An International Intercomparison of Fixed Points by Means of Sealed Cells has been conducted under the auspices of the Comité Consultatif de Thermométrie (CCT) between 1978 and 1984. Forty-one sealed cells, realizing the triple point of seven different substances, defining both primary fixed points of the IPTS-68 and secondary fixed points in the temperature range from 14 K to 90 K, were supplied by nine laboratories. They were measured in eleven national laboratories around the world, against the fixed points realized in these laboratories (both in open cryostats or in other sealed cells). Some 150 independent series of data were produced, from almost 300 melting experiments, representing some 2,300 equilibrium temperature values. The basic sets of results are presented, concerning the agreement between different cell realizations and the comparison of national IPTS-68 realizations. Data connecting the results of this intercomparison with that performed at NPL in 1975 using calibrated thermometers are also given.

Within the International Temperature Scale of 1990 (ITS-90) the platinum resistance thermometer (PRT) is used to realize the scale from approximately 13,8 K to 1 235 K. Such a temperature range is wider than the corresponding range in the International Practical Temperature Scale of 1968 (IPTS-68) because the PRT is used up to the freezing point of silver (1 234,93 K). In this way, the ITS-90 can be realized with much more precision than the IPTS-68, particularly between 901 K and 1 235 K where the standard Pt-10% Rh vs Pt thermocouple was previously used.This paper describes some important steps in the construction of the PRT reference function and the criteria for the selection of the PRT interpolating equations of the ITS-90. In contrast to previous international scales, the PRT range of the ITS-90 is based on two reference functions, one from 13,8 K to 273,16 K and the other from 273,15 K to 1 234,93 K. The two reference functions were obtained from two real PRTs.A set of interpolating equations is used to account for the differences of other real PRTs from those on which the reference functions are based. They provide flexibility of calibration and high precision as expressed, for instance, by non-uniqueness and sub-range inconsistency not exceeding 0,5 mK over the range from 13,8 K to 693 K. Such good properties are the consequence of the choice of suitable forms for the interpolating equations and of fine adjustments in the values assigned to some defining fixed points.

The construction of a constant-volume gas thermometer for use between 10 K and 300 K is described. The thermometer features gas-flow temperature control and the capability for continuous operation. Relative PV isotherms giving values for thermodynamic temperature and the second virial coefficient for 4He were measured at 27 K, 43 K, 54 K, 63 K and 84 K referenced to 20 K on NPL-75 and at 172 K referenced to 273.15 K. Further measurements at the isotherm temperatures referenced to 273.15 K were used to redetermine the reference temperature at 20 K. Constant-volume gas-thermometry measurements were carried out at 72 temperatures between 13.81 K to 287 K. The results agree well with NPL-75 from 13.81 K to 27.1 K but differ from IPTS-68 significantly from 27.1 K to 273.15 K.

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A constant-volume gas thermometer has been used to measure the thermodynamic temperature, and also the second virial coefficient of helium 4, at the triple point of neon (24.5 K). This was carried out by measuring a relative isotherm at 24.5 K referenced to the temperature of the boiling point of hydrogen on the NPL75 temperature scale. The thermodynamic temperature values measured for the triple points of natural neon and neon 20 were (24.5570 ± 0.0011) K and (24.5394 ± 0.0011) K respectively. The result for natural neon agrees well with the corresponding temperature value of (24.5565 ± 0.0011) K inferred from NPL75 and may be related to other laboratory realizations of the neon triple point through the recent international intercomparison of sealed triple-point cells.