Interpolating equation of industrial platinum resistance thermometers in the temperature range between 0 °C and 500 °C

Abstract

This paper compares four interpolation equations for the calibration of 18 industrial platinum resistance thermometers (IPRTs) in the temperature range between 0 °C and 500 °C. They are the quadratic Callendar–van Dusen (CVD) equation, the deviation function specified in the International Temperature Scale of 1990 (ITS-90), and the third- and fourth-order polynomials of resistance as a function of temperature. It was found that when the upper limit of the calibration range was higher than 240 °C, the third- and fourth-order polynomials resulted in a smaller standard error-of-fit than either the CVD equation or ITS-90 deviation function did, and of the latter two functions, the ITS-90 deviation function worked better. When the upper limit of the temperature range was 500 °C, the fourth-order polynomials showed distinctly better performance than the others. The standard error-of-fit for the fourth-order polynomial in the temperature range between 0 °C and 500 °C was on average ≈1/3 compared to the CVD equation, ≈1/2 compared to the ITS-90 deviation function and ≈70% compared to the third-order polynomial. When the upper limit was 100 °C, the difference among the four equations was insignificant. Consideration is briefly given to using interpolation as specified in the ITS-90 but with additional check points, and also to the use of the CVD equation below 0 °C.