Research On Novel NDIR Breathing CO₂ Monitoring System With New Light Chamber Structure

Human respiratory CO₂ concentration monitoring is one of the important contents of clinical monitoring.Changes in CO₂ concentration can reflect changes in human metabolism,circulation,breathing,airway or ventilation system function.And it is widely used in intensive care,emergency medicine,telemedicine,anesthesia monitoring and other fields.In view of the low sensitivity and signal-to-noise ratio of the mainstream non-dispersive infrared?NDIR?breathing CO₂ monitoring system,in this dissertation,the light chamber structures of compound parabolic concentrator,straight cylinder concentrator and conical concentrator were applied to the monitoring system.The three kinds of optical chamber structures were optimized and simulated by ZEMAX,their simulated optical efficiency was obtained,and the physical processing was carried out according to the simulated data.Experiments on the focusing effect of the compound parabolic concentrator chamber with the highest efficiency were carried out to verify the consistency between the simulation results and the experimental results.In addition,in this dissertation,the monitoring system was designed based on these three kinds of optical chamber structure,and the key devices such as light source,detector and microprocessor were selected.The light source modulation circuit,voltage following circuit,signal amplification circuit and system power supply circuit were designed.The algorithms of signal acquisition,filtering and denoising used in this dissertation were described.In order to compare the performance improvement of the monitoring system with different optical chamber structure,the third-order least square curve fitting method was used to calibrate the CO₂ concentration of the monitoring system with different optical chamber structure.And for the purpose of investigating the performance and practicability of the monitoring system in the clinical environment,the human real-time respiratory CO₂ monitoring experiment was carried out on the monitoring system with the best performance.The ZEMAX simulation results show that the optical efficiency of the optical cavity with compound parabolic is 4.3%,which is 89.77 times higher than that of the straight cylinder concentrator and 3.58 times higher than that of the conical concentrator.The experimental results show that the sensitivity and signal-to-noise ratio of the system with compound parabolic concentrator chamber are 8.94 and 24.65,which is 3.8 times and 1.9 times higher than that of the system with straight cylinder concentrator,at the same time,1.6 times and 1.4 times higher than that of the system with conical concentrator.In addition,the system with compound parabolic concentrator chamber runs stably,responds quickly,and can reflect the capnogram of the tested person in real time,which meets the expected performance requirements.