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Research On The Spectrum Absorptive Optical Gas Detection Theory And Technology

Posted on:2013-12-12Degree:DoctorType:Dissertation
Country:ChinaCandidate:K K ZhangFull Text:PDF
GTID:1228330377959252Subject:Communication and Information System
Abstract/Summary:PDF Full Text Request
Optical gas sensor based on spectrum absorption detects the gas concentration bymeasuring the intensity changes of transmitted light or reflected light. In recent years, opticalgas sensor based on spectrum absorption has been widely used in the field of the gasconcentration detection for it doesn’t need to carry on the manual sampling and thepretreatment, and it has the advantages of high-sensitivity, good-selectivity and fast-responsetime.First of all, the line profile, the line width and the line broadening of the gas absorptionlines are analyzed based on the theory of molecular spectroscopy. According to Beer-Lambertlaw, in the near infrared region, the three methane absorption lines in the R3transitions of2v3band are chosen and the line parameters are confirmed to measure the gas concentration usingspectrum absorption. Wavelength modulation spectroscopy theory and harmonic detectiontheory are studied, and the frequency-intensity modulation signal is analyzed based on thefrequency modulation signal using Fourier series expansion and Taylor series expansion. Therelationship between harmonic signals and modulation coefficients is studied to determine thebest wavelength modulation coefficient, and etalon effects is analyzed in order to restrainetalon noise originating from multiple reflections in the optical path.Secondly, the quadrature digital lock-in algorithm is studied based on the basic principleof lock-in amplifier. In order to suppress the impact of phase adjustment error and analog drift,the first harmonic signal and the second harmonic signal are extracted using quadrature digitallock-in amplifier to improve the detection accuracy of the system. The characteristics andshortcomings of the second harmonic detection scheme are analyzed, and the methaneconcentration is measured using the first harmonic signal. The first harmonic detection systemhas a simple structure, and inhibits the influence of non-linear intensity modulation and laserwavelength shift on the system detection accuracy. The first harmonic detection schemereduces the stability requirements of the laser, which ensures system reliability and long-termstability.Once more, the influence of temperature fluctuation on the line strength, the line width,the absorption coefficient and the concentration measurement is analyzed. The advantages anddisadvantages of temperature self-correction scheme and temperature calibration coefficientscheme are discussed, and the measurement error from temperature fluctuation is reducedusing temperature correction scheme. The principle of measuring the gas temperature using tunable diode laser absorption spectroscopy (TDLAS) technology is analyzed, and two CO2absorption lines at6241.402828cm-1and6242.672190cm-1are selected by HITRANdatabase to study temperature measurement scheme. TDLAS temperature measurementscheme based on the first harmonic detection is proposed, and the feasibility of TDLAS gastemperature measurement is studied.Finally, according to the first harmonic detection scheme, a methane concentrationdetection system is designed, and laser, photodetector and absorption cell are analyzed. The5×10cm optical path transmission-type cell is designed based on the research of the spectrumabsorption of transmission-type cell and reflective-type cell, and the best wavelengthmodulation coefficient is determined to minimize the impact of modulation coefficientfluctuations. Optical gas detection system based on spectrum absorption detects theconcentration of methane using the first harmonic detection scheme. The results show thatthere is a good linear relationship between the methane concentration and the system output,and the system has a good performance of the system sensitivity, resolution, repeatability andstability.
Keywords/Search Tags:spectrum absorption, wavelength modulation, harmonic detection, first harmonic
PDF Full Text Request
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