ITS-90 non-uniqueness and evaluation of characteristics of new 1000 type platinum resistance thermometers for low-temperature measurement

For use in a low-temperature region below the triple point of water (TPW), the National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (NMIJ/AIST) developed a precise comparison system of capsule-type standard platinum resistance thermometers (CSPRTs) for dissemination of the International Temperature Scale of 1990 (ITS-90). This study compared eight CSPRTs with a reference CSPRT using the comparison system to study non-uniqueness at low temperature. We also evaluated characteristics of a new type of low-temperature PRT with about 1000 omega of resistance at TPW (PT-1000s), which was recently developed by Netsushin Co, Ltd. to obtain better temperature sensitivity of resistance at a low temperature. The PT-1000 sensor is made from a high-grade platinum wire, similarly to standard PRTs of the ITS-90. We calibrated four PT-1000s based on the ITS-90 and evaluated their characteristics by comparison with a reference CSPRT. Results indicate that the character of the temperature scale of PT-1000s is very close to that of CSPRTs down to 14 K.