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Thermodynamic and practical temperature scales

The normal hydrogen scale


Preparation of metre prototypes was begun. Each national prototype metre was furnished with two mercury-in-glass thermometers, calibrated at the BIPM. The thermometers were made, to the order of the BIPM, by an instrument maker in Paris called Tonnelot. The thermometers were made of verre dur, a particularly good glass from the point of view of stability, and a reproducibility of measurement of a few thousandths of a degree was possible. It became urgent to establish a uniform scale of temperature against which they could be calibrated.


Chappuis, at the BIPM, worked to relate the readings of the very best mercury-in-glass thermometers to absolute (i.e. thermodynamic) temperatures. In the first part of his study he considered in detail the constant-volume gas thermometer, using, in turn, hydrogen, nitrogen and carbon dioxide as the working fluids. The estimated uncertainty of his measurements was better than one-hundredth of a degree over most of the range studied, to 100 °C.


The CIPM adopted the constant-volume hydrogen scale (called the normal hydrogen scale), based upon fixed points at the ice point (0 °C) and the steam point (100 °C) as the practical scale for international metrology. This decision was ratified by the 1st CGPM in 1889.


Chappuis continued his work at the BIPM with an investigation of the constant-pressure gas thermometer using the same three gases. He concluded that the constant-volume thermometer provided a more convenient practical standard than did the constant pressure thermometer. At the instigation of Griffiths, of Kew Observatory, UK, the work was pursued, using a constant- volume thermometer, to extend the temperature range to higher temperatures. In collaboration with Callendar, Griffiths had been developing a platinum resistance thermometer which was stable to at least 600 °C. Callendar and Griffiths used the boiling point of sulphur, which they deduced to be 444.53 °C, as a third fixed point for calibration, and proposed to the BIPM that a comparison be made between their platinum resistance thermometers and the constant-volume gas thermometer of Chappuis.


This comparison was carried out by Chappuis, in collaboration with Harker of the Kew Observatory. It involved the establishment of a constant-volume nitrogen scale up to the boiling point of sulphur. The Chappuis/Harker measurement of the sulphur point led to a value of 444.70 °C, in very close agreement with the earlier result of Callendar and Griffiths.

The International Temperature Scale of 1927


Many freezing and boiling points were measured during the last two decades of the nineteenth century. Callendar gave a detailed review of gas thermometry at the 1899 meeting of the British Association for the Advancement of Science (BAAS), when he made a proposal for a practical temperature scale. He proposed that a platinum resistance thermometer be adopted as the defining instrument of the scale, and that it be calibrated at the freezing point of water and the boiling points of water and sulphur. Further, he proposed that a particular batch of platinum wire be selected from which the thermometers defining the scale be manufactured.

It was his intention that such a scale be called the British Association Scale of Temperature and that it be related to the ideal temperature scale through chosen gas thermometer measurements of the sulphur point. It is not clear why the British Association did not take up his proposals.


The Physikalish-Technische Reichanstalt (PTR, later to become the PTB), Berlin, addressed a circular letter to the BIPM, the National Physical Laboratory (NPL), Teddington, and the Bureau of Standards (BS, which in 1934 become the National Bureau of Standards, NBS, and in 1986 the National Institute of Standards and Technology, NIST), Washington, suggesting that the thermodynamic scale be adopted as the International Temperature Scale, and that a practical realization of it be the 1899 proposal of Callendar. Both the NPL and the BS agreed, the constants of the platinum were specified, and it was proposed that above the upper limit (1100 °C) the scale be defined in terms of the optical pyrometer.


At the 5th CGPM, every encouragement was given to this initiative and a Resolution was adopted, asking the Directors of the three laboratories to meet with the aim of coming to a firm agreement on such a scale. The planned meeting did not take place, however, owing to the outbreak of the First World War.


By the time discussions resumed, the three national laboratories had put into operation a platinum resistance thermometer scale covering the range from –38 °C, the freezing point of mercury, to 444.5 °C, the boiling point of sulphur, using a quadratic interpolation formula. During the course of a visit to the NPL and the PTR by a representative of the BS, the basis of an international scale was agreed upon. It was to consist of a platinum resistance thermometer to cover the range up to 650 °C, calibrated at 0 °C, 100 °C and the boiling point of sulphur at 444.5 °C. Between 650 °C and 1100 °C the scale was to be defined by a Pt-10 % Rh/Pt thermocouple calibrated at the freezing points of zinc, antimony, silver and gold and using a cubic interpolation formula. Above the gold point, 1063 °C, an optical pyrometer was proposed.

This informal agreement was followed by wider discussions in which the BIPM and the University of Leiden also participated.


In 1925 a draft proposal was drawn up, to be put to the CIPM in 1927. In this the range of the platinum resistance thermometer was extended down to -193 °C, and the cubic equation of the thermocouple was replaced by a quadratic equation with calibration points at the freezing points of antimony (630 °C), silver (960 °C), and gold.


The 7th CGPM adopted the International Temperature Scale of 1927, which differed very little from the draft of 1925. It was planned to hold an International Thermometry Conference in 1928, at which the question of the status of the International Temperature Scale would be examined in more detail. This Conference, however, did not take place.

The evolution of ITS-27 and its modifications in 1948


The CIPM established a Consultative Committee on Thermometry and Calorimetry to advise it on matters concerned with these subjects. Since then, it has been the Consultative Committee on Thermometry (CCT) that has largely taken the initiative in matters concerned with the evolution of the International Temperature Scale.


The first revision of the ITS took place in 1948. In this revision, the only change below 0 °C was the disappearance of the extrapolation below the oxygen point, to –190 °C, which had been found to be unreliable. The IPTS-48 extended down only to –182.97 °C. The junction between the resistance thermometer and the thermocouple was changed from 660 °C to the freezing point of antimony, 630.5 °C, and the temperature assigned to the silver point was increased slightly, from 960.5 °C to 960.8 °C. It was also decided to drop the name "degree Centigrade" for the unit and replace it by degree Celsius.

The 1958 4He and 1962 3He vapour pressure scales

The saturated-vapour pressure/temperature relation for liquid helium provides such a good and reproducible scale that its use as such long pre-dates any internationally agreed scale in the helium range; in fact, it even pre-dates ITS-27. However, it proved difficult to reach international agreement on a helium vapour pressure scale.


The CIPM adopted a Table of 4He vapour pressure against temperature data, proposed to it by the CCT. The Table was based upon gas thermometry data smoothed by magnetic thermometry and, below 2.2 K, by thermodynamic calculations. It covered the range from 0.5 K to 5.23 K and became known as the 1958 4He Scale, temperatures measured on it being denoted by T58.


Shortly after the adoption of the 1958 4He Scale, a further proposal was made in respect of a vapour pressure scale for 3He. This was based upon comparison of the vapour pressures of 3He with the 1958 4He Scale above 0.9 K, and with thermodynamic calculations below 0.9 K. The Scale was accepted by the CIPM and become known as the 1962 3He Scale, temperatures measured on it being denoted by T62.

Complete revisions have since been made of both the 3He and 4He scales, making them consistent with the results of gas, noise, acoustic and magnetic thermometry carried out since the original versions of the scales were adopted.

The International Practical Temperature Scale of 1968 (IPTS-68)


The 10th CGPM finally adopted a proposal that Kelvin had made back in 1854, namely that the unit of thermodynamic temperature be defined in terms of the interval between the absolute zero and a single fixed point. The fixed point chosen was the triple point of water, which was assigned the thermodynamic temperature of 273.16 °K.

The proposal had already been made in 1948, but at that time there was still a divergence of view as to what value should be assigned to the absolute zero. The question was finally resolved by the CGPM in 1954.


In 1961 it was agreed that the NPL and the Physicotechnical and Radiotechnical Measurements Institute (PRMI), Moscow, would undertake a comparison of platinum resistance thermometers calibrated on four of the most important gas thermometer scales. These were the NPL(1961), NBS(1955), PRMI(1954) and Pennsylvania State University PSU(1954) scales. The results of the comparison provided the basis for the eventual low-temperature part of IPTS-68.

The NBS-55 scale is of particular note since it is an example of the way in which a so-called "wire-scale" can be successfully operated. NBS-55 is a scale based upon gas thermometry carried out in 1939. It was originally maintained on a group of six platinum resistance thermometers, and was known as NBS-39. In 1955 an arbitrary shift of 10 mK was made over the whole of the scale and the name was changed to NBS-55. The successors to these six original NBS-39 thermometers continued to be used to maintain an NBS version of IPTS-68.


The CCT defined a reference function W for interpolation between a number of low-temperature fixed points. The CCT-64 was published as a table under the title "Provisional reference table CCT-64 of W against T for platinum resistance thermometers in the range 12 K to 273.15 K".


A CCT Working Group proposed a 1966 Provisional Scale, taking into account further gas thermometry results for the oxygen boiling point and the hydrogen triple point.


The second revision of the Temperature Scale took place, and resolved the curious situation that thermodynamic temperatures were defined in quite a different way from International Practical Temperatures. In the IPTS-68, both thermodynamic and practical units were defined to be identical and equal to 1/273.16 of the thermodynamic temperature of the triple point of water. The unit itself was renamed "the kelvin" in place of "degree Kelvin" and designated "K" in place of "°K".

In outline the IPTS-68 was made up of four parts:

    (a) between 13.81 K and 273.15 K;
    (b) 0 °C to 630.74 °C;
    (c) 630.74 °C to 1064.43 °C; and
    (d) above 1064.43 °C.(*)

In part (a) the Scale was defined in terms of a set of six low-temperature fixed points together with a reference function. In the range (b) the Scale was defined in terms of the old Callendar quadratic equation, but modified to take account of new gas thermometry values for the fixed points. In part (c) the defining instrument was the Pt-10 % Rh/Pt thermocouple, calibrated at 630.74 °C and the freezing points of silver and gold, and using a quadratic interpolation formula. Part (d) was defined in terms of the radiation emitted by a black body and described by Planck's equation.

*. Thermometrists generally refer to temperatures below 0 °C in kelvin, and those above in degrees Celsius.

The 1976 Provisional 0.5 K to 30 K Temperature Scale (EPT-76)


In 1976 the CIPM approved a new low-temperature scale called the 1976 Provisional 0.5 K to 30 K Temperature Scale, or EPT-76. Its purpose was to provide a unified Scale upon which temperature measurements could be made in this range, pending the revision and downward extension of the IPTS-68. It was defined in terms of the temperatures assigned to eleven fixed points within the range 0.5 K to 30 K, together with the differences between T76 and the following existing scales: IPTS-68; the 4He-1958 and 3He-1962 vapour pressure scales; NPL-75 and the NBS version of IPTS-68 which was defined by difference from NBS-55. In contrast to IPTS-68, the EPT-76 could thus be realized in a number of ways; either by using one of the above scales and the tabulated differences given in the text of EPT-76 or by using a thermodynamic interpolating thermometer, such as a gas thermometer or magnetic thermometer, calibrated at one or more of the specified reference points of EPT-76.

The International Temperature Scale of 1990 (ITS-90)


The International Temperature Scale of 1990 (ITS-90) came into effect on 1 January 1990, replacing the IPTS-68 and the EPT-76.

The ITS-90 differs from the IPTS-68 in a number of important respects:

  • it uses the triple point of water (273.16 K), rather than the freezing point of water (273.15 K), as a defining point;

  • it extends to lower temperatures: 0.65 K instead of 13.8 K;

  • it is in closer agreement with thermodynamic temperatures;

  • it has improved continuity and precision;

  • it has a number of overlapping ranges and sub-ranges;

  • in certain ranges it has alternative but substantially equivalent definitions;

  • it includes the helium vapour pressure scales;

  • it includes an interpolating gas thermometer as one of the defining instruments;

  • the range of the platinum resistance thermometer as defining instrument has been extended from 630 °C up to the silver point, 962 °C;

  • the Pt/10 % Rh-Pt thermocouple is no longer a defining instrument of the scale;

  • the range based upon the Planck radiation law begins at the silver point instead of at the gold point, but options exist for using any one of the silver, gold or copper points as reference points for this part of the scale.

The Provisional Low Temperature Scale of 2000


To extend the range of ITS-90 down to 0.9 mK, a Provisional Low Temperature Scale (PLTS-2000) was developed and approved in 2000 by the CIPM. It is based on 3He melting pressure because of the sensitivity and reliability with which this may be measured over a wide range: it covers more than three decades of temperature apart from a narrow region around a pressure minimum at 315.24 mK. This pressure minimum is one of four natural features that can be located and used as fixed points of pressure and temperature.