Gas Monitoring with a Fabry-Perot Based Bolometer: Cross-Sensitivity to Water Vapor

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 is much higher. To reduce the impact of H2O absorption, a new method using a glass plate as infrared filter is introduced. The presented theoretical and experimental analysis outlines the resulting cross-sensitivity for absolute humidity and thus explores the associated limits of the system’s applications. 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.