Methane Concentration Detecting Instrument Based On Infrared Absorption Theory

A methane concentration detecting instrument is developed based on infrared absorption theory. Methane has a wavelength-selecting absorption to infrared radiation, and the absorption is functional with concentration of methane. The detecting instrument realizes the measurement of the gas concentration, with converting infrared radiation to voltage signal. This instrument has many advantages such as fast respondence, inflammation-proof and explosion-proof, large dynamic range and anti-poison.With analyzing and comparing with many gas measurement methods and according to precision and working environment required, we select infrared spectrum absorption method as the theoretical way of this instrument. Refer to the characteristics of the incandescent lamp, LED and laser diode, incandescent lamp is selected as the light source. IRL715 is an incandescent lamp whose accessible wavelength range amounts from the visible up to about 4.4μm. Using the pyroelectric IR detector PYS 3228 TC G5.2/G20 as the gas sensor, the detector employs two pyroelectric chips in a single sensor housing of TO-5 package size with two independently selectable IR windows. The center wavelengths of two infrared filters are 3.39μm and 4μm, which are in front of IR windows.According to the selected light source and detector characteristics, design a reasonable source modulation circuit and signal processing circuit. PIC16F877, which is a 8-bit SCM, is taken as the core of the instrument. Sampling of peak value of sine wave, concentration calculation, and light and beep alarm etc are completed. With the SCM, seven extra peripheral modules such as system power circuit, light source modulation circuit, basic amplifier, peak detector circuit, LED display, light and beep alarm circuit, two-line transfer circuit are also designed, and the functions are realized through programming with PIC16F877. While the detected gas concentrations are over the warning limit, which was set before, light and beep alarm signal is triggered.Experiment is carried out, and experimental data curve, error analysis, and calibration result are given. The results prove that the methane concentration detecting instrument based on infrared absorption theory has a good stability and accuracy.