An Optical Measurement of the Deviation of International Practical Temperatures T68 from Thermodynamic Temperatures in the Range from 730 K to 930 K

The difference (T-T68) between the thermodynamic and international practical temperatures has been measured in the range from 730 K to 903 K. The systematic uncertainty amounts ±20 mK and the random uncertainty ±3 mK at 903 K and 99% confidence level. As a part of this investigation the thermodynamic freezing temperature of aluminium was determined as 933.450 K (±21 mK systematic and ±7 mK random uncertainty at 99% confidence level). The values measured of (T-T68) deviate significantly from those of Coates and Andrews (1982). However, they agree very well with results of Bonhoure (unpublished). All measurements of this work have been performed by means of a photoelectric dc-pyrometer equipped with an unbiased Si-photodetector of excellent linearity. The effective wavelength was set to approximately 974 nm by a dielectric multilayer interference filter of 40.6 nm half-width. The measurements will be continued.