Miniaturized Photoacoustic Sensors for the Detection of Trace Gases in Ambient Air

Measuring the concentration of trace gases in ambient air is becoming increasingly important in a wide range of applications. However, current sensor solutions are often too large and complex to be suitable for large-scale use. This work emphasizes how the use of photoacoustic sensing can be used to overcome these drawbacks, and illustrates this with two sensors for different atmospheric trace gases. Subject of the first part is the design of a miniaturized sensor for ambient CO2. A special focus is set on the dimensioning, the construction and the properties of the photoacoustic detector. For the evaluation of the detectors, a setup for a Fourier-transformation spectrometer is proposed which allows to determine the spectrally resolved detectance. The knowledge of the spectral detectance enables comprehensive statements on the gas filling, other gases, photoacoustic conditions and more. With the manufactured detectros and two different infrared LEDs, two fully integrated miniaturized sensors were designed. With dimensions of 7.5 mm × 8 mm × 17mm they are among the smallest optical gas sensors published so far. The second part of this work is dedicated to a resonant photoacoustic one-chamber sensor, which contains a 450nm LED light source and a T-shaped resonator cell. Beside investigations on the resonance modes of the cell and the optical design, a novel approach for finding the resonance frequency with a photoacoustic wall-signal is proposed. Finally, both sensors were characterized with respect to their gas sensing properties such as sensitivity, linearity, resolution and cross sensitivities. Based on the results, their suitability for the application is evaluated.