Platinum resistance thermometry: the conceptual difference between calibration uncertainty and measurement uncertainty

The calibration uncertainty of a standard platinum resistance thermometer (SPRT), as reported by a calibration laboratory in terms of the resistance-ratio with respect to the triple point of water (TPW), is strictly zero at the temperature of the TPW. This does not mean that when the instrument is used to measure a temperature close to the TPW the measurement uncertainty is zero. This point is being explicitly emphasized in this short note for the benefit of colleagues at calibration laboratories and users of SPRTs.