Methane Photoacoustic Sensor By Use Of Double-pass Photoacoustic Cell

Methane?CH₄?is a colorless and odorless gas,but a flammable and explosive gas,which is one of the main greenhouse gases.The detection of methane is of great significance to environmental prevention,coal mine and industrial safety production.Photoacoustic spectroscopy technique has a lot of advantages,such as high detection sensitivity,large dynamic range,wavelength independence,which has been widely applied to many fields so far.This thesis presents a methane photoacoustic sensor system by use of double-pass photoacoustic cell.The performance of the sensor system is optimized and assessed.An excellent result is obtained.The process of photoacoustic effect includes gas infrared absorption,heat generation,sound excitation and weak signal detection and extraction,etc.In this thesis,these theories are introduced in detail.Photoacoustic cell is the core component of a photoacoustic system.This thesis takes the photoacoustic cell with a standard cylindrical resonator as an example.The distribution of sound field and the amplitude of the photoacoustic signal in the cell are calculated.These works provide a basis for the design and optimization of the resonant photoacoustic cell,as well as the design of double pass configuration.In this experimental part in this thesis,a methane photoacoustic sensor system is developed by combining a double-pass resonant photoacoustic cell with a DFB laser.The central wavelength of the DFB laser is at 1.65?m,which corresponds to the 2?₃ band of methane absorption spectra.The performance of the methane photoacoustic sensor system is optimized and assessed.A detection sensitivity of 0.21ppm is achieved,corresponding to normalized noise equivalent absorption coefficient?NNEA?of 2.1×10^-8cm^-1·W·Hz^-1/2.The experimental results from single-and double-optical path configurations are compared,which are consistent with the theory due to the fact that the absorption path of the gas doubles,and the amplitude of the photoacoustic signal doubles.The methane photoacoustic sensor system has simple construction and low cost,due to the use of cost-efficient DFB laser,which can meet the requirements of atmospheric environmental monitoring,mine gas detection and industrial process control.