Versatile infrared gas measurement system with tunable microstructured Fabry-Pérot Filter

Based on the combination of non-dispersive infrared (NDIR) transmission and photoacoustic absorption spectroscopy (PAS) measurements, the detection of multiple gases and their mixtures was investigated using an integrated, microstructured Fabry-Pérot Filter (FPF). The proposed versatile system can be utilized as IR spectrometer for a wide range of gas concentrations using IR transmission spectroscopy (IRTS) measurements for high concentrations and PAS for low concentrations. The system is based on low cost components: a thermopile detector for the IRTS signal, condenser microphones for the PAS signal and a microstructured IR source. The source is modulated electrically with a frequency in the range of 1 to 10 Hz, i.e. using a nonresonant operating mode of the gas chamber. The investigated, commercially available FPF allows the analysis of gases with gas specific absorption bands between 3.9 and 5.0 μm. Our IRTS system, therefore, can be adapted for many application scenarios in which two or more gases have to be detected over a wide concentration range. One example is the isotope specific detection of carbon dioxide (CO₂) for the urea breath test.