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.
A noise thermometer has been developed at IMGC and has been used to realize the Thermodynamic Scale (TS). Differences between TS, as realized at IMGC, and the IPTS-68 were determined in the range 630-962OC and the results are presented.The discussion of results primarily deals with the factors introducing measurement uncertainties and with the comparison of noise thermometer results with those of recent radiation thermometry and gas thermometry experiments.Data fitting provides the TS versus IPTS-68 differences as a continuous function of the Celsius temperature in the range 630-1064°C. For this, data from monochromatic pyrometry experiments are taken into account in order to cover the range from the freezing point of silver to the freezing point of gold.
The freezing point of silver is being considered as a joining point between platinum resistance thermometry and optical pyrometry. Therefore the freezing points of high purity samples of silver have been investigated. An important impurity effect arises from the depression of the freezing point of pure Bg caused by residual dissolved oxygen contents in some samples. Melting range parameters were determined on both oxided and carefully deoxided samples to aid in the selection of the purest sample. In addition the presence of dissolved oxygen is indicated by a slope inversion on the melting curve. Evaluations of the freezing point depression for small oxygen contents are given. A graphite sample crucible, 11. ith graphite radiation and gettering discs interspersed in the thermal insulation of the thermometer well, is a convenient arrangement to prevent the contamination of the sample in the presence of traces of oxygen, but is not sufficient for purifying heavily oxided samples; such samples require a vacuum treatment at about 1050" C. The pressure effect on the freezing temperature of silver has been determined.
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