MID-IR LED-based, photoacoustic CO2 sensor
The technology used to implement CO2 sensors depends on the requirements in terms of sensitivity, price and robustness. The most common technology for highly sensitive tasks are based on tunable diode laser spectroscopy, while so-called non-dispersive infrared (NDIR) photometers  are used in less demanding scenarios such as control air conditioning systems. Most NDIR systems use thermal emitters as light source which are readily available at low cost but require compensation for cross-sensitivities toward other gas species. The detector technology employed in these systems ranges from photodiodes to thermopiles and pyroelectric detectors, all of which require the use of spectral filters to avoid cross sensitivities. Here we present a low-cost photoacoustic-based detector comprised of a microphone in a hermetically sealed chamber filled with CO2. To excite sound waves a MID-IR LED emitting radiation in the strong CO2 absorption region around 4.2 mm is used for the first time.