Calibration of industrial grade platinum resistance thermometers by ITS-90 fixed points

We calibrated several industrial-grade platinum-resistance-thermometers (IPRT) using the ITS-90 fixed points in the range from the triple point of mercury to the freezing point of zinc. Calibration parameters were calculated for the ITS-90 deviation-function (DeltaW(90)) and for the Callendar-Van Dusen (CVD) model. Some of the IPRTs used in this exercise fulfil the ITS-90 acceptation criteria either W(Hg) or W(Ga), while others clearly do not fulfil them. However, all these instruments follow closely the ITS-90 interpolation model. National Laboratories of some developing countries as well as most of secondary and industry laboratories around the world use IPRTs as reference standards. The calibrations of these thermometers by fixed points and by using the ITS-90 model reduces the instrumental uncertainty from about 50 mK (typical of the CVD model) to 7-10 mK thus, this procedure may improve the measurement capabilities of those laboratories.