An evolutionary approach to updating the international temperature scale

2021

The International Temperature Scale of 1990 (ITS-90) underpins much of modern thermometry, yet efforts to comprehensively characterize and improve its reproducibility have been limited. Here, a low-uncertainty data set drawn from 13 standard platinum resistance thermometers is used to quantify all three main manifestations of scale irreproducibility over the range 13.8033 K to 273.16 K. Four overlapping ITS-90 subranges are investigated, along with three prospective approaches to temperature scale improvement. The current version of the Guide to the Realization of the ITS-90 is found to overestimate scale realization uncertainties, and recommendations are made to revise the guidance provided therein. Replacing the ITS-90 deviation function of subrange 3 with a new version further improves the interpolation quality of this subrange by a factor of two, and calibrating thermometers at the triple point of xenon rather than the triple point of mercury cuts the realization uncertainty of subrange 4 in half. New linear subranges spanning from the triple point of water to the triple points of mercury or xenon could be added to the scale without compromising the performance of the existing subranges. Combining new interpolation equations with calibration at the triple point of xenon provides the basis for a much more reproducible temperature scale than the ITS-90, with magnification of fixed point uncertainties nearly eliminated for all of subranges 1-4 above the triple point of oxygen.

Section: Interpolation and Scale Reproducibilitymentioning

confidence: 99%

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

2021

“…In principle, a new reference function could take any form. However, Hill & Steele recognized that from an engineering perspective, the simplest substantial change that could be made to ITS-90 is to improve thermodynamic accuracy by changing the values of the constants defining the reference function, W r (T 90 ), without changing its functional form [76,77]. Such a change might have a minor effect on software in commercial instrumentation, perhaps only affecting tables of constants, and may be less disruptive than changes to the underlying mathematical of the reference function.…”

Section: New Reference Functionmentioning

confidence: 99%

Standard platinum resistance thermometer interpolations in a revised temperature scale

White

2020

The thermal metrology community is considering revising the International Temperature Scale of 1990 (ITS-90), motivated by the opportunity to improve the thermodynamic accuracy, reproducibility, and ease of use of the scale, as well as health and safety concerns with the use of mercury in the current scale. This paper considers the mathematical structure of the standard platinum resistance thermometer (SPRT) interpolations of the ITS-90 and identifies (i) mathematical features that are advantageous and should be retained, (ii) opportunities for improvements, and (iii) the research required to maximise the benefits from such improvements. The improvements considered include minor adjustments that leave ITS-90 intact, numerical adjustments to reference resistance ratios that preserve the structure of ITS-90, fixed-point replacements, new subranges, and large-scale changes in the mathematical structure of the interpolations. A significant research effort will be required to implement some of the changes. Overall, an improvement in the thermodynamic accuracy by a factor of about ten is relatively easily realised, but improvements in reproducibility of the scale of even a couple of tens of percent will be hard won. For most users, the costs and inconvenience of a substantial scale revision may outweigh the benefits.

“…the potential influence on the overall data set of unmodelled systematic effects intrinsic to any one primary thermometry method. The fruits of this effort may enable the formulation of a new scale with much better thermodynamic accuracy than ITS-90 [15,16], while simultaneously advancing technical development of modern primary thermometry techniques to facilitate direct dissemination of T [17] according to the new definition of the kelvin [18][19][20].…”

Section: Thermodynamic Temperature Of the Triple Point Of Xenon Measu...mentioning

confidence: 99%

Thermodynamic temperature of the triple point of xenon measured by refractive index gas thermometry

2020

Measurements of the refractive indices of helium and argon using a quasi-spherical microwave resonator and upgraded experimental protocol are reported at the temperatures of the triple points of water and xenon. The results at the triple point of water are used to determine the compressibility of the resonating cavity, which is bounded by the copper shell of the resonator. This experimentally-determined compressibility is consistent with the literature value for copper, but with much smaller measurement uncertainty. The measured compressibility is extrapolated to the triple point of xenon, and combined with the refractive index results at that temperature to determine the thermodynamic accuracy of the International Temperature Scale of 1990 (ITS-90): (T − T 90 ) = (−6.9 ± 1.7) mK at ITS-90 temperature T 90 = 161.405 96 K, corresponding to a xenon triple point thermodynamic temperature of T = (161.3991 ± 0.0017) K. The experimental compressibility is further extrapolated to the triple points of argon, oxygen and neon, and used to re-analyze earlier refractive index gas thermometry measurements made using the same resonator, yielding updated values with reduced uncertainties: (T − T 90 ) = (−4.1 ± 1.6) mK at T 90 = 83.8058 K, (T − T 90 ) = (−2.0 ± 0.8) mK at T 90 = 54.3584 K, and (T − T 90 ) = (−0.61 ± 0.49) mK at T 90 = 24.5561 K. The ITS-90 thermodynamic accuracy results of the present refractive index gas thermometry study agree with those previously reported by acoustic gas thermometry and dielectric constant gas thermometry.