A Non-dispersive Infrared Gas Monitoring System Based On A Relaxed Ferroelectric Single Crystal Pyroelectric Infrared Detector

With the increase of air pollution worldwide and people’s importance to air pollution,the development of gas sensors and gas detection instruments has become the focus of global attention.Domestic and foreign research institutes and companies have also developed various pollutant emission detection technologies and equipment,among them,the online detection of stationary pollution sources,especially CO₂ and VOCs,is of particular concern.At present,the main gas detectors in the market are mainly catalytic combustion,electrochemical,photoelectric and infrared absorption.The infrared absorption gas detector becomes the main direction of market development due to its good stability,long service life,short response time,high precision and not easy to be toxic.We designs and develops a non-disperse infrared（NDIR）gas detection system to meet the requirement of development and applications of high-precision gas detection system including designing the optical path structure of the gas detection system,fabricating high-performance pyroelectric infrared detector based on relaxor-based ferroelectric single crystals and applying to the optical path system,designing the hardware structure of gas detection system,writing the modularization program using C language,testing the complete gas detection system,analyzing the experimental results and error,as well as calibration.Finally,the working conditions of the infrared light source of the optical path were determined to be 3.3V with a square wave driving at 5Hz,and the airtight chamber is designed with multi-reflective gold-plated chamber.The high-performance pyroelectric infrared detectors with specific detectivity of 1.5?10⁹ cmHz^1/2/W,have been fabricated by optimization of the properties of the relaxor-based ferroelectric single crystals and the preparation process of sensitive elements,which is more than 2times that of commercial detectors based on TGS、LiTaO₃ single crystals.The system circuit is mainly composed of light source drive circuit,signal processing circuit and single chip circuit.The transistor is used as the switch to control the excitation signal of infrared light source.The LCD screen is used as the display unit to sample,process,analysis and display efficiently.In the body of the system program,the system boot first advanced into the program first,and then each module initialization,waiting for the interrupt trigger an ADC acquisition,the digital signal processed,finally,the density was calculated and outputted.An inversion algorithm of CO₂ gas concentration for the system is developed by modifying the Lambert-Beer law between the concentration of CO₂ gas and the output signal of the detector.For the system performance,we first established the calculation model of the accuracy of NDIR gas detection system,estimated the theoretical accuracy of the system about 7 ppm,which is far higher than the precision of the main products on the market.In addition,the actual precision of the system is determined to about23ppm.We analyzed in detailed the factors that affected the actual accuracy of the system andfound that the characteristic of the frequency spectrum,instability of light emission power,the mismatch of light source and detector,stray light radiation and temperature drift were the main factors lead to lower actual precision.The non-dispersed infrared gas detection system designed in this thesis based on pyroelectric infrared detectors using relaxor-based ferroelectric single crystals has realized the on-line detection of CO₂ gas concentration,with precision up to 23 ppm and range of 0-2000 ppm（can be increased to 5000 ppm）.The system not only has high precision,but also has low cost and small volume.It will have a broad application prospect in the fields of medical prediagnosis,intelligent wearable,factory exhaust monitoring and household inspection.