Real-time correction of self-heating effect of capsule-type SPRTs used in gauge block calibration at KRISS

Abstract

Standard Platinum Resistance Thermometer (SPRT) is the most accurate temperature sensor directly calibrated by the fix point cells given in the International Temperature Scale 1990 (ITS-90). However, when it is used for temperature measurement of an object in high precision dimensional metrology, self-heating effect may cause error in the measured temperature, and thus also in the measured length. The two-current method, which is the well-known method to compensate the self-heating effect of SPRTs is not practical for dimensional metrology because it takes several tens of minutes to measure temperature. A new method has been p6roposed by the authors which can enable a real time correction of the self-heating effect of SPRTs. The method utilizes the fact that the resistance of SPRT is linear with respect to its temperature within the short range around 20 °C. The correction value for the AC bridge resistance ratio corresponding to the self-heating effect is measured in advance by using the two currents of 1 mA and 1.414 mA. This correction value can be directly added to the AC bridge readout in real time to compensate for the self-heating value in length measurement applications. Since no current change is required during actual length or temperature measurements, there is no need to wait se4veral tens of miutes like in the case of using the two-current method during the length measurement. In this paper, we present the measurement results of the self-heating effect of Minco’s capsule type SPRTs used in the gauge block interferometer of KRISS.