Copper Fixed-Point Measurements for Radiation Thermometry at National Research Council

Due to its high transition temperature relative to other fixed points defined in the International Temperature Scale of 1990 (ITS-90) and its relatively low cost compared to silver and gold, copper is often chosen as the fixed point used to define the ITS-90 above 1235 K at national measurement institutes. Measurement of the copper freezing point can be done in a variety of furnaces. Although there are a large number of copper fixed-point designs, we expect the freezing temperatures to be the same. The difference between realizing different sized fixed points and the use of different furnaces in which to realize them is explored here. A traditional, large aperture fixed-point containing over 600 g of copper is compared to a hybrid-type fixed point containing only 15 g of copper and a commercial fixed point. Three types of furnaces including a heat-pipe furnace, a compact furnace, and a high-temperature blackbody were used to realize the copper freezing point. Between the fixed-point types, only the length of the plateau differed. However, a significant difference was found between the freezing temperatures determined in the different furnaces, and this difference was independent of cell type.