Comparative studies of photo-acoustic gas analyzers based on tunable CO2 lasers with external and intracavity detector arrangement

Abstract

The results of comparative studies of laser photo-acoustic (PA) gas analyzers with external and intracavity arrangement of resonant differential photo-acoustic detector (PAD) are presented. An automated tunable waveguide RF-excited CO2 laser was used in the PA gas analyzers. During the experiments, the optimal design of the resonant differential PAD was determined, in which the various models of microphones were tested. Studies of the effect of gas flow rate through the PAD, measurements of gas absorption spectrum (CO2) at different PAD resonant frequencies have been carried out. Two versions of transportable PA gas analyzers with different detector arrangements are developed and tested: outside and inside the cavity of tunable CO2 laser. For an integration time of 1 s, the minimum detectable absorption for the external PA gas analyzer is αmin = 7.5 × 10−10 cm−1, for the intracavity PA gas analyzer αmin ~ (3.5 ± 0.4) × 10−10 cm−1. It is shown that the value of the minimum detectable absorption αmin is practically independent of the external or intracavity location of the differential PAD, i.e. from the power of CO2 laser radiation in the range of ~1.5–100 W.