Mr. Moldover et al., Thermodynamic temperatures of the triple points of mercury and gallium andin the interval 217 K to 303 K, J RES NAT I, 104(1), 1999, pp. 11-46
We measured the acoustic resonance frequencies of an argon-filled spherical
cavity and the microwave resonance frequencies of the same cavity when eva
cuated. The microwave data were used to deduce the thermal expansion of the
cavity and the acoustic data were fitted to a temperature-pressure surface
to deduce zero-pressure speed-of-sound ratios. The ratios determine (T-T-9
0), the difference between the Kelvin thermodynamic temperature T and the t
emperature on the International Temperature Scale of 1990 (ITS-90). The aco
ustic data fall on six isotherms: 217.0950 K, 234.3156 K, 253.1500 K, 273.1
600 K, 293.1300 K, and 302.9166 K and the standard uncertainties of (T-T-90
) average 0.6 mK, depending mostly upon the model fitted to the acoustic da
ta. Without reference to ITS-90, the data redetermine the triple point of g
allium T-g and the mercury point T-m with the results: T-g/T-w = (1.108 951
6 +/- 0.000 002 6) and T-m/T-w = (0.857 785 5 +/- 0.000 002 0), where T-w
= 273.16 K exactly. (All uncertainties are expressed as standard uncertaint
ies.) The resonator was the same one that had been used to redetermine both
the universal gas constant R, and T-g. However, the present value of T-g i
s (4.3 +/- 0.8) mK larger than that reported earlier. We suggest that the e
arlier redetermination of T-g was erroneous because a virtual leak within t
he resonator contaminated the argon used at T-g in that work. This suggesti
on is supported by new acoustic data taken when the resonator was filled wi
th xenon. Fortunately, the virtual leak did not affect the redetermination
of R. The present work results in many suggestions for improving primary ac
oustic thermometry to achieve sub-millikelvin uncertainties over a wide tem
perature range.