Design And Experiment Research Of Carbon Dioxide Detection Based On Infrared Spectrum Absorption Theory

With the development of science and technology and the improvement of people's living standards, the production scale of the industry has been expanded rapidly. At the same time, there are more and more carbon dioxide emissions. Carbon dioxide is one of the most important components of the air. It will lead greenhouse effect and land desertification if the content of carbon dioxide is too high. However, carbon dioxide is also playing an important part in the environment of the growth of animals and plants. Carbon dioxide the material of photosynthesis, whether its content in the air is proper will influence the growth of plants. Nowadays, with the growth of people's environmental protection consciousness, there are more and more requires of carbon dioxide detection in fields of air condition, agriculture, medical, automobiles, environmental protection, and so on. Therefore, to design a kind of carbon dioxide detection system has very important significance.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 CO₂ absorption coefficient; C is gas concentration; L is the absorption path length. Transform the formula, it can be written as followIf measured CO₂ gas and system structure is fixed, CO₂ concentration C can be obtained by measuring I and I0 .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 CO₂ 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 lightλgas andλr ef . The absorption of CO₂ at wavelengthλgas is very strong and it is weak at wavelengthλr ef . The radiation at wavelengthλgas is detecting signal, and the radiation at wavelengthλref is reference signal.The IRL715 light goes into gas cell, and then it is divided to two beams by beam splitter. Through two filters whose centre frequencies areλg as andλref, two beams reach the photodetector and are transferred to electric signals. Then, the signals will be amplified, filtered, and then be sent into the MCU after A/D conversion.We want to design a high sensitivity CO₂ sensor, which is cheap and apt to use commonly. According to the characteristic of CO₂ molecule, we choose CO₂ strong absorption line and use IRL715, 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 CO₂ concentration, and finish data-processing, saving and displaying.In order to reduce the power dissipation and volume of the system and according to the three principles of equipment design which are integration, micro, and low power dissipation, we select a kind of high integration MCU, C8051F020. It is a totally integrated signal processor, and a genuine work independently system on chip.At room temperature, we do the calibration experiment using 4 kinds of standard concentration carbon dioxide gas. And we get a fitting function, y=-0.002511x+10.0606, and then we fixed it into the MCU program, so that we can detect the concentration of current carbon dioxide gas. At the same time, we do the inversion equation experiment to analysis the sensitivity of the system.At the same experiment condition, we detect the same concentration of carbon dioxide gas many times, and get a relative standard deviation of 0.12%, then we analysis the repeatability of the system.In the experiment, we design an infrared gas detection system based on open gas cell, and it can be used for 0～10% CO₂ detection. There are some advantages to use an open gas cell, such as free flowing, quick response, real-time control. We use constant-current source to make sure the light source power is more stable. We design a DC amplifier circuit,a bandpass circuit and a zero-regulation circuit. The system is a kind of simple, reliable, low cost system. Experiments show that the system has a high signal-to-noise ratio and sensitivity.