Sensor uses sound waves for temperature measurement

A sensor that uses sound waves to measure temperature could replace thermometers that lose accuracy in harsh environments such as nuclear power stations.

Scientists at UK measurement institute the National Physical Laboratory (NPL) are using the long-established principle that sound travels faster through warm air to create a cheap and robust thermometer that doesn’t need recalibrating or replacing.

They hope the device would be used to measure extremely high temperatures or in locations where it would be difficult to change the thermometer, such as in nuclear reactors.

Existing industrial temperature sensors typically use thermocouples or thermistors, which are electrical devices that produce a certain voltage or electrical resistance, respectively, depending on the temperature.

But changes in the materials over time produce an effect called drift, where the voltage and resistance levels change and the thermometers lose their accuracy.

Acoustic thermometry instead measures the speed at which sound waves travel to calculate how hot a substance is — a method that is commonly used to assess ocean temperatures.

It can measure temperature over a wide range from cryogenic levels to more than 1,000ºC and doesn’t suffer drift so it remains highly accurate, potentially to measurements of milli-Kelvins.

‘Potential uses include any hostile environment [for example] inside a furnace above 1,000°C,’ said Michael de Podesta, who is leading NPL’s acoustic thermometry research.

‘Any contact thermometer used in this environment degrades as soon as it is used and is usually placed in the environment inside a protective tube. The practical acoustic thermometer consists just of the tube itself.’

NPL’s model uses a speaker that transmits acoustic pulses through on a gas-filled tube — which can be as long as necessary and bent into complex shapes — to a microphone at the other end.

Measuring how long it takes the sound waves to travel the length of the tube allows the device to calculate the temperature of the gas.

‘The principle is very simple but the application is pretty complicated,’ said Dr Rob Simpson of NPL’s engineering measurement team. ‘The difficult part is working out to how to produce the sound and then how to listen to it.’

The company wouldn’t reveal what materials they were using for the device but suggested that a ceramic tube would enable the device to work at higher temperatures.

A 2010 journal article on NPL’s website details experiments that used tubes filled with inert gases such as argon.

NPL is looking to work with industry partners to develop the technology and hopes to produce a demonstrator device in the next six to 12 months.