Fraunhofer Institute for Physical Measurement Techniques IPMGases can be detected very precisely and selectively by means of photoacoustic spectroscopy (PAS). Alexander Graham Bell already described the basic measurement principle in 1880: If a gas sample in a measuring cell is irradiated with a pulsed light source, specific gas molecules will absorb the light and the gas sample will heat up. According to kinetic gas theory, a local increase in temperature leads to an increase in pressure. Periodic modulation of the intensity of the incident light beam causes periodic pressure fluctuations whose frequency matches the modulation frequency of the light source. These acoustic waves, i.e. the photoacoustic signal, can be detected with sound transducers (e.g. commercial MEMS microphones). The signal amplitude correlates with the strength of the absorption and thus provides information about the gas concentration in the measuring cell.
Photoacoustic gas sensors
Gases can be detected very precisely and selectively by means of photoacoustic spectroscopy (PAS).
© Fraunhofer IPM
Photoacoustic spectroscopy
Laser-based photoacoustic spectroscopy enables highly sensitive detection of trace gases in the low ppb range.
© Fraunhofer IPM
Photoacoustic spectroscopy
Resonant and non-resonant measuring cells
The geometry of the measuring cell is essential for the sensitivity of the photoacoustic detector. The detector can either be operated resonantly or non-resonantly. In resonant operation, the modulation frequency of the light source matches the acoustic resonance frequency of the measuring cell. This significantly amplifies the photoacoustic signal and thus increases the detection sensitivity.
In non-resonant operation of a photoacoustic detector, the modulation frequency of the light source (typically a broadband IR emitter) is far below the acoustic resonance frequency of the measuring cell. Non-resonant systems can be operated at different frequencies and are more stable with regard to changing pressure and temperature during operation.
LED, laser or IR-emitter as light source
Depending on the target gas and application, different geometries of measuring cells and different light sources are selected. Cost-effective measurement systems that use rapidly modulatable LEDs as light sources, for example, can already determine target gas concentrations very sensitively (in the 1 ppm range). If lasers are used as light source, the selectivity and sensitivity of the measurement systems are improved, often reaching values in the low ppb range.
Example: Photoacoustic SO2 measuring system
For a compact SO2 measurement system used for emission measurement in the exhaust gas scrubber on ships, Fraunhofer IPM employs a resonant measurement cell with a UV LED as light source. Thus, the required high resolution of 1 ppm in the measuring range up to 50 ppm SO2 is achieved.
Example: Minaturized photoacoustic CO2 sensor
For a miniaturized photoacoustic CO2 sensor used for the detection of indoor air quality, a non-resonant measuring cell was combined with a broadband IR emitter and a MEMS microphone. This allows a particularly small sensor with a sensitivity of 50 ppm and a wide measuring range up to 5000 ppm CO2.
Example: Laser-based photoacoustic measuring system for leak detection
A laser-based PA module with a resonant measuring cell has been developed for detecting gas leaks, for example from gas pipes in the ground. The operation of the measuring cell is actively stabilized, and concentrations of methane (CH4) and ethane (C2H6) are achieved with a resolution of a few ppb in each case.