Trace Ammonia Gas Detection Based On QEPAS

The trace ammonia detection have played a positive role in air quality monitoring, industrial waste gas monitoring and prevention of human diseases. Currently ammonia detection methods are based on traditional electrochemical methods. However, traditional electrochemical method cannot achieve rapid detection or continuous online detection, has been unable to meet the current detection needs. The absorption spectroscopy technology with high sensitivity, rapid detection, continuous online detection, has become an important means for trace gas detection. But this technology exist some shortcomings, such as complex equipment, bulky, susceptible to background noise. To solve these problems, we propose an ammonia detection method based on the quartz enhanced photoacoustic spectroscopy. This technology has small volume, perfect selectivity, fast response, and background noise immunity, has become a hot topic in gas detection field. By studying this subject, have practical significance for trace gases detection.This paper describes the gas infrared absorption theory and principles of the photoacoustic signal excitation, detailed analysis gas photoacoustic effect. By contrast with conventional photoacoustic spectroscopy, analysis the feasibility, principles and characteristics of using a quartz tuning fork as the sound energy accumulation device and acoustic sensor. For the problem of weak output piezoelectric signal, proposed a detection method based on wavelength modulation and harmonic detection technology. For the problem of absorption peaks overlap of different types of gases, detailed analysis the near-infrared ammonia absorption lines, exclude the interference of other gases, choice the1531.65nm as the detected absorption lines finally.Designed and built the gas detection system based on QEPAS. Achieve laser wavelength modulation by using DFB lasers and laser controller. For the problem of laser output beam divergent and irregular, designed a collimated laser optical filter, which improved the laser beam profile. In order to enhance the photoacoustic signal amplitude, proposed and designed a micro-resonator and sealed gas chamber. Made a signal generation, control and collection system based on Lab VIEW, achieved the detection process automation.This paper tests the properties of quartz tuning fork, and experimentally determined the optimal modulation depth and modulation factor. Choose a1531.65nm DFB laser as the light source, completed the DFB laser temperature and current tuning characteristic experiment. Detected different concentrations of ammonia, obtained a linear relationship between the amplitude of the photoacoustic signal and the gas concentration. In order to improve the amplitude of the photoacoustic signal, proposed the method to enhance the laser output power, experimentally obtained the linear relationship between the photoacoustic signal amplitude of the laser output power. In order to improve the detection system SNR, proposed the method to reduced pressure, get higher amplitude of the photoacoustic signal and improved the SNR. Detected different concentrations of NH3gas samples under different pressure, compared and analyzed the test results. Finally, tested the system performance, obtained the maximum sensitivity of the detection system is146ppb, the detection system response time is about40s,4hours of continuous detection of the relative error is2.34%.