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Highest temperatures precisely fixed

With the aid of radiation thermometry based on PlanckÕs radiation law, PTB could determine the melting temperatures of novel eutectic metal-carbon alloys (M-C alloys) up to nearly 3000Ê¡C for the first time with low measurement uncertainties. To improve industrial high-temperature measurements, these M-C alloys shall in future be used to determine high-temperature fixed points.

View into a Re-C cell during melting at 2475 °C. The cell was
developed within the scope of the HIMERT EU project.

The measurement of high and highest temperatures is decisive for the efficient use of resources e.g. in power engineering or aerospace industry. A service temperature that is too high or too low by 20 °C reduces, for example, the service life of a turbine operated at 1500 °C by 50 %, and safety can also depend on precise high-temperature measurements, e.g. when space shuttles re-enter the Earth’s atmosphere.

The currently valid international temperature scale of 1990 (ITS-90) was defined with the aid of a series of fixed points with fixed temperatures. The highest fixed-point temperature of 1085 °C is the setting temperature of copper; beyond this temperature, the ITS-90 depends on extrapolation with increasing measurement uncertainty. Up to now, the precision of high-temperature measurements has therefore been limited by the lack of even higher, reproducible melting and setting temperatures.

The Japanese State Institute’s proposition in 1999 to use M-C alloys for such high-temperature fixed points, since the carbon alloys of different metals have melting points from 1150 °C to over 3000 °C, gave a prospect of improvement. To prepare the transfer of these materials to industrial application, PTB, in cooperation with the Japanese and the Russian State Institutes, investigated them in detail within the scope of an EU-project. For the first time, PTB succeeded in precisely determining the melting temperatures of M-C alloys for temperatures up to 2880 °C. For Re-C (melting temperature 2475 °C) for example, an expanded measurement uncertainty of less than 1 °C could be achieved with a reproducibility between different fixed-point cells of less than 0.2 °C.

Currently, PTB is taking part in a global project for the improvement of the International Temperature Scale by implementation of the new high-temperature fixed points.

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