A Cryostat for Automated mK-Level Thermometer Calibrations from -202 °C to 250 °C

The National Measurement Institute of Australia (NMIA) has developed a vacuum cryostat capable of calibrating precision electronic thermometers with a transfer error less than 2 mK over the range from -202 A degrees C to 250 A degrees C. The calibration of precision temperature measurement probes such as platinum resistance thermometers is usually performed in circulated fluid baths to achieve mK-level calibration uncertainties, and requires the use of several baths to cover the commonly used range of -80 A degrees C to 250A degrees C. Below -80 A degrees C, dry-well systems cooled by liquid nitrogen are available down to -196 A degrees C, but achieve poor uniformity and stability. The increased use of cryogenic preservation in the biomedical area has seen an increase in demand for precision calibration of electronic thermometer systems, in particular, down to a few degrees below the boiling point of nitrogen (-196 A degrees C). This has prompted NMIA to develop a new design of a dry-well calibrator, based around a 380 mm long, 50 mm diameter, oxygen-free copper block insulated by gold-plated radiation and guard shields. Temperatures down to -202 A degrees C are achieved by controlling the flow of liquid nitrogen through a restricting orifice into an evacuated heat exchanger. Computer control of the nitrogen flow and of several immersion heaters achieve a temperature stability of a few mK at all temperatures over the operational range, requiring typically 60 min to equilibrate at each new setpoint. Radiative transfer limits operation to 250 A degrees C where the uniformity is 0.5 mK center dot cm(-1) (and becoming negligible at lower temperatures). A significant design innovation is the thermometer entry region, which has a purge system to keep the wells free of condensed moisture or atmospheric gases without the need for a seal. As the block is only 50 mm from the face of the cryostat, thermometers as short as 250 mm can be calibrated. The system is now in regular use at NMIA providing fully automated calibrations of precison temperature measurement systems without the need to use multiple temperature enclosures.