(Published in Proceedings of TEMPMEKO ’99, J. Dubbeldam and M. de Groot, eds., pp. 418-423) PRIMARY ACOUSTIC THERMOMETER FOR USE UP TO 800 K D. C. Ripple, D. R. Defibaugh, K. A. Gillis, and M. R. Moldover National Institute of Standards and Technology, U. S. A. ABSTRACT Primary acoustic thermometers determine the thermodynamic temperature of a monatomic gas from measurements of the speed of sound in the gas. Here, we describe the design and construction of an acoustic thermometer designed to operate at temperatures up to 800 K with uncertainties of a few millikelvin. Features of our acoustic thermometer include: construction that minimizes sources of gas contamination; a gas handling system for continuous purging of the resonator during the acoustic measurements; monitoring the purity of the gas exiting the resonator; determination of the thermal expansion and dimensional stability of the resonator cavity by in situ measurements of microwave resonance frequencies; use of novel acoustic transducers; and measurement of the temperature of the resonator shell on the International Temperature Scale of 1990 (ITS-90) with up to five long-stem standard platinum resistance thermometers (SPRTs). We are currently in the process of implementing this thermometer at NIST, and results will be presented at a later date. 1. INTRODUCTION AND DESIGN PHILOSOPHY Thermodynamic temperature measurements using monatomic gases form the basis for the ITS-90 from 273.16 K to 730 K, and provide a reference point for radiometric determinations at higher temperatures [1]. Unfortunately, the best previous measurements [2,3] using Constant Volume Gas Thermometry (CVGT) have discrepancies of 12 mK at 500 K and rising to 30 mK at 730 K, which are much larger than the combined measurement uncertainty. Thus we were motivated to develop an acoustic thermometer as an alternative technique for determining the thermodynamic temperature above 500 K. In our thermometer, the speed of sound of a monatomic gas is determined from measurements of the frequencies of acoustic resonances in a gas-filled spherical shell of volume V. Kinetic theory relates the speed of sound u to the thermodynamic temperature T. Measurements of the frequencies of microwave resonances within the same shell determine the thermal expansion of the resonator cavity. The equation linking the measured frequencies to T, neglecting small corrections [4, 5], is T T uT uT VT VT f T f T f T f T f T f T w w w / a a w m w m a a w ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) , =       =             =             2 23 2 2 2 (1) where T w is the triple point of water, 273.16 K, and f a and f m are acoustic and microwave resonance frequencies. Distinct advantages of acoustic thermometry over earlier CVGT work include higher precision, the ability to conduct experiments with continuously flowing gas, and the ability to use microwave resonances to characterize the volume of the resonator cavity in situ. Recent acoustic thermometry results [4,5] at NIST have determined thermodynamic temperature to a standard uncertainty of 0.6 mK in the temperature range 217 K to 303 K. The present NIST effort seeks to greatly expand the temperature range of precision acoustic thermometry and to benefit from the lessons learned while conducting the lower temperature measurements. The NIST acoustic thermometer, shown in Fig.1, has the following features: A. Operation up to 800 K. Discrepancies between the NBS/NIST CVGT data become significant at temperatures above 500 K. Measurements at the zinc freezing point (692.677 K) are desirable, because the determined value of (T – T 90 ) at the fixed-point temperatures does not depend on the nonuniqueness of the SPRTs, which is a measure of the interpolation error between fixed points on the ITS-90. B. Continuous purging of the resonator cavity. Contamination of the gas in the resonator is proportional to its residence time, or inversely proportional to flow rate. Continuous purging reduces gas residence time approximately two orders of magnitude relative to the residence time in CVGT experiments. C. Direct measurement of impurities in the gas exiting the resonator. D. Simultaneous microwave and acoustic measurements. At elevated temperatures, creep of the spherical shell is a significant possibility. Microwave measurements that are concurrent with the acoustic measurements test for creep at each datum point.