Influencing Factors On Harmonic Characteristics And Research On System Calibration By Harmonic Method

The detection of water vapor concentration is of great significance in the field of EHV transmission systems,natural gas and energy use.Based on the theory of tunable diode laser absorption spectroscopy(TDLAS),the theoretical model of wavelength modulation spectroscopy(WMS)is analyzed in detail.The second harmonic method is used to detect the concentration of water vapor.The system structure and working mode of detecting the water vapor concentration by harmonic method are described in detail.The influences of different factors on the second harmonic characteristic are discussed,including influence of the amplitude of modulation signal,the range of scanning signal,the temperature control of laser and the phase difference between the reference signal and the modulation signal.And,calibrate the system by using the absorption simulation signal.The content and innovation of this paper mainly include:1.Summarize the importance of water vapor detection in EHV transmission,natural gas and energy utilization;introduce the common methods of water vapor detection;and the advantages and development of TDLAS.2.The linearity and the broadening mechanism of the gas absorption line are discussed in detail.Based on the Bill-Lambert law,the theoretical model of wavelength modulation spectroscopy is studied;And the principle of phase lock amplification technology is introduced.3.The main components of the system and the working process are described in detail in the case of TDLAS harmonic method.The use of anhydrous lasers and anhydrous detectors reduced the system background noise.The use of Herriot structure of the gas cell increased the absorption of optical path effectively.The use of AD630-based phase-locked amplifier extracted the second harmonic accurately.4.The influences of different factors on the second harmonic characteristic are discussed,including influence of the amplitude of modulation signal,the range of scanning signal,the temperature control of laser and the phase difference between the reference signal and the modulation signal.The harmonic waveform can be optimized by adjusting relevant factors.5.The method of calibrating the detection system through the absorption simulation generator is proposed for the first time.According to Bill Lambert's law,calibrate the system by using the driving signal with absorption features.The performance of the system can be optimized according to the calibration results.