The NIST Primary Acoustic Thermometer will measure the difference between the International TemperatureScale of 1990 and the Kelvin Thermodynamic Scale throughout the range 273 K to 800 K with uncertainties of only afew millikelvins. The acoustic thermometer determines the frequencies of the acoustic resonances of pure argon gascontained within a spherical cavity with uncertainties approaching one part in 10 6 . To achieve this small uncertainty atthese elevated temperatures we developed new acoustic transducers and new techniques for the maintenance of gaspurity and for temperature control. The new electro-acoustic transducers are based on the capacitance between a flexiblesilicon wafer and a rigid backing plate. Without the damping usually provided by polymers, mechanical vibrationscaused unstable, spurious acoustic signals. We describe our techniques for suppression of these vibrations. Our acousticthermometer allows the argon to be continuously flushed through the resonator, thereby preventing the build up ofhydrogen that evolves from the stainless-steel resonator. We describe how the argon pressure is stabilized whileflushing. The argon exiting from the resonator is analyzed directly with a customized gas chromatograph. Because theacoustic resonator was so large it has an outer diameter of 20 cm a sophisticated furnace, based on surrounding theresonator with three concentric aluminum shells, was designed to maintain thermal uniformity and stability of theresonator at a level of 1 mK. We describe the design, modeling, and operational characteristics of the furnace.
Proceedings Title: Temperature, International Symposium | Eighth | Temperature: Its Measurement and Control in Science and Industry; Volume Seven | AIP
Conference Dates: October 21-24, 2002
Conference Title: AIP Conference Proceedings
Pub Type: Conferences
acoustic, argon, ITS-90, speed of sound, temperature, thermodynamic