An acoustic gas thermometer for the redefinition and realisation of the kelvin
Creators: name: Underwood, Robin James
Publisher: name: University of Surrey
Year: 2023

description: Primary acoustic gas thermometers (AGTs) use measurements of the speed of sound in pure, monatomic gases to accurately infer thermodynamic temperature. As a primary thermometer, they can perform measurements that are directly traceable to the definition of the temperature unit, the kelvin, as opposed to an empirical temperature scale such as the International Temperature Scale of 1990 (ITS-90). Over the past two decades, AGT measurements have provided valuable insight into the deviations of the ITS-90 from thermodynamic temperature. They also performed a crucial role in the 2019 redefinition of the kelvin in terms of the Boltzmann constant, kB. In this international project, AGTs and other primary thermometers were used to determine a consensus value for kB under the previous kelvin definition. This consensus value was subsequently adopted as a fixed value, thus rooting the kelvin definition in a fundamental constant of physics.<br>
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description: The work described in this thesis involved developing an AGT of unprecedented accuracy at the National Physical Laboratory, UK (NPL). The AGT was based around a precision-engineered quasispherical resonator (QSR), within which the speed of sound in pure argon could be determined from the acoustic resonances. One of the key experimental challenges was determining the internal volume of the QSR with part per million accuracy. To achieve this, two completely independent methods were developed, involving water pyknometry and microwave resonance measurements. Other challenges involved building a thermostat which could control the QSR temperature between 118 K and 323 K with a spatial uniformity on the order of 0.1 mK (1 mK = 0.001 K).<br>
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description: The results of the measurements were a determination of kB with a relative uncertainty of only 0.7 × 10−6, and improvement of a factor 2 on the best results at the time. The NPL measurement had significant weighting in the fixed value of kB assigned in the redefinition, and differed from this value by only 0.3 × 10−6. Following the Boltzmann constant measurements, the NPL AGT was repurposed for measurements of the differences between the ITS-90 and thermodynamic temperature, (T −T90). The results were, and still are, the most accurate measurements of T in the temperature range 118 K to 323 K, and revealed previously unseen detail in the function (T − T90).<br>
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description: In the next few decades, it is anticipated that primary thermometers such as AGTs will largely replace empirical temperature scales as the preferred route of disseminating temperature standards. The techniques developed in this work have improved the accuracy of AGT, and will help accelerate this transition. Nevertheless, the ITS-90 and its successors will continue to coexist alongside thermodynamic temperature standards for the foreseeable future. The (T − T90) results of this work, together with those from international collaborators, will enable users to convert between these two scales with minimal uncertainty.<br>