High temperature fixed pointsinvestigation of influence parameters on the temperature uncertainty by thermal modelling

Zusammenfassung

Abstract In the field of temperature standards and calibration, accurate determination of the high temperature eutectic melting point is very important. However due to the difficulty in performing experimental measurements at such high temperatures thermal modelling is an essential tool to understand the sources of uncertainty in the realisation of these fixed points. Once the temperatures of these fixed points is established, they can easily be used in any laboratory that has suitable facilities as reliable high temperature fixed points (HTFP). The difference between the ideal fixed point temperature at the liquid-solid interface and the temperature measured by the radiation thermometer is caused by several factors: heat loss to the surroundings, thermal resistance of the crucible walls, temperature profile in the furnace, design of crucible and furnace and the nominal temperature of the fixed point. It is these factors that are most easily investigated through thermal modelling. In this work the uncertainties of these factors for radiation thermometry of high temperature eutectic fixed points is estimated. In particular, this modelling is applied to the melting temperature of the Co-C, Pt-C and Re-C alloys which are the subject of intense research by the world metrology community to form the basis of new temperature references. These uncertainties (in terms of a temperature difference) has been evaluated by numerical simulation using the FLUENT (finite volume method) computational package to specific cruciblefurnace configurations designed by the National Metrology Institute of Japan (NMIJ), the National Physical Laboratory (NPL, UK) and Laboratoire National D'Essais (LNE, France). There have also been studies undertaken with the National Institute of Metrology of China (NIM), the Spanish Metrology Institute (CEM) and the National Institute of Standards and Technology (NIST, United States of America) all with the aim of understanding uncertainty contributions to facilitate the accurate realisation of high temperature fixed points. The first chapters deal with general aspects about thermometry and the analysis and validation of the approach of numerical modelling. The next chapters are focused in modelling and evaluating the uncertainty factors in the temperature drop associated with the HTFP. As a result of this work, the influence of cracks and holes inside imperfectly formed metal-ingots is evaluated, uncertainties associated with the values of emissivity and thermal conductivity of graphite are calculated and also the influence of the blackbody tube length and the back-wall thickness or the use of different furnaces and temperature profiles is estimated.