Measurement Of CO₂ Concentration Based On TDLAS

With the rapid development of science and industry,the environmental pollution has become increasingly serious.The greenhouse effect,which caused mainly by the carbon dioxide gas,will do harm to both the environment of earth and human health.Therefore,effective monitoring of the gas is very important.The detection of carbon dioxide gas concentration,can also lay the foundation for the detection of other harmful gases,which can improve the level of gas concentration measurement.In this paper,the basic principle of Tunable Diode Laser Absorption Spectroscopy(TDLAS)technology is analyzed.It is pointed out that the gas concentration can be calculated according to the different absorption characteristics of different gases.The spectrum intensity and spectral line shape function are briefly introduced.According to the analysis of HITRAN database,a carbon dioxide line at 1572.3351 nm was selected for measurement.The direct analysis method and the wavelength modulation method are introduced in detail.The wavelength modulation method is simulated by MATLAB.Based on the theory of TDLAS,the experiment system platform is set up,including three important components: light source system,air chamber,receiving and data processing system.Each part of the components is introduced in detail.Then,the key parameters of the wavelength modulation method are selected and set based on experiment.By analyzing the stability and peak value of the received signal,the appropriate high-frequency modulation parameters are selected.At the same time,the key parameters of the lock-in amplifier are experimentally selected.So that the harmonic signal can be detected more accurately.Using the experiment system,both the direct absorption method and wavelength modulation method were used to measure the concentrations of different carbon dioxide gases.The direct absorption method can directly showed the light intensity that absorbed by the gas.But the accuracy is not good and the lower limit is high.However,using the wavelength modulation method,with the lock-in amplifier,the second harmonic signal can be detected.The peak of the second harmonic signal shows a linear relationship.The concentration of the unknown gas can be deduced by the known concentration of standard gas,using the gas concentration inversion algorithm.The measurement results have high accuracy and the lower limit can be reduced.