International Journal of Thermophysics

, Volume 35, Issue 6–7, pp 971–984 | Cite as

Microwave-Dimensional Measurements of Cylindrical Resonators for Primary Acoustic Thermometry

  • R. J. Underwood
  • G. J. Edwards


Acoustic gas thermometry relies on the fundamental relationship between the speed of sound in a monatomic gas and its thermodynamic temperature. The speed of sound is calculated from the resonance frequencies of a cavity whose dimensions or thermal expansivity must be measured with high accuracy. For quasi-spherical cavities, the use of microwave resonances is a successful and proven dimensional measurement technique. The simplicity and economy of cylindrical resonators makes them an attractive alternative to quasi-spherical resonators, particularly for high-temperature thermometry. This article summarizes the basic theory of cylindrical microwave resonators, and describes methods for obtaining cavity dimensions from the mode frequencies. The perturbing effects of cavity shape deformations, the wall to end-plate junction, coupling probes and non-conducting surface layers are discussed. The results of an experiment with a simple aluminum cavity are presented, which demonstrate the superior performance of the TE0\(pq\) modes over the more commonly used TM0\(pq\) modes.


Acoustic gas thermometry Microwave resonators Thermal expansivity 



This work was partly funded by the EMRP. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. Crown copyright 2013. Reproduced by permission of the Controller of HMSO and the Queen’s printer for Scotland.


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Copyright information

© Crown Copyright 2014

Authors and Affiliations

  1. 1.National Physical LaboratoryTeddingtonUK

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