Versatile Gas Detection System Based on Combined NDIR Transmission and Photoacoustic Absorption Measurements

Based on a low-cost micro-machined infrared (IR) source with subsequent gas specific filter, photoacoustic absorption (PA) measurements were investigated in a nonresonant operating mode of a gas cell. In addition, nondispersive IR (NDIR) transmission measurements based on a thermopile detector were used to expand the measurement range, but also for an inherent self-monitoring scheme of the system. Compared to other 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 transmission as well as absorption measurements-were evaluated using discrete Fourier transformation. By combining PA and NDIR transmission measurements a wide concentration range is achieved, e.g., 100-20 000 ppm of carbon dioxide for a measurement cell with a length of 9 cm. Whereas the NDIR transmission measurement is suitable for monitoring higher concentrations, the PA measurement offers better detectivity at lower gas concentrations. Furthermore, the modular system allows the realization of a self-monitoring scheme by comparing the signals measured with both approaches, thus omitting the need for a second filter at a reference wavelength.