Versatile gas measurement system based on combined NDIR transmission and photoacoustic spectroscopy

Based on a low-cost, micromachined IR source with subsequent gas specific filter, photoacoustic spectroscopy (PAS) was investigated in a non-resonant operating mode of a gas chamber. In addition, non-dispersive infrared (NDIR) transmission measurements based on a thermopile detector were used to expand the measurement range, but also for monitoring purposes. Compared to many existing systems using expensive laser sources for stimulation of acoustic pressure waves in a resonant mode of operation, the approach is based on low-cost components. The IR source was modulated in the frequency range between 1 and 10 Hz. To achieve enhanced resolution and a good signal to noise ratio the sensor raw data - both for IR transmission as well as PAS - were evaluated using discrete Fourier transformation (DFT). By combining PAS and NDIR a wide concentration range is achieved, e.g. 50 to 20,000 ppm for CO₂ for a measurement chamber with a length of 9 cm. Whereas the NDIR transmission measurement is suitable for monitoring higher concentrations, the PAS measurement is more sensitive to lower gas concentrations. Furthermore, the modular system allows the realization of a self-monitoring scheme at least for medium concentrations by comparing the signals measured with both approaches.