Recently, researchers demonstrated a new type of gas sensor that utilises thermoacoustic properties instead of traditional resistive and optical methods. What challenges do typical gas sensors face, what did the researchers demonstrate, and how could it be used?

What challenges do gas sensors currently present?

Sensing technology has always been one of the major foundations of electronics. It can be argued that some of the first electronic projects were geared towards sensors, with examples including radio and TV. Fast forward to modern times, and sensors continue to be a critical part of everyday life, whether it is sensors used in security, industrial, commercial, residential, or infrastructure.

One area that has become of increasing concern is the state of the environment and how atmospheric pollutants affect health. Of all pollutants that can be monitored, CO2 is of particular interest as elevated levels of CO2 result from emissions, and CO2 generally correlates to other pollutants and particulates. But detecting CO2 is not entirely easy to monitor as it is an unreactive gas, meaning that it does not readily react with common sensor technologies (such as chemical resistive elements).

Thus, CO2 sensors rely on more advanced techniques like those found in spectrometers. Infrared light is shone through a gas sample, and the resulting spectrum is analysed. While these sensors work, they are generally expensive and difficult to implement on a scale. Considering that environmental IoT solutions are planning to deploy CO2 technologies en masse, such optical sensors would prove to be too expensive.

Researchers exploit thermoacoustic to create a CO2 sensor

Recently, researchers from the University of Cambridge have created a new CO2 sensor that utilises thermoacoustics instead of traditional chemical resistive or optical techniques.

For the sensor to work, it relies on two physical phenomena: thermoacoustic behaviour and acoustic properties of different gasses. Simply put, a MEMS microheater is driven by an amplifier with a sine wave which creates a temperature change that is also sinusoidal. The rapid thermal changes in the microheater cause it to emit sound (thermoacoustics) and this soundwave is then sent toward a CMOS active microphone.

Image courtesy nature.com

However, the emitted sound wave from the microheater passes through a gas sample before arriving at the MEMS microphone, and the acoustic properties of the gas affect how the thermoacoustic sound wave travels. The received signal is then passed into a differential amplifier that compares the received sound wave to a reference microphone used to minimise noise. The output of this comparator feedback into a lock-in amplifier is used to alter the microheater in a closed loop.

Image courtesy nature.com

The result of the new device is the ability to detect CO2 concentration in the air as the acoustic properties of CO2 differ from that of standard air. Furthermore, the setup was developed using off-the-shelf technology and CMOS circuitry, meaning that it can be readily manufactured. Therefore, the new CO2 sensor could provide a cost-effective solution to monitoring CO2 and other gases in the future.

How could thermoacoustic gas sensors be used?

It is clear that the ability to reliably detect CO2 without the need for expensive sensing technology will allow future environmental IoT monitors to track atmospheric concentrations of CO2, which will give city planners better insight into the environment and the health impact of emissions. Furthermore, such CO2 sensors could even find their way into rooms that can be used with automated systems to reduce the amount of stale air (another factor contributing to poor health).

However, another primary application of such a sensor is in volatile gas detection. Some gas sensors utilise chemical resistive elements whose resistance changes in the presence of a gas. However, a heater element is needed to help the gas molecules react with the sensing element for these to work. As such, these sensors can be dangerous to install in environments where concentrations of the gas can reach explosive levels.

Thus, using a miniature thermoacoustic sensor could be a major game-changer in volatile gas detection by providing a cheaper alternative to traditional sensors while simultaneously eliminating the risk of ignition.