INTERPOLATION EQUATIONS FOR INDUSTRIAL PLATINUM RESISTANCE THERMOMETERS

The Callendar – Van Dusen (CVD) equations of the IEC-751 for the Industrial Platinum Resistance Thermometers (IPRTs) do not exactly follow the International Temperature Scale of 1990 (ITS-90), within the required accuracy. Indeed, there is a demand for calibrations of IPRTs to uncertainties of around 10 mK for temperatures from 0 to 250 °C and better than 0.1 K between 250 °C and about 500 °C, while the use of the CVD equations do not allow an interpolation uncertainty better than ± 0.2 K over a range larger than 0 - 250 °C. To solve this problem, two new reference equations, one below 0 °C and the other above 0 °C, are proposed to be used instead of the CVD equations as reference for the IPRTs. These equations will be of a higher order than the CVD equations and their lower order coefficients are equal to the constants A, B and C of the CVD. The use of these new reference equations allows an interpolation accuracy at the millikelvin level, limiting the number of calibration points in every range to five.