Practical acoustic thermometry with twin-tube and single-tube sensors

Accurate measurement of high temperatures in a nuclear environment presents unique challenges. All secondary techniques inevitably drift because the thermometric materials in thermocouples and resistance sensors are sensitive not just to temperature, but also their own chemical and physical composition. The solution is to use primary methods that rely on fundamental links between measurable physical properties and temperature. In the nuclear field the best known technique is the measurement of Johnson Noise in a resistor (See Paper 80 at this conference). In this paper we describe the measurement of temperature in terms of the speed of sound in a gas confined in a tube — an acoustic waveguide. Acoustic thermometry is the most accurate technique of primary thermometry ever devised with the best uncertainty of measurement below 0.001 °C. In contrast, the acoustic technique described in this work has a much larger uncertainty, approximately 1 °C. But the cost and ease of use are improved by several orders of magnitude, making implementation eminently practical. We first describe the basic construction and method of operation of thermometers using twin-tubes and single tubes. We then present results using a twin-tube design showing long-term stability (i.e. no detectable drift) at 700 °C over periods of several weeks. We then outline how the technique may be developed for different nuclear applications.