Research On Temperature Influence And Comprehension Method Of Methane Detection Based On TDLAS

Gas detection technology is used in many fields.The current gas detection technology has a variety of means,and laser spectrum method because of its precision and real-time advantages develops a lot.However,when the temperature of the measured gas changes in a wide range,the physical properties of the gas would change,resulting in inaccurate measurement of laser spectroscopy detection method.Aiming at this problem,this paper presents a temperature compensation algorithm that relies on the second harmonic signal,and does not need to measure the actual temperature of the measured gas.This paper studies the research through methods of theoretical analysis,algorithm modeling and verification test.The influence of temperature on methane detection is analyzed by theoretical modeling.Three kinds of absorption models are established,and choose Lorenz Profile as the absorption line profile.Then,the influence of temperature on the absorption coefficient is analyzed from the aspects of density,line width and line strength,and the influence of temperature on the second harmonic signal in harmonic detection is analyzed.When the temperature changes from-30 degrees Celsius to 120 degrees Celsius,the gas molecular density value decreases by 38.6%,line strength decreases by 45%,line width decreases by 29.7%,absorption coefficient decreases by 51.7%,second harmonic signal at the center frequency is reduced by 51.6%.At present,the solution to the temperature effect,generally take the empirical formula or theoretical formula method for compensation correction,but the premise is to obtain the measured gas temperature.Or simultaneously testing two absorption lines to obtain temperature and concentration at the same time.This paper presents a new method to compensate the effect of temperature.The second harmonic signal contains the concentration information of the measured gas,using the difference between the two frequency points in the second harmonic to calculate the concentration,and the dependence of the method on the temperature is relatively smaller.In the algorithm model,with a modulation factor of 1,for example,when the temperature changes from-30 degrees Celsius to 120 degrees Celsius,using the second harmonic signal at center frequency point to calculate concentration,the relative error can reach 51.6%.When the new method proposed is used,the relative error is 12.7%,which greatly reduces the dependence of the measurement result on temperature.The algorithm is used to verify the test,and the concentration data of the same concentration of methane gas at different temperatures are collected.The results of the measurement are basically the same as the theoretical model.On the basis of these data,the compensation algorithm is used to verify that the effect of temperature on the measurement results is reduced by 50%,and the effect is improved.Theoretical results and experimental results have a certain gap,try to analyze the causes of this phenomenon,the impact of environment noise problems can not be ignored.