The Nist Industrial Thermometer Calibration Laboratory

The NIST Industrial Thermometer Calibration (ITC) Laboratory provides calibrations of industrial thermometers over the range from –196 °C to 550 °C. The different types of thermometers include liquid-in-glass (LiG), thermistors, thermocouples (<200 °C), and industrial platinum resistance thermometers (IPRTs). Calibrations are performed by both comparison with a standard platinum resistance thermometer (SPRT) calibrated on the International Temperature Scale of 1990 and by a limited number of fixed-points. The comparison baths are liquid nitrogen (–196 °C), alcohol (–100 °C to 1 °C), water (0.5 °C to 95 °C), oil (95 °C to 275 °C), and salt (275 °C to 550 °C). The fixed points used are the melting point of ice (0 °C), the triple point of water (0.01 °C), and the melting point of gallium (29.7646 °C). The temperature measurement system for the reference SPRT it is a commercially-available ac resistance ratio bridge, for the IPRTs and thermistors it is a dc voltmeter with a constant current source, for the thermocouples it is a dc voltmeter, and for the LiGs it is a digital video camera. All of these temperature measurement systems are integrated via three computer-controlled data acquisition systems that semi-automate calibrations. This paper presents the methods, equipment, and uncertainties associated with the calibration of industrial thermometers in the ITC Laboratory.