Logo PTB

Developing of new (approximating) methods for the realisation of the temperature scale

Noble metal thermocouples of type S, R and B (platinum-rhodium alloys) and pure metal thermocouples made of gold and platinum (Au/Pt) and made of platinum and palladium (Pt/Pd) can be calibrated at the melting and freezing points of pure metals up to temperatures of 1600 °C. However, in the temperature range above about 1000 °C only a few suitable fixed points of pure metals are available (Ag, Pd). Therefore, eutectic (and peritectic) fixed points made of metal-carbon alloys as fixed-point materials are developed and metrologically investigated. Its phase transition temperatures are presented in figure 1.

Innovative approaches are applied to develop and investigate novel thermocouples usable at temperatures above 1600 °C, for instance on graphite basis.

Figure 1: Phase transition temperatures of eutectic (peritectic) metal-carbon alloys

 

The use of the additional metal-carbon fixed points improves the accuracy and traceability of temperature measurements above 1000 °C by reducing the measurement uncertainties caused by interpolation and extrapolation when using pure metal fixed points solely. An additional advantage of metal-carbon fixed points is the fact that the graphite of the crucible material is simultaneously a part of the fixed-point material. In this way the risk of the fixed-point material to become contaminated by the crucible is significantly reduced. A schematic diagram of a double-wall fixed-point cell usable for metal-carbon eutectics is shown in figure 2.

Figure 2: Eutectic fixed point cell usable for the calibration of thermocouples