The dose makes the poison – this also applies to air pollutants. Air contains many gases in addition to its main components, oxygen and nitrogen, some of which are present in very low concentrations. Increased levels of these trace gases can impair our well-being and damage our health. For example, carbon dioxide (CO₂) causes poor concentration and discomfort at concentrations as low as 0.5 percent by volume. The World Health Organization has set a limit of only 5 ppb for toxic nitrogen oxide (NO₂), which is produced during combustion processes, among other things. This corresponds to 5 molecules of NO₂ per billion air molecules. Comprehensive monitoring of critical trace gases in indoor spaces, tunnels, underground parking garages, and traffic-heavy urban or industrial areas has so far failed due to expensive, complex, or insufficiently sensitive measurement technology.

Classic measuring principle – modern components

Dr. Christian Weber of the Fraunhofer Institute for Physical Measurement Techniques IPM developed miniature photoacoustic sensors using the principle of photoacoustic spectroscopy (PAS). These sensors can detect CO₂ and NO₂ gases at very low concentrations for a fraction of the cost of previous systems. "In my doctoral thesis, I essentially revived the 150-year-old photoacoustic measurement principle," says Weber. "Today, high-performance LEDs or semiconductor lasers are available as light sources, and inexpensive micromechanical microphone chips serve as detectors. This makes it possible to build small, low-cost sensors that require little energy."

Turning light into sound: the photoacoustic effect

Photoacoustic gas detection involves irradiating the gas with light of a suitable wavelength, which the gas then partially absorbs. The amount of light energy absorbed is proportional to the number of absorbing molecules in the gas mixture. This energy is then converted into heat energy via several processes. The gas expands, causing an increase in air pressure and the generation of a sound wave. This sound wave can be measured using a microphone. If pulsed light is used, a periodic signal is generated, equivalent to an acoustic tone. The higher the gas concentration, the louder the acoustic signal. This allows precise statements to be made about the gas concentration.

CO₂ detector: significantly higher sensitivity due to indirect photoacoustics

For his doctoral research, Weber developed two sensor architectures for the target gases CO₂ and NO₂. He relies on an indirect photoacoustic method for CO₂ detection. An LED sends infrared light through a short measurement path into a hermetically sealed photoacoustic detector filled with CO₂. The catch: As its concentration rises, the CO₂ in the measuring volume between the light source and the detector cell absorbs the exact wavelengths of light that would usually activate the detector. Consequently, the signal becomes weaker. The detector thus precisely detects the narrow absorption lines to which CO₂ reacts. "This design makes our sensor four to six times more sensitive than classic filter photometric solutions and allows us to shorten the light path to eight millimeters," says Weber. The sensor module, measuring 8 mm × 7 mm × 17 mm, is smaller than most optical gas sensors on the market. With a one-minute measurement interval and an average power consumption of only 24 μW, the sensor can operate on a battery for several years without drift or calibration.

Resonant photoacoustics: NO₂ detection limit of 3 ppb

The nitrogen dioxide detection sensor is based on direct photoacoustics. NO₂ absorbs light in the violet-to-blue spectrum. Although strong LED light sources can generate a measurable signal, it is very weak at low concentrations. Weber uses an acoustic resonator to amplify the signal and generate an audible tone. The pitch at which the resonance oscillates is influenced by environmental parameters, such as pressure and temperature. To avoid measurement errors, the excitation frequency must exactly match the resonance frequency of the measuring cell. Weber has found an elegant solution to this problem. Instead of generating the sound via a loudspeaker, he uses a second LED that focuses its light directly onto the wall of the resonant cell. Absorption of the radiation on the wall transfers some heat to the gas, creating a sound wave that excites the cell resonance. This method is completely linear and free of its own resonances and can be easily and inexpensively integrated into sensor technology. The process has already been patented and licensed. In test measurements, the sensor achieved a detection limit of 3 ppb, a sensitivity previously only possible with complex, expensive laser or chemiluminescence measurement systems.

The principles of photoacoustic trace gas detection can be adapted to detect other gases. Based on these findings, a methane leak detection system has been developed and is now in productive use.