Bilateral ITS-90 comparison at WC-C peritectic fixed point between NIM and NPL

Abstract

Improvements in the realization of the ITS-90 over the temperature range from the melting point of gallium to the freezing point of silver at NIM Abstract. The WC-C peritectic fixed point, nominal melting and freezing temperature 2747 qC, shows extremely good metrological potential. Elsewhere, we published a prototype scale comparison of the ITS-90 between NPL, NIM and CEM, using high temperature eutectic fixed points (HTFPs) of Co-C (1324 qC), Pt-C (1738 qC), and Re-C (2474 qC). In this paper we present the further results of the bilateral comparison of the ITS-90 at an even higher temperature, 2747 qC, between NIM and NPL using WC-C peritectic fixed points. A NIM single zone high temperature furnace, model Chino IR-80, was modified to extend its temperature to 2800 qC. Then, an NPL researcher, on secondment to NIM, filled two WC-C cells in the modified furnace in a vertical position. The two WC-C cells were then realized in the same furnace, in an horizontal position. Their melting temperatures, defined by the inflection point of the melting curves, were measured by a linear pyrometer, model NIM-PSP. NIM's ITS-90 scale was assigned to the two cells, which were then transported to NPL. The realization of NPL's ITS-90 was then assigned to the two cells by using a model HT9500 Thermogauge furnace to realize the fixed points and a linear pyrometer, model LP3, to determine their temperature. The difference from the mean value of the NIM and NPL ITS-90 values for the WC-C points was derived. This allowed us to compare ITS-90 as realized by the two institutes and to determine the uncertainty in the scale comparison..

Performance of WC–C peritectic and Ru–C eutectic fixed points

“…As mentioned in section 2.1, the NIM-WC-4# cell has previously been shown to have a melting temperature 0.40 °C lower than the NPL-WC-1 cell [11]. Taking this difference into account, the NPL-WC-1 cell melting temperature on the NMIJ scale is derived, which is shown in table 3.…”

“…The influence of impurities on the liquidus and solidus of W-C-X system was evaluated by use of Thermo-Calc software, and the results also showed the advantage of the WC-C system against impurities [19]. a Melting temperature of the NPL-WC-1 cell was derived from the result of the NIM-WC-4# and the result in [11].…”

“…The final mass fraction of carbon was 5.0% with the ingot mass of 34.3 g and 50.9 g for the NPL-WC-1 cell and the NIM-WC-4# cell, respectively. The melting temperature of the NIM-WC-4# cell was 0.40 °C lower than that of the NPL-WC-1 cell in the previous comparison [11], so the superior cell, the NPL-WC-1 cell, was chosen as the comparison artefact. However, this cell was not available at the time of the comparison, so the comparison was carried out with the NIM-WC-4# cell, which served to link the result to the NPL-WC-1 cell.…”

“…WC-C cells constructed at NMIJ have already been used as a transfer standard for a bilateral comparison of the ITS-90 realization between NIM and NMIJ [10]. NIM and the National Physical Laboratory (NPL), UK, performed a similar comparison with WC-C cells constructed at NIM [11]. The results of both these comparisons confirmed that WC-C cells have high stability and repeatability that makes them suitable for scale comparisons at high temperatures above 3000 K. High reproducibility of WC-C cells constructed at one and the same laboratory was also demonstrated at VNIIOFI [12].…”

Investigation of ternary and quaternary high-temperature fixed-point cells, based on platinum–carbon-X, as blind comparison artefacts

“…HTFPs can be used to compare high temperature scales and this has been done on a number of occasions [10][11][12]. However, as the temperatures of the HTFPs become better known the credibility of the comparison has been compromised because the temperatures are known a priori.…”