Measurement of CH4 by differential infrared optical absorption spectroscopy

To solve the problem that the current use of optical methods used to detect various gases existing in sensors and signal processing was too complicated, detection sensitivity was not high, detection of the type of gases was single. We used optical phase-shifted grating laser window adjust the differential infrared absorption spectrometry and studied spectrum adjustment and detection signal processing problem. CH₄ in cigarette main smoke of different kind brands was measured continuously by differential optical absorption spectroscopy (DOAS). By using RM200 rotating disk smoking machine with 20 smoking flue, 20 kinds of gas were inserted into the rotating disk and ignited every 3 s. By use of DOAS technique, the measurement was completed with in 6~7 min after the gas was transported into a white cell with 31.5 m path length which was connected to the smoking machine. This technique can improve the measuremental temporal resolution compared with traditional method. At the beginning, the pressure in white cell was 5.2times10⁴ Pa and it increased to 1.03times10⁵ Pa while the measurement finished. The result shows the concentration of CH₄ in main gas is between 0.89 mg/m³ and 1.54 mg/m. Pool in the white cell entrance placed wide infrared wavelength LED light source, light source after a large-diameter phase-shifted fiber grating access to white cell, through the piezoelectric ceramic (PZT) phase-shift controlled phase-shifted fiber Bragg grating to infrared LED into a laser modulation of narrowband continuous changes in light, wavelength corresponding to various gases such as CH₄ absorption spectrum. White cell output pool signals entered into the photoelectric detector and ADC further, controlled by computer-related demodulation digitally. It can achieve high sensitivity digital lock-in amplifier function and be suitable for high-resolution detection of trace gases. Through simulation studies and practic- al tests, The results show that the piezoelectric ceramic (PZT) control phase-shifted fiber Bragg grating spectral scan can effectively suppress the background optical interference, the elimination of Rayleigh scattering and Raman scattering. Phase-shifted fiber Bragg grating modulation and digital lock-in amplifier detection sensitivity of the combination improve three orders of magnitude. The methods solve the problem a wide range of gas detection by one kind of equipment. The device is small size, low cost, easy to carry, can measure a wide range of gas content, and has an important application value for environmental monitoring. If the white pool with underwater ocean can detect trace gases, for rivers and the marine environment and resources to detect environmental monitoring is also of great significance.