Use of Heat-Pipes as new temperature standards for contact thermometry

Project Description

During recent studies started and prompted by IMGC, an encouraging agreement has been obtained between measurements of the immersion characteristics in a mercury Gas-Controlled Heat-Pipe (GCHP) and the Clausius-Clapeyron profile.
This result opened the perspective of chaining GCHPs, in order to thermodynamically relate different temperature ranges, with the possibility of defining one temperature range in terms of another, namely of a lower one, where the uncertainty can be kept smaller. Indeed, by controlling the helium pressure over the GCHP working substance by means of a Temperature Standard Platinum Resistance Thermometer (SPRT) inserted in the thermometer well of a mercury GCHP, any temperature in the range between 240 °C and 400 °C was "amplified" in a sodium GCHP connected to the same helium line, so realising very reproducible temperatures in the whole range between 660 °C and 962 °C. This device, called “Temperature Amplifier” allowed a temperature reproducibility between 660 °C and 962 °C that surpasses that of the High Temperature Standard Platinum Resistance Thermometer (HTSPRTs) defined for use in this range by the ITS-90.

This technique, which is being developed now also in other Laboratories and is rapidly spreading, opens the prospect of realizing the ITS in a different, less uncertain, way at the higher temperatures and up to the silver point: any accurate reproducible temperature range may be used to generate another temperature range, the relationship between the two ranges being thermodynamically related by the vapor-liquid transitions of pure substances .This is an advantage especially at high temperatures, where the use of GCHPs will be useful to overcome the well known ITS-90 non uniqueness and the insufficient reproducibility of the HTSPRTs.The Project aims at the realization and/or circulation of a prototype of “Temperature Amplifier”, based on two coupled GCHP with the dedicated furnaces, initially of mercury and sodium . It will be provided of the innovative pressure controller and all relevant electronic equipment already available from IMGC. Other control-system prototypes are aimed to be developed during the project. The temperature of the two working-fluid phase transitions will be measured using SPRTs and HTSPRTs calibrated at the ITS-90 fixed points. The thermodynamic relation between the two phase transition curves will be separately evaluated by the involved laboratories where the apparatus will be hosted. The results obtained will be fundamental for the proposal of a possible different future definition of the ITS using this new temperature standard for contact thermometry, namely in the temperature field between the present ITS-90 Al and Ag fixed points.

The project is expected to last three years from its start.
The project is expected to extend to further participants outside the EU.

Final Report 2014-06-06

As reported in the 2012 report, the EUROMET 772 project became part of the EMRP SIB10 NOTED. all the activities planned for the 772 are now structured as tasks and deliverables of the SIB10. The 772 is therefore closed.

Thermometry (T)
Andrea Merlone, INRIM (Italy)
Coordinating Institute
INRIM (Italy)