Total radiation thermometry

A total radiation thermometer (comprising a cryogenic radiometer which measures the total radiation emitted from a black-body cavity) has been developed at NPL as an alternative instrument to the traditional gas thermometer for measuring thermodynamic temperature. The total radiant exitances, E(T) and E(T0), of a black body at temperatures T and T0 respectively, can be expressed as a ratio E(T)/E(T0) = T4/T04, where T0 = 273.16 K, the temperature of the triple point of water. Hence, by measuring the ratio E(T)/E(T0) a value of T can be determined. The uncertainty in T using this thermometer is about 1 mK, equivalent to measuring the black-body radiation with an uncertainity of about 1.5 parts in 105. If at the same time the black-body temperature is measured by a platinum resistance thermometer calibrated on IPTS68, a value of T68 can be realized, and differences T-T68 can be deduced. In this paper values of T-T68 will be presented in the temperature range - 130 to +110 degrees C and compared to recent gas thermometry in the same range