An indirect determination of the thermodynamic temperature of the fixed point of copper was made at INRIM by measuring four cells with a Si-based and an InGaAs-based precision radiation thermometer carrying approximated thermodynamic scales realized up to the Ag point. An average value TCu = 1357.840 K was found with a standard uncertainty of 0.047 K. A consequent (T − T90)Cu value of 70 mK can be derived which is 18 mK higher than, but consistent with, the presently available (T − T90)Cu as elaborated by the CCT-WG4.
The design of a compact blackbody furnace that allows temperature fixed points to be realized to a high degree of accuracy is described. Using this apparatus, the standard uncertainty in the calibration of a precision infrared thermometer at the fixed-point temperatures of indium, tin and zinc was estimated to be about C. This furnace can be used as a travelling standard for intercomparisons as well as laboratory apparatus for the calibration of precision infrared thermometers.
Four independent radiation temperature scales approximating the ITS-90 at 900 nm, 950 nm and 1.6 µm have been realized from the indium point (429.7485 K) to the copper point (1357.77 K) which were used to derive by extrapolation the transition temperature T 90 (Co-C) of the cobalt-carbon eutectic fixed point. An INRIM cell was investigated and an average value T 90 (Co-C) = 1597.20 K was found with the four values lying within 0.25 K. Alternatively, thermodynamic approximated scales were realized by assigning to the fixed points the best presently available thermodynamic values and deriving T (Co-C). An average value of 1597.27 K was found (four values lying within 0.25 K). The standard uncertainties associated with T 90 (Co-C) and T (Co-C) were 0.16 K and 0.17 K, respectively. INRIM determinations are compatible with recent thermodynamic determinations on three different cells (values lying between 1597.11 K and 1597.25 K) and with the result of a comparison on the same cell by an absolute radiation thermometer and an irradiance measurement with filter radiometers which give values of 1597.11 K and 1597.43 K, respectively (Anhalt et al 2006. The INRIM approach allows the determination of both ITS-90 and thermodynamic temperature of a fixed point in a simple way and can provide valuable support to absolute radiometric methods in defining the transition temperature of new high-temperature fixed points.
The high-temperature extension of the fixed-point technique for primary calibration of precision infrared (IR) thermometers was investigated both through mathematical simulations and laboratory investigations. Simulations were performed with Co-C (1,324 • C) and Pd-C (1, 492 • C) eutectic fixed points, and a precision IR thermometer was calibrated from the In point (156.5985 • C) up to the Co-C point. Mathematical simulations suggested the possibility of directly deriving the transition temperature of the Co-C and Pd-C points by extrapolating the calibration derived from fixed-point measurements from In to the Cu point. Both temperatures, as a result of the low uncertainty associated with the In-Cu calibration and the high number of fixed points involved in the calibration process, can be derived with an uncertainty of 0.11 • C for Co-C and 0.18 • C for Pd-C. A transition temperature of 1,324.3 • C for Co-C was determined from the experimental verification, a value higher than, but compatible with, the one proposed by the thermometry community for inclusion as a secondary reference point for ITS-90 dissemination, i.e., 1,324.0 • C.
The behavior of the Co-C eutectic fixed point was investigated at INRIM. Several cells of different design and volume, and filled with cobalt of different purity were constructed and investigated with both Pt/Pd thermocouples and radiation thermometers. The melting behavior was investigated with respect to the melting rate, the pre-freezing rate, and the annealing time. The melting temperatures, as defined, were not significantly affected by the different testing conditions, even if the shape and duration of the plateaux were influenced. Several tens of melt and freeze cycles were performed with the different cells. The spread in the results for all of the different conditions was very limited in extent, giving rise to a standard deviation of less than 0.04 • C; a repeatability of better than 0.02 • C was found with both Pt/Pd thermocouples and radiation thermometers. The results of our measurements are encouraging and confirm the suitability of Co-C as a reference point for the high-temperature range in a possible future temperature scale. Investigations of long-term stability remain ongoing.
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