High Precision Trace Gas Concentration Detection Based On Tunable Laser Absorption Spectrum

Tunable laser absorption spectroscopy?TDLAS?has been widely used in gas detection since 21 century.However,under the condition of weak on-line intensity or low pressure and low concentration,the absorption signal is so weak and the signal-to-noise ratio is so high that the detection accuracy is low.At present,the main means to improve the signal to noise ratio of detection results are:the wavelength modulation method starting from the signal harmonic,the multi-path absorption method starting from the optical path.According to Beer-Lambert's law,the intensity of absorption signal can be effectively enhanced by raising the optical path of absorption.The techniques used in multi-path absorption include White cell,Herriott cell,and ring-down cavity and so on.Because of the stability and simple structure of Herriott cell,this paper simulates and designs an integrated multi-path measurement system of Herriott cell structure by using this method as the method of extending optical path.Based on the research results at home and abroad,the problems of short optical path,poor airtightness and portability of universal multipath system are studied and improved in this paper,and the experimental detection is carried out.The contents and conclusions of the study include:1.Based on Beer-Lambert law and Herriott reflector principle,the general expression of light reflection in Herriott cell is derived.The relationship between the time of reflections and incident angle,mirror spacing,spherical mirror focal length and incident position is studied.2.A set of integrated multi-optical path high-precision gas detection system is simulated and established.The total optical path of the system is 20.28m,the length of single path is 204mm,and the total number of light reflection is 100 times.The leakage rate of the system is 56Pa/h under near vacuum condition and 15Pa/h under low pressure condition?10kPa?.The system is integrated into a 5U chassis and a reference light path is installed.The system has its own current stabilizer and can measure the pressure of the air chamber in the system automatically.The system integrates BNC data interface,FC light interface and ball stop valve.It can adapt to all kinds of gas detection and output data.3.Direct absorption method is used to measure the concentration of water vapor of the environment and 2005ppm standard NH₃ gas respectively in this paper.It also discusses the measurement results and analyzes the accuracy of measurement and the application range of each measurement method.The results are as follows:1)the detection result of NH₃ standard gas by 2005ppm is 1946.5ppm,and the error is 2.9%compared with the standard gas.2)at the same time,the measured results of the system are compared with those of the ordinary gas chamber.The measurement result of the standard 2005 ppm NH₃of the ordinary gas chamber is 2754.4ppm,and the error is 37.4%compared with that of the standard gas.3)the water vapor concentration in the air is measured,and the result is24461.9ppm.The average output of 16 groups of data collected in each acquisition period is 1 s,a total of 3600 sets of data are collected for 1 hour,and the detection accuracy is 68.4e^-3ppm calculated according to the variance of Allan.It is proved that the system can effectively improve the spectral absorption under the same conditions,improve the signal to noise ratio?SNR?of the absorbed signal,reduce the measurement error,and accurately measure the gas concentration in the case of trace gas or low pressure weak spectral line intensity.At the same time,the system does not need light path adjustment,it can be measured simply by filling and pumping and adjusting laser controller,which is very suitable for field measurement of trace gases.