Investigation of fixed point of copper in the metal-in-graphite blackbody cavity using standard photoelectric linear pyrometer

The phase transition temperature of high purity copper has been used as fixed point temperature to calibrate standard thermometers of both contact as well as non-contact type. In the present work the freezing point of copper has been measured by a spectral radiation pyrometer using copper-in-graphite black body crucible fabricated in the form of a cavity with an aperture of 10 mm. A recently procured detector based photoelectric pyrometer, LP4 has been used as transfer standard in the measurement process. The output signal of photocurrent due to radiation from the source of copper blackbody cavity at wavelength of 650 nm in vacuum has been measured during the period when metal changes its phase from solid to liquid and liquid to solid. The stability of fixed point has been determined from the melting and freezing curves plotted between the photocurrent or corresponding temperature and the time period of the whole cycle. The uncertainty in the measurement of copper point has been evaluated to be ± 0.530 ºC and thus the copper freezing point has been determined to be 1083.41 ºC (± 0.53 ºC, at k =2) differing from the assigned value in the international temperature scale of 1990 (ITS-90) by 1.21 ºC lower while the melting point of copper has been determined to be 1084.23 °C differing by 0.39 °C lower respectively than the defined value of ITS-90