Detection Of Trace Methane Concentration Based On Tunable Semiconductor Laser Absorption Spectroscopy (TDLAS)

Tunable diode laser absorption spectroscopy(TDLAS)technique with real-time response,high precision,high sensitivity and enables online non-contact monitoring features,according to insufficient the traditional technology of methane gas has,TDLAS technique is introduced to designed a monitoring system to test methane concentration.And through theoretical analysis,device selection,measuring,concluded that the monitoring system can reach concentrations of detection limit values.The monitoring system study the methane gas,and check out HITRAN database,choose the best wavelength measurement of methane gas.These key elements of research provide theory for the source selection and determination of structure and the plan design.And the second harmonic signal to calculate gas concentrations of comparative analysis of the two selected to meet the requirements of high precision measurement scheme,the system’s calculated theoretical measuring range value is 0.9ppm.In devices selection aspects,according to measurement requirements and the above theory analysis,design out whole TDLAS methane detection system program,for program in the involves of laser,and light drive,and scan signal and modulation signal occurred circuit,and photoelectric detector,and long light drive gas pool,and lock phase zoom device,and data acquisition card,devices parameter for analysis,according to mea.surement of precision requirements and the program implementation of feasibility,purchase out meet requirements of experiment devices and design making out experiment by needed of signal occurred circuit,part devices.In the system build process,on the part of the device for debugging purposes,observe their work correctly,and identify and resolve device issues while you work to finalize the whole experimental system set up.For the measurement of the concentration of methane gas provide a good platform.After times experiment exploration,and the analysis,obtained the measurement system of best modulation conditions following:through laser injected voltage,and work temperature on itself power and Center wavelength effect of results analysis,according to measurement precision and methane gas absorption peak requirements,obtained laser of best injected voltage for 2.67 V,work temperature for 22 ℃;through three species different waveform scan signal(50 HZ)on laser scan wavelength of effect,obtained best scan signal waveform for saw tooth wave;after times experiment validation,Gets the best signal is the sine wave frequency 10 KHz.Under the optimum curing conditions proved the feasibility and accuracy of second-harmonic detection method,and the lower limit of the methane concentration measurement system value is 1.1 ppm,the results close to the theoretical value.These conclusions indicate that the device is a reasonable choice,low error detection method,to meet the expected accuracy.The system has a certain practicality,can provide references for other gases,high-precision measurement.