A Heat-Pipe is basically a closed chamber, filled with a small quantity of the so-called "working fluids", depending on the field of temperature ad application needed, having a capillary structure placed or machined all along the inner walls. One end of the tube is externally heated, causing the inside liquid to vaporise and the vapour to move to the opposite end of the pipe. This part of the Heat-Pipe is cooled, in order to condense the vapour back to liquid. The condensed fluid returns back to the heated part of the pipe, thanks to the capillary forces, thus closing a liquid-to-vapour and vapour-back-to-liquid cycle. The Gas-Controlled Heat-Pipe (GCHP) is a special kind of Heat-Pipe equipped with a gas-controlled line that enables a direct control of the inner pressure.
Gas-controlled heat-pipes (GCHPs) are used at IMGC for the study of vapour-liquid phase transitions for several pure substances (water, mercury, potassium, sodium), between 100 °C and 1000 °C. Phase transition curves are used in primary thermometry for contact thermometers calibrations and for vapour pressure scales study. By means of accurate pressure control systems, studied and developed at IMCG and based on the use of a PRT as sensor, temperature stability within few tenth of millikelvin is achievable. Temperature uniformity in the measuring zone is generally within few millikelvin.
The "Temperature Amplifier" has been realized at IMGC and is based on a couple of GCHPs connected to the same pressure line, one using mercury and the other using sodium as working fluids. Each temperature in the mercury GCHP, between 240 °C and 400 °C, correspond to a unique temperature in the sodium GCHP, between 660 °C and 962 °C. As a consequence, the very high reproducibility of SPRTs below 400 °C, in the mercury GCHP, can be transferred to the temperature range above 660 °C, in the sodium GCHP, surpassing the reproducibility of HTSPRTs and showing a reproducibility better than that of the Al freezing point. The results achieved at IMGC open the perspective of redefining one temperature range in terms of another. Several metrological laboratories (NMIs) in the world expressed a growing interest in the field. New GCHPs have been manufactured under IMGC design and are now available, both for calibration purposes or for research. A new generation of "Temperature Amplifier" (TA), based on two new manufactured GCHP and an innovative pressure controller, is in course of realization. A new laboratory, devoted to the study and application of the GCHPs, will be setup at IMGC.
A Euromet project on the circulation of a TA is started and will be coordinated by IMGC, while several contracts with other NMIs and university are opened both in the field of metrological research and for calibration devices studies, based on the use of GCHP. The CCT officially expressed an interest in the use of vapour pressure curves for the definition of temperature standards.
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