − 0.125°C

The ITS-90 has been constructed in such a way that, throughout its range, for any given temperature the numerical value of T₉₀ is a close approximation to the numerical value of T according to best estimates at the time the scale was adopted. By comparison with direct measurements of thermodynamic temperatures, measurements of T₉₀ are more easily made, are more precise and are highly reproducible.

Definition of the ITS-90

Between 0.65 K and 5.0 K T₉₀ is defined in terms of the vapour-pressure temperature relations of ³He and ⁴He.

Between 3.0 K and the triple point of neon (24.5561 K) T₉₀ is defined by means of a helium gas thermometer calibrated at three experimentally realizable temperatures having assigned numerical values (defining fixed points) and using specified interpolation procedures.

Between the triple point of equilibrium hydrogen (13.8033 K) and the freezing point of silver (961.78 °C) T₉₀ is defined by means of platinum resistance thermometers calibrated at specified sets of defining fixed points and using specified interpolation procedures.

Above the freezing point of silver (961.78 °C) T₉₀ is defined in terms of a defining fixed point and the Planck radiation law.

The defining fixed points of ITS-90 and some selected secondary reference points

    * Defining point of ITS-90.
       All except the triple points and the hydrogen boiling point at 33 330.6 Pa are at a pressure of 101.325 Pa (1 standard atmosphere).

References

Metrologia (1990) 27, 3–10 and 107; Metrologia (1994) 31, 149–153

T.J.Quinn

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