Effect of wall adsorption on the photoacoustic detection of ammonia with a 1.53-μm DFB diode laser

Abstract

The adsorption of ammonia molecules (and other polar molecules such as nitrogendioxide, water, etc.), on the walls of the photoacoustic (PA) detector influences the accuracy of the concentration measurement. This effect becomes larger for small concentrations and depends on the direction of the concentration change. This means that a hysteresis of the PA signal is observed between measurements carried out by decreasing and by increasing the concentration. The adsorption problem was reduced considerably by performing the measurements in a continuous flow. For measurements with high flow rate a new photoacoustic detector was developed. The differential design provides good flow and electronic noise suppression. The PA signal has been generated by tuning a 40-mW, 1.53-μm GEC-Marconi DFB laser to the absorption peak of ammonia at 1527 nm. The dependence of the PA signal on the flow rate of ammonia–nitrogen mixtures with different concentrations was investigated. In another experiment the hysteresis of the PA signal was investigated by increasing and decreasing the concentration in several steps, but maintaining the same total flow rate. It was found that a higher flow rate results in a smaller error of the concentration measurement and a shorter response time of the PA detector.