Refractive index gas thermometry between 13.8 K and 161.4 K

Abstract

We have measured the refractive index of helium using a quasi-spherical copper microwave resonator at five different temperatures in the interval between the triple point of hydrogen at 13.8 K and the triple point of xenon at 161.4 K for pressures up to 380 kPa. From these results and additional measurements of the refractive index of neon near 54.4 K, 83.8 K and 161.4 K we determine the differences (T − T 90) between the thermodynamic temperature T and its approximation T 90 by the International Temperature Scale of 1990 (ITS-90). We have estimated the isothermal compressibility of copper and the effective compressibility of our microwave resonator by different methods including resonant ultrasound spectroscopy (RUS) and microwave measurements with helium at 273.16 K. We compare the results of these compressibility estimates and discuss the associated uncertainty. From the refractive index measurements, we estimate the second density virial coefficient of helium and neon which are found in good agreement with the ab initio calculations of the same properties.

ITS-90 reproducibility, xenon fixed point substitution and new interpolating equations between 13.8033 K and 273.16 K

“…If SPRT calibrations via primary thermometry could be extended up to 54.3584 K, then the importance of subrange 2 would fade and the excellent new version of subrange 3 described in section 7 could be used above the triple point of oxygen. Thermodynamic temperature measurements with uncertainties of 0.3 mK-0.4 mK between 24.5561 K and 54.3584 K, competitive with the defined scale CU values shown for this temperature range in figure 20, have already been demonstrated by dielectric-constant gas thermometry [8] and should be achievable by acoustic gas thermometry and refractive-index gas thermometry in this region as well [46,[50][51][52]. One issue that would remain to be addressed if switching from thermodynamic temperature dissemination to defined scale dissemination at 54.3584 K is the 2 mK thermodynamic inaccuracy of the ITS-90 at this point [8].…”

“…However, exhaustive attempts to improve subranges 1 and 2 may not be worth the effort. The recent redefinition of the kelvin has increased focus on primary thermometry [45], and several techniques now exist that can realize thermodynamic temperature between 13.8033 K and 24.5561 K with uncertainties of 0.5 mK or less [46][47][48][49][50], competitive with the defined scale CU values shown for this temperature range in figure 20. Given the difficulty of realizing the FP17 and FP20 points, the select group of NMIs capable of realizing ITS-90 subrange 1 may, in time, choose to directly disseminate thermodynamic temperature below the triple point of neon instead [5,43].…”

Towards Realising the Redefined Kelvin

“…These thermometers are being continuously improved to increase their accuracy in the determination of thermodynamic temperature T, and their results and methods are cross checked by mutual comparisons of the differences T -T 90 [63]. Recent such determinations include: the AGT result obtained at LNE-Cnam at 25 K [41] which served as a thermodynamic temperature reference for single-pressure RIGT between 5 K and 25 K at TIPC-CAS [57]; the RIGT results obtained between 14 K and 161 K at NRC and INRiM [49,64] and the DCGT results obtained at PTB between 4 K and 200 K [63,65]. With minor exceptions, the thermodynamic properties which are derived from these experiments show remarkable consistency with the available theoretical estimates.…”

Acoustic measurement of the triple point of neon T Ne and thermodynamic calibration of a transfer standard for accurate cryogenic thermometry

“…In figure 1, two literature values of T Ne are based on refractive-index gas thermometry (RIGT) [5,6] and one is based on dielectric constant gas thermometry (DCGT) [4]. These values of T Ne were obtained during wide-range measurements of the differences T − T 90 and their estimated uncertainties u(T − T 90 ).…”

“…Values of T Ne from the literature and from this work. Literature values are identified, by technique, first author, year, and reference: AGT Pitre et al (2006) [3]; DCGT Gaiser et al (2017)[4]; RIGT Rourke (2020)[5]; RIGT Madonna-Ripa et al 2021[6]; AGT this work.…”