A Research And Manufacturing Of CO₂ Sensor Based On NDIR Technology

The oxygen that the human body breathes is transported to the lungs through the blood circulation to participate in the metabolism of the human body,and the generated carbon dioxide is discharged to the outside of the body through respiration.Respiratory carbon dioxide,a metabolic product of the human body,can reflect changes in the body's metabolism and ventilatory system function.Therefore,the concentration of carbon dioxide in the breathing gas has been considered to be the same basic vital signs as body temperature,respiration,pulse,blood pressure,arterial oxygen saturation,etc.,and is widely used in anesthesia monitoring,intensive care,emergency medicine and the like.The traditional application of CO₂ gas monitor is huge,high in cost and long in response time.It is difficult to meet the daily application requirements of CO₂ gas in the detection of motor function and metabolic function.This thesis has carried out research on carbon dioxide detection technology.The NDIR?Non-Dispersive Infrared?technology is used to prepare gas sensors.The main work is as follows:1.In the air chamber design process,the optical path simulation work is synchronously performed,and the air chambers with rectangular and circular cross sections are analyzed and explored,and the air chambers with circular cross sections are obtained by irradiance analysis.The air chamber of the cross section has a higher convergence effect on infrared light.The air chamber of the infrared gas sensor was selected using a crisscross structure,and an optical path length ranging from 10 mm to30 mm was obtained for detecting a desired length range of human exhaled carbon dioxide?4%-6%?.2.In this paper,the gas sensor is calibrated under different temperature and humidity conditions,and the fitting curve is obtained.Through further analysis of the fitting curve,the concentration calibration result and the fitting curve are written into the MCU,and finally obtained in the temperature range of-20°C-50°C,25%RH-99%RH humidity range,0%-8 Within the%concentration range,the absolute error is less than 0.5%,and the relative error is 3%of the CO₂ sensor.3.In addition,the higher frequency modulation frequency is explored,and the response time of the gas sensor is related to the modulation frequency of the light source.Higher modulation frequency can bring faster response speed.After the source modulation frequency is increased from 2 Hz to 4 Hz,the response time of the sensor is reduced from 3 s to 1 s.4.Through fluid simulation,it is found that the process of interaction between the gas to be tested and the gas chamber also affects the accuracy and immediacy of concentration detection.This paper provides a stable flow rate for the gas to be measured by introducing a piezoelectric micropump that controls the flow rate of 350 ml/min.The inside of the gas chamber causes an airflow column due to the entry and exit of the air holes on one axis,which causes the effective optical path length of the gas sensor to be shortened.A spoiler structure consisting of 14 1mm*1mm square through holes is added to the air inlet to quickly and evenly distribute the measured gas entering the air chamber throughout the air chamber.