Multiple Fixed-Point Cell

Abstract

The paper describes the construction and investigation of multiple fixed-point cells usable for the calibration of thermocouples at temperatures above 1100 \(^{\circ }\)C. These fixed-point cells made of pure graphite are characterized by a simple construction as well as by a flexible application. The cylindrical basic mount is equipped with a central hole for the insertion of a thermocouple, and with eight drill holes containing exchangeable cartridges which surround the central bore axially symmetrically. The cartridges are filled with different metal–carbon (Me–C) eutectics: cobalt–carbon (Co–C), nickel–carbon (Ni–C), palladium–carbon (Pd–C), and rhodium–carbon (Rh–C). The melting temperatures of the different Me–C eutectics of the cartridges were compared to the melting temperatures of commonly used Me–C eutectic fixed-point cells of the Physikalisch-Technische Bundesanstalt by using a Pt/Pd thermocouple (Co–C, Ni–C) and Type B thermocouples (Pd–C, Rh–C). The uncertainties (\(k = 2\)) of the emfs measured at the inflection points of the melting curves are in the order of a few \(\upmu \)V which correspond to temperature equivalents between 0.3 K and 0.6 K. Furthermore, the difference between the melting temperatures of the Co–C and Ni–C cartridges was found to be 4.2 K by using simultaneously two sets of four cartridges filled with the two materials and placed alternately in the eight outer holes of one basic mount.