基于QEPAS技术的水蒸气浓度检测系统

发布时间：2018-03-26 05:20

本文选题：气体传感　切入点：石英增强光声光谱技术　出处：《山东大学》2017年硕士论文

【摘要】：水蒸气浓度检测技术是当今气体传感领域中的一项重要课题。在天然气运输、电力传输、食品安全、工业冶炼以及精密器械维护等方面,水蒸气的浓度要求非常苛刻,需要严格控制,所以迫切需要对水蒸气浓度检测技术进行深入地研究。石英增强光声光谱(quartz-enhanced photoacoustic spectroscopy)技术简称QEPAS技术,是近年来痕迹气体检测研究中一种新的方法,与传统光声池方法以及其他传统检测方法相比,在体积、成本、灵敏度和抗干扰方面都有着一定的优势,尤其是它不受激光器、准直器等内部含有的微量水蒸气的影响,克服了吸收光谱法固有的水底问题。本文通过对基于QEPAS的水蒸气浓度检测系统的研究,阐述了相关原理,实现了系统的功能,优化了光路与信号检测电路,采用的锁频法解决了检测过程中出现的问题和矛盾,最终达到了低浓度水蒸气的准确标定。QEPAS技术原理涉及到红外光谱气体的吸收理论以及由于吸收产生的多次谐波理论。本文通过气体分子红外吸收、气体分子的吸收定律、线性函数、波长调制,逐步阐述和推导了 QEPAS技术的原理,然后又对石英音叉的压电特性等机理进行了详细描述。根据QEPAS原理与相关的电路知识,本文详细介绍了系统各部分的功能、组成和设计思路,组建起了 QEPAS水蒸气浓度检测系统,对程序、系统参数设定完毕后,在大气中检测出水蒸气的浓度,初步实现了系统的功能。为了提升系统的检测精度,对影响系统信噪比的激光器调制幅度、锁相放大电路参数进行了优化,使信噪比提升了 1倍。对共振管和音叉的特性做了细致的研究,确定了最优的共振管结构和尺寸,发现了音叉在共振管结构中中心频率变化很小而带宽变化明显。确定了 QEPAS水蒸气浓度检测的新方法——锁频法。针对激光驱动信号扫描法与锁频法进行理论分析与实验验证,证明了锁频法在光声光谱系统中的巨大优势,并将锁频法应用到了系统中,二次谐波信号的信噪比得到了质的飞跃,空气中的信噪比达到了 500。标定了 QEPAS水蒸气浓度检测系统的检测极限为200ppm,精度在某些浓度下可以达到6ppm,最后探究了系统的稳定性以及石英音叉特性的长期稳定性。
[Abstract]:Water vapor concentration detection technology is an important subject in the field of gas sensing. In the fields of natural gas transportation, electric power transmission, food safety, industrial smelting and maintenance of precision instruments, the concentration requirements of water vapor are very stringent. It needs strict control, so it is urgent to study the detection technology of water vapor concentration. Quartz enhanced photoacoustic spectroscopy (QEPAS) technique is a new method in trace gas detection in recent years. Compared with the traditional photoacoustic cell method and other traditional detection methods, it has some advantages in volume, cost, sensitivity and anti-interference, especially it is not affected by the trace water vapor contained in the laser, collimator, etc. In this paper, the principle of water vapor concentration detection system based on QEPAS is discussed, the function of the system is realized, and the optical circuit and signal detection circuit are optimized. The frequency locking method is used to solve the problems and contradictions in the detection process. Finally, the accurate calibration of low concentration water vapor is achieved. The principle of QEPAS technology involves the absorption theory of infrared spectrum gas and the theory of multiple harmonics due to absorption. In this paper, the absorption law of gas molecule is obtained by infrared absorption of gas molecule, the absorption law of gas molecule, and the absorption law of gas molecule. Linear function, wavelength modulation, the principle of QEPAS technology is expounded and deduced step by step, and then the piezoelectric characteristics of quartz tuning fork are described in detail. According to the principle of QEPAS and related circuit knowledge, In this paper, the function, composition and design idea of each part of the system are introduced in detail, and the QEPAS water vapor concentration detection system is constructed. After the program and system parameters are set up, the water vapor concentration is detected in the atmosphere. In order to improve the detection accuracy of the system, the modulation amplitude of the laser and the parameters of the phase-locked amplifier circuit which affect the SNR of the system are optimized. The signal-to-noise ratio is doubled. The characteristics of the resonant tube and the tuning fork are studied carefully, and the optimal structure and size of the resonant tube are determined. It is found that the center frequency of tuning fork changes very little and the bandwidth changes obviously in the resonant tube structure. A new method of detecting QEPAS water vapor concentration, frequency locking method, is established. The theoretical analysis and experimental verification of laser driven signal scanning method and frequency locking method are carried out. It is proved that the frequency locking method has great advantages in the photoacoustic spectrum system, and the frequency locking method has been applied to the system, and the signal-to-noise ratio of the second harmonic signal has achieved a qualitative leap. The signal-to-noise ratio in air reaches 500ppm. The detection limit of QEPAS water vapor concentration detection system is 200 ppm, and the precision can reach 6 ppm at some concentrations. Finally, the stability of the system and the long-term stability of quartz tuning fork are discussed.
【学位授予单位】：山东大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TP274

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