Determining the uncertainty in the reference signal at the silver fixed point for radiance temperature measurements

UME radiance temperature scale is derived from the UME Silver fixed point black body and is maintained and disseminated by the UME standard radiation thermometer. According to ITS-90, temperature scale is derived from the Planckian ratio equation by comparing the thermometer signal at an arbitrary temperature with the thermometer reference signal at the fixed point. Therefore, the uncertainty associated with the reference signal is transferred to radiance temperatures calculated. Using the Wien's approximation, the temperature uncertainty of the reference signal must be lower than 30 mK in order to keep the uncertainty component due to reference signal within 100 mK at 2000 degreesC. Uncertainty in the reference signal obtained at the silver point originates from two sources. These are the uncertainties associated to the fixed point itself and, the uncertainties associated to the radiance thermometer. For the first part of the uncertainty, metal impurity, emissivity, cavity uniformity, aperture uniformity of the fixed point are considered. For the second part, spectral responsivity function, size of source effect, residual response out of spectral responsivity function, time drift of the standard radiation thermometer are considered. Finally, to assess the total uncertainty associated with the reference signal, we have also considered the standard uncertainty of the freezing plateaus obtained during the melting and freezing cycles.