Abstract
Nitrogen dioxide (NO2) is a poisonous trace gas that requires monitoring in urban areas. Accurate measurement in sub-ppm concentrations represents a wide application field for suitable economical sensors. We present a novel approach to measure NO2 with a photoacoustic sensor using a T-shaped resonance cell. An inexpensive UV-LED with a peak wavelength of 405 nm as radiation source as well as a commercial MEMS microphone for acoustic detection were used. In this work, a cell has been developed that enables a “non-contact” feedthrough of the divergent LED beam. Thus, unwanted background noise due to absorption on the inside walls is minimized. As part of the development, an acoustic simulation has been carried out to find the resonance frequencies and to visualize the resulting standing wave patterns in various geometries. The pressure amplitude was calculated for different shapes and sizes. A model iteratively optimized in this way forms the basis of a construction that was built for gas measurement by rapid prototyping methods. The real resonance frequencies were compared to the ones found in simulation. The limit of detection was determined in a nitrogen dioxide measurement to be 200 ppb (6 σ) for a cell made of aluminum.
Highlights
NO2 is a trace gas poisonous for humans, animals and plants
Electrochemical sensors show a high sensitivity to NO2, are small and cost effective, but require a frequent replacement due to their limited lifespan
Compared to the modes found in simulation, the measured ones are lower due to the extended resonator length caused by the microphone mounting
Summary
NO2 is a trace gas poisonous for humans, animals and plants. Due to the impact of sunlight, NO2 splits into likewise poisonous NO and a free oxygen radical. The latter forms ozone (O3 ) together with the atmospheric. Accurate measurements in the ppb range represent a wide application field, e.g., monitoring at inner-city traffic junctions, in tunnels and underground car parks [4]. To serve these claims, the sensor must be sensitive and selective, and economical. IR absorption photometers on the other hand need an absorption path of several meters to gain a sufficient sensitivity, which makes them very large and costly
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