A New NIST Automated Calibration System for Industrial-Grade Platinum Resistance Thermometers

Industrial-grade platinum resistance thermometers (IPRTs) are calibrated at NIST using a newly automated calibration system. The computer-controlled data-acquisition system is used to calibrate IPRTs over the range from –196 °C to 550 °C using fixed-points (ice point, triple-point of water, and Ga melting point) and comparison baths (cryogenic, water, oil, and salt). A standard platinum resistance thermometer, calibrated on the International Temperature Scale of 1990, is used with a commercially-available 30 Hz automatic-balancing resistance-ratio bridge to determine the temperature of the comparison baths during measurements. A dc measurement system consisting of an 81⁄2 digit multimeter, constant-current source, and a reference resistor is used to determine the resistances of the IPRTs under test. The automated calibration system and uncertainties will be discussed.