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Australian Government: National Measurement InstituteAustralian Government: National Measurement Institute
National Measurement Institute
      

Temperature Research

NMI is responsible for maintaining and disseminating Australia's standard for temperature (the kelvin). Recent research and development has been directed towards improving the accuracy of the temperature scale at high temperatures, developing thermocouples with greater accuracy at high temperatures and investigating the use of thermoluminescence to measure high temperatures.

High Temperature Fixed Points

The accuracy of the temperature scale is much lower above 1000°C, largely due to the lack of convenient reference points. NMI aims to improve the accuracy of the temperature scale in this region by the development of fixed point blackbodies such as platinum (1768°C), alumina (2053°C) and zirconia.

Elemental Thermocouples

Until recently, the most stable thermocouples in the region above 500°C were thought to be those based on platinum and a platinum/rhodium alloy, having a calibration uncertainty of the order 0.5°C at 1000°C. Recently, NMI and other national measurement institutes have been conducting research into the development of a new generation of high-stability thermocouples, such as gold versus platinum or platinum versus palladium, with the aim of achieving an order of magnitude improvement in accuracy.

The first phase of the study, that of studying the thermoelectric behaviour of pure wires of Au, Pt and Pd at temperatures up to 1000°C and when affected by stress, such as handling, has been completed. The next phase, of examining practical versions of the Au-Pt and Pt-Pd thermocouples, is underway.

RE Bentley (2001) Thermoelectric Changes in Au and Pt: Metals used in Elemental Thermocouples. Meas. Sci. Tech. 12(5), 627–634
RE Bentley (2001) Thermoelectric Instability in Pd within the Elemental Thermocouple Pt/Pd. Meas. Sci. Tech. 12(8), 1250–1260

Thermoluminescence

The temperature dependence of the decay time of stimulated fluorescence in rare earth doped ceramic crystals is a means of measuring temperature. The technique has been widely used in industry at the 0.5 to 2°C accuracy level in optical fibre-based thermometers. Research at NMI aims to extend the use of this technique to 1200°C or higher at the 0.1°C accuracy level.