National Physical Laboratory

Primary Acoustic Thermometry

NPL is embarking on a project to use primary acoustic gas thermometry to measure thermodynamic temperature in the range from – 40 °C to + 156 °C. This will provide new knowledge about the thermodynamic accuracy of the ITS-90, and in the longer term it will provide NPL with a capability for the direct dissemination of thermodynamic temperature over a wider range.

The method measures the velocity of sound, c, in the gas (argon or helium) and relates this to the temperature through the equation

c2 = γRT / M

where γ is the ratio of the principal specific heats, R is the molar gas constant and M is the molar mass of the gas. The velocity is measured in a spherical acoustic resonator, a cavity which resonates when excited by sound at a precise frequency. By measuring c first at the triple point of water, Ttp, and then at other temperatures Tx, these can be determined from the ratio of the velocities, since cx2 / ctp2 = Tx / Ttp.

Alternatively, if γ and M are both accurately known, the product RT can be deduced. In this way it is proposed to determine R, and hence the Boltzmann constant, k, from the measurement at the triple point of water.

Acoustic Thermometry

The conceptual simplicity of the method conceals many difficulties in achieving the desired uncertainties of only a few parts in 106. These include the design and manufacture of the cavity with very tight tolerances, stabilising its temperature, determining its radius (to obtain the velocity from the resonant frequency), and the thermal expansion, determining the purity and isotopic composition of the gas and measuring its pressure, etc.

The video below shows Michael de Podesta explaining Acoustic Thermometry for the 2013 Royal Society Summer Science Exhibition.

Last Updated: 12 Jul 2013
Created: 11 Sep 2007

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