The ‘Mini‐Coil’ Method for Calibration of Thermocouples at the Palladium Point

The melting point of palladium (1554.8 °C on the ITS‐90) has widespread use as a reference point for the calibration of thermocouples to 1550 °C. The ‘wire bridge’ method is widely used to realize this reference temperature. At NML (CSIRO) a more convenient technique, the ‘mini‐coil’ method, in which a small coil of Pd wire is squeezed over the thermocouple junction has been developed and used for some years. In the study presented here the ‘mini‐coil’ method is assessed for any possible systematic errors, such as arising from the argon gas purge or the heating rate, and is compared with the ‘wire bridge’ method. The two methods are found to give equivalent results; however the larger mass of the Pd used in the mini‐coil method gives longer, better defined and more reproducible melting plateaus (reproducibility is 0.5 μV). Analysis of possible Pt‐Pd inter‐diffusion is examined by comparing melting plateaus obtained after heating the thermocouple and Pd sample for various periods just below the Pd melting ...