Experiment Research About The Detection Of Methane Concentration By LED

Methane, as a main component of the atmospheric hydrocarbones of both natural and antropogenical origin (in accordance with different data of 85 to 95% of total hydrocarbons amount) is in the list of priority gases of the atmosphere to be continuously monitored in global scale and locally. On the other hand, extended development of gas-oil industry, so as coal industry, by many countries all over the world leads to increasing of the number of incidents related with methane leakages and consequences of these incidents. It forces us to solve the problem of the detection of methane. In this paper, we introduce how to detect the methane concentration by LED, using this method we can reduce the cost of measurement.This paper is divided into two parts, theory and experiment. In the part of theory, I put emphasis on explaining how to calculate the absorption coefficient of absorption band, the methods detecting methane concentration. In the experimental part I measured the methane concentration in the mixture of methane and air in a way similar to the differential absorption technology. I compared the calculation results with the actual methane concentration and the results are satisfying.The experimental setup is shown in Fig. 1. As we know, when a beam of light passes Fig. 1. Experimental setupthrough the gas to be tested, the light intensity changes into (1) is the light intensity absorbed by methane gas and is proportional to the concentration of methane. Then the light was collected and guided to a Fabry-Perot interferometer, Consider a particular case in which the cavity length d is much longer than the coherence length of the original light source but smaller than the coherence length of the negative lasers i.e. . The response of the interferometer to the original source is simply an intensity superposition of the multireflected beams and can be written as , where is the light intensity from the original source and R represents the intensity reflectivity of the cavity mirrors. For simplicity, we have assumed that the reflectivities of the two cavity mirrors are the same. The response of the Fabry-Perot interferometer to the negative laser will be a periodic function of d, i.e., , where . So after passing throught the Fabry-Perot interferometer, the light intensity can be written as (2) When we change the length of the cavity d, the light intensity coming out of the Fabry-Perot interferometer will change with it periodically. Assume when d=d1, we have the minimal light intensity Imin, as (3)When d=d2, I reaches maximum Imax, as (4)After reorganization, we can get (5)So if we know Imin and Imax, we can calculate the methane concentration.In the experiment we measured Methane –air mixtures with methane ratios of 10%, 5%, 2%, 1%, 0.5%. The versus the methane concentration is shown in Fig. 2. Fig. 2. versus the methane concentration. The circular marks represent the actual methane concentration measured by a U-shaped barometer. The line represents the methane concentration calculated by equation(5).