Practical acoustic thermometry is suitable for use at temperatures from -200 °C to above 1000 °C, and is accurate, affordable and exceptionally robust.

Acoustic thermometry is the most accurate technique of thermometry ever devised. Typically, in a laboratory setting, ultra-pure gas is trapped inside a spherical resonator and the frequencies of acoustic resonances are measured. If the dimensions of the resonator are known then the speed of sound can be calculated, and from this the temperature can be directly inferred.

Although very accurate, this version of the technique is utterly impractical. Instead, to capture the advantages of acoustic thermometry in a more useable way, scientists at NPL are working to develop practical acoustic thermometry, PAT.

In PAT, the speed of sound in a gas is measured inside acoustic waveguides - what we would call 'tubes'. In simplest terms, a pulse of sound is sent down a tube filled with pure gas and the time for the sound to reflect off one end of the tube is measured. This transit time is related to the average speed of sound in the tube, and hence to the average temperature. If two tubes of different lengths are used side by side, then the difference in the transit time is related to the temperature of just the extra length.

The technique has several advantages:

• The physical principles on which the technique is based are very well understood.
• Most high temperature thermometers need to be placed inside tubes a few millimetres in diameter. PAT consists entirely of tubes a few millimetres in diameter. This should make PAT intrinsically cheap and robust.
• The material of the waveguide can be chosen to survive whatever conditions the thermometer will meet. The argon or helium gas inside the tube will survive to thousands of degrees.
• The waveguides do not have to be straight. Sound travels down tubes even if they are curved, like light down an optical fibre.

We have published results demonstrating that the system works up to 1000 °C and are now working to improve the practicality and understand the factors that limit accuracy.

Download 'Practical Acoustic Thermometry with Acoustic Waveguides'

Contact

For further information, please contact Michael de Podesta