Investigation of W(T₉₀) Functions for Low‐α PRTs in the Sub‐ranges above 0 °C

The most practical method of interpolation for industrial platinum thermometers is the use of the Callendar‐Van Dusen (CVD) second‐order equation. Standard tables R(T) for IPRTs of different purity of the platinum wire were established by a number of Standards (IEC, DIN, GOST, JIS). From fitting the coefficients B versus A of the CVD equations, obtained for 166 thermometers with a resistance ratio at 100 °C, W(100), ranging from 1.380 to 1.392, the linear function B(A) was generated in this work, which makes it possible to establish the second‐order reference function for PRTs of any nominal W(100) value. It is important that substantial distortion of the interpolating CVD curves was found for the IPRTs with film platinum sensing elements. Although the ITS‐90 interpolation method is supposed to be applied only to PRTs that have a strain‐free sensing element made of platinum wire of a very high purity (W(Ga)>1.11807), it has become usual practice to use the ITS‐90 function for industrial thermometers. As shown in this paper, a systematic difference occurs between temperature values calculated by means of the CVD equation and the ITS‐90 interpolation technique, which does not depend on the purity of the platinum wire in a large W(100) range, but highly depends on the temperature sub‐range. For individual calibration of an IPRT in the sub‐range 0 to 230 °C, it is possible to use only one calibration point besides 0 °C.