Measuring CO2 concentration with a Fabry–Perot based bolometer using a glass plate as simple infrared filter

Recently, Fabry–Perot IR-absorbing structures have been proven suitable as low cost detectors in non-dispersive infrared (NDIR) gas-sensors for measuring diluted gases (e.g., CO2) in a pure nitrogen (N2) atmosphere. To identify the monitoring capability of the devised prototype system in ambient air, the cross-sensitivity to other gases, mainly water vapor, has to be explored. The absorption coefficient associated with the individual infrared absorbing bands of water vapor is small compared to that of CO2; however, the atmospheric concentration of vapor much is higher. To improve the impact of CO2 absorption compared to other gases a new method using a normal glass plate as infrared filter is introduced. The presented theoretical and experimental analysis investigates the achievable response to CO2 as well as the resulting cross-sensitivity for absolute humidity, where also the dependence of the characteristics on the temperature was considered. In particular a model for the bolometer is combined with ray tracing simulations for a connected sample chamber yielding the response of the entire IR-absorption sensor system, which is compared with measurements.