Applications of a Visible‐LED‐Based Resonant Photoacoustic Device

The analysis of the NO2 content in the exhaust gases of diesel engines is performed by a new simple resonant photoacoustic system. It consists of a special design of transversal illumination from LEDs attached to a cylindrical glass cell. LEDs emitting at 464 and 400 nm allow carrying out differential measurements, to take into account possible interferences due to other absorbing species. The detection was accomplished through a microphone placed at the cell end and a personal computer that performs synchronic detection through a standard sound card. The calibration constant of the setup was obtained for NO2‐air concentrations between 5 and 400 ppmV. During this calibration procedure, the NO2 molecule density was found to change with time due to adsorption. In order to quantify this phenomenon, a theoretical model of the adsorption process was developed under the hypothesis of uniform surface adsorption. To verify its validity, the evolution of the photoacoustic signal amplitude from a low concentration ...