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The NIST Facility for Primary Realization of the International Temperature of 1990 Below 84 K Objectives: The first objective of this program is to realize the International Temperature Scale of 1990 (ITS-90) below 84 K and to transfer this scale to the user community, which consists primarily of NASA, aerospace industries, and superconducting magnet and infrared equipment manufacturers. The second objective is to compare NIST's realization with those performed by other national laboratories. NIST's realization is being performed using the newly-developed state-of-the-art low-temperature facility, which has the capability of realizing the scale with a documented inaccuracy not greater than 0.1 mK. The facility is to be used for three purposes. The first is to provide primary calibration for capsule-type platinum and rhodium-iron resistance thermometers in the range 0.65 K to 24.5661 K. These thermometers will subsequently be used to transfer the ITS-90 to the NIST user community in a separate facility. The second purpose of the facility is to investigate and document the subrange inconsistencies of the ITS-90 in this range that may be associated with the overlapping definitions of the scale as well as the inherent performance differences of the individual scale defining instruments. The third purpose is to perform triple-point realizations in the region 24.5661 K to 83.8058 K in open cells; these will be used in conjunction with sealed triple point cells in another facility to provide primary calibration for standard platinum resistance thermometers (SPRTs) in this range. ITS-90 Definition: Realization of the ITS-90 below 84 K requires the use of the triple points of argon, oxygen, neon, and equilibrium hydrogen as fixed points for the calibration of SPRTs. Two vapor-pressure fixed points of equilibrium hydrogen are also defined for the calibration of SPRTs. In addition, the ITS-90 specifies the use of an interpolating constant- volume gas thermometer to realize the scale from 3.0 K to 24.5661 K, and 3He and 4He vapor pressure thermometers to realize the scale from 0.65 K to 5.0 K. In the region below 25 K, ITS-90 realization requires the accurate measurement of pressure. Apparatus: In the low-temperature facility, the ITS-90 is realized in a gold-plated copper block which contains triple point cells for argon, oxygen, neon, and equilibrium hydrogen (see Figure 1). The triple-point cell for equilibrium hydrogen is also used for its vapor-pressure fixed points. The block also contains a gas thermometer and vapor-pressure cells for 3He and 4He. The block 3He recirculating refrigerator. The pressure measurements are made using a differential capacitance diaphragm gage (DCDG) in series with a gas-lubricated piston gage (PG) that is operated in the absolute mode (see Figure 2); this combination allows for pressure measurement with an accuracy of better than 22 ppm and a repeatability of better than 3 PPM For pressures less than 1.3 kPa (the lower limit of the PG) the DCDG, operated in the absolute mode, is used to measure pressures. The accuracy of the pressure-measurement system allows realization of the ITS-90 to well within 0.1 mK. The resistance of the capsule-type thermometers is measured with a commercial automatic a.c. resistance-ratio bridge which is referenced to the NIST resistance standards. The resolution of the thermometry is within ±0.1 mK and within ±0.02 mK over most of the range. Above: Photographs of the copper block containing the realization cells, mounted in its cryostat. For more information, contact Christopher Meyer at christopher.meyer@nist.gov |
Last Updated on: 2/25/04
