System Design And Experiment Of Mid-infrared Methane Detection

Methane is the main component of gas. Now there are several methane sensors are used commonly in coal mine in china, such as carrier catalysis,interference of light,thermal conductivity sensors. However, these detection technology fall behind, they are limited to detect, hard to use, and easy to make mistakes. In recent years, with the development of laser devices and laser technology, many researchers pay more attention to the methane detection based on infrared laser diode or infrared light emitting diode. The sensitivity and the measurement range of this detection reach a high level. However, because this detection costs almost ten thousands yuan, it is difficult to be used commonly in coal mine. So we design a new infrared methane sensor based on the principle of infrared absorption spectra, and do lab tests.1. The principle of infrared absorption According to the principle of infrared absorption, when source emitting wavelength is equal to gas absorption wavelength, resonance takes place. Absorption intensity is dependent on gas concentration. By measuring the absorption intensity, gas concentration is obtained.When a branch of parallel light cross measured gas and its intensity is I0, if there is one or several absorption lines in source spectra, light intensity falls down. According to Beer-Lambert law, the relationship between emergent light intensity I and incident light intensity I0 isWhere g (λ) is methane absorption coefficient; C is gas concentration; L is the absorption path length. Transform the formula, it can be written as follewsIf measured methane gas and system structure is fixed, methane concentration C can be obtained by measuring I and I0.2. Infrared detection method However, in practical application, the width of gas in the infrared absorption spectrum is only several nanometers. It is several in a thousand of light intensity of source, even weaker than the noise source. Gas absorption signal is drown in the noise, so it is difficult to measure gas concentration directly used spectral absorption method, especially when gas concentration is low. In the infrared absorption gas-sensor system, there are a number of factors to affect sensitivity, including the power fluctuations of light source,the optical interference from environment,the photodetector noise,the time-drift and the temperature-drift of circuit and so on. These factors can make system sensitivity fall down, and they are random. So if we want to get the accurate detection of methane concentration, we must take appropriate steps to overcome these problems.In this design, we use a dual-wavelength differential absorption method. In this method, we choose broadband light source, and two different wavelength filters for filtering, so we can get two closed wavelength lightλ_gas andλ_ref. The absorption of methane at wavelengthλ_gas is very strong and it is weak at wavelengthλ_ref, meanwhile, they must be far from absorption lines of other gas molecule (C₂H₂, NH₃, H₂O and so on). The radiation at wavelengthλ_gas is detecting signal, and the radiation at wavelengthλ_ref is reference signal.As shown in figure 1, the LED light goes into gas cell, and then it is divided to two beams by beam splitter. Through two filters whose centre frequencies areλ_gas andλ_ref, two beams reach the photodetector and are transferred to electric signal.According to Beer-Lambert lawWhere I_gas is the intensity before beam pass through the gas cell at wavelengthλgas; Ig′as is the intensity after beam pass through the gas cell at wavelengthλgas; I ref is the intensity before beam pass through the gas cell at wavelengthλref; Ir′e f is the intensity after beam pass through the gas cell at wavelengthλref;αis absorption coefficient of measured gas at wavelengthλgas; C is gas concentration; l is the length of optical path;βi is light intensity effect from many uncertain factors (i=1,2),β1≈β2.Equation (3) is divided by Equation (4), then3. System structure designConsidering the need of coal mine and environment monitor, we want to design a low concentration,high sensitivity methane sensor, which is cheap and apt to use commonly. According to the characteristic of methane molecule, we choose methane strong absorption line and use LED, and design gas detecting system for experiment based on the principle of broadband source differential absorption detection, including light source drive and stability power circuit,gas cell design,filters switch and detection circuit and so on. Using microcomputer we design single optical path detection and dual-optical path differential detection for methane concentration, and finish data-processing,saving and displaying.System design chart is shown in figure 2. The whole system consists of infrared emitting part,sensor and photo-electric conversion,signal processing circuit,SCM control and data collection part.A pulse signal which comes from SCM or oscillator modulates current source, which can control LED. This technology takes instead of modulation by complicated mechanism. After infrared radiation crosses the gas cell, beams are filtered by filter. Gas concentration signal and reference signal are obtained by photo-electric detector. Because signal is much weaker than background noise, it is necessary to use bandpass circuit for filter. The two output signals from detector need be further processed. Bandpass circuit consists of a low-pass filter and a high-pass filter. It is effective to restrain noise and interference. DC amplifier takes the signal from bandpass filter and makes it stronger. Useful signal of gas concentration is obtained. Then the micro-processing units can control the sound and light alarm,show gas concentration on LCD and transmit the signals by wireless.4. Experiment and data processing At experiment, we use an ellipsoid reflector. From the experiment, detection output is 1.75 times stronger than before. As shown in figure 3, LED and PIN detector are placed on two focus of ellipsoid reflector.Because of ellipsoid reflector, LED radiation focus on detector. Collection efficiency increases. Anther advantage of this design structure is to avoid coherent noise.At the calibration experiment ( 20℃, 101.325kaPa), we measure the output signal of 31 standard concentrations, as shown in figure 4, we use Matlab to do curve fitting, and get a fitting formula as follows According to equation (6), concentration inversion formula can beSave this formula into SCM. When sensor exports the value I g′as / Ir′ef, we can put this value into concentration inversion formula and get current gas concentration. According to concentration inversion formula, do experiment again and again, 10 sets of data are obtained, as shown in table 1. The comparison curve between measured value and standard value is shown in figure 5. 5. ConclusionAt the experiment, we design an infrared gas detection system based on open gas cell, and it can be used for methane detection. There are some advantages to use an open gas cell, such as free flowing,quick response,real-time control. We choose MEMS light source by electromodulation, the structure is simpler than chopper; because pulse current comes from constant-current source, light source power is more stable. We design DC amplifier circuit,bandpass circuit,peak-detector circuit, the system is simple,reliable,low cost. Experiments show that the system has a high signal-to-noise ratio and sensitivity.