A fully opened photoacoustic resonator based system for fast response gas concentration measurements

This paper describes a fast response gas concentration measuring photoacoustic (PA) system based on a completely open cylindrical acoustic resonator. Contrary to conventional PA systems, the operation of the resonator presented here does not require the use of gas sampling unit. Thus adverse effects of adsorption/desorption processes that always occur on the walls of sampling units (longer response time and lower measurement accuracy) can be avoided completely. High sensitivity is ensured by efficient PA signal generation in resonant operation (through modulated laser excitation of a specially selected high-frequency combination acoustic mode of the resonator) and external noise suppression (through dual microphone configuration based differential detection scheme). The developed PA system was tested in water vapor concentration measurements, in which it was operated in parallel with a fast water vapor analyzer (LI-COR 840 with a response time of 0.5 s). Results of test measurements prove that the minimum detectable water vapor concentration with the proposed photoacoustic system is approximately 80 ppm and its response time is shorter than that of the reference instrument; which makes it a promising candidate for being used in highly demanding applications such eddy covariance flux measurements.