Research On Tuning Diode Laser Spectrometry Method For Nitric Oxide

Internal combustion engine-type vehicles and ships have emitted a large amount of NO pollutants,which has brought immeasurable damage to the environment and economic development.Although the total amount of NO emitted by internal combustion engine-type vehicles and ships is large,considering the large number of them,the concentration of NO emitted by a single internal combustion engine-type vehicle or ship is low.Therefore,it is very meaningful to study the rapid real-time online monitoring of trace amounts of NO gas.However,the conventional methods for measuring NO gas,such as gas chromatography,are time-consuming and complex,and the gas concentration cannot be monitored online in real time;Fourier transform infrared spectroscopy is costly,bulky,and susceptible to other gases;Differential optical absorption spectroscopy has low resolution.In view of these shortcomings,this paper uses tunable diode laser absorption spectroscopy to measure NO gas.This method has the advantages of high sensitivity,fast response,good anti-interference and real-time online monitoring of gas.This paper introduces the theory of infrared selective absorption of gas molecules and the theory of gas infrared absorption spectroscopy,which leads to Lambert-Beer law.Based on this,combined with wavelength modulation technology and harmonic detection technology,the important of formula and conclusions for tunable diode laser spectroscopy is derived.It provides theoretical guidance for the development of NO gas measuring equipment.In this paper,the characteristic absorption peaks of NO gas in the mid-infrared region are analyzed,and the 5250.0 nm DFB-ICL laser is selected as the light source for gas detection.VIGO's mercury cadmium telluride detector was selected based on the center wavelength of the selected laser.Subsequently,an optical path system consisting of an off-axis parabolic mirror and a retroreflector was designed to measure gas for the low concentration and dispersion of NO gas.On the one hand,the path length of the light absorption is increased,and on the other hand,the system can easily measure the gas.Next,the overall structure of the NO gas measurement system is planned and introduced.Based on the previous theory and planning,this paper developed power supply module,function signal generation module,signal superposition and voltage-current conversion module,serial communication module,data acquisition module,lock-in amplifier and other hardware devices,and developed the corresponding software.Subsequently,the paper designed the experiment,the stability test of the power supply,the stability test of the drive current,the ADC data acquisition test,the direct absorption test and the second harmonic absorption test,and the experimental results were analyzed.The experimental results show that the second harmonic detection technology can better detect trace gases compared to direct absorption measurement techniques.The concentration of NO gas has a good linear relationship with the amplitude of the second harmonic.