The Research And Implementation Of Portable Photoionized VOCs Detector

In recent years,people had paid more and more attention to environmental pollution,and the detection of air quality had also become an important research topic.Among many gas pollution concentration detectors,the photoionization detector was a detector with fast response time,high detection accuracy,wide detection range,and non-destructive gas detection.Compared with other traditional detectors,it had the advantages of high sensitivity,portability,and good accuracy.Based on the principle of photoionization technology,this thesis designed a portable photoionization detector for rapid detection of volatile organic compounds.The detector used STM32 single-chip microcomputer as a microcontroller.The detector completed photoionization chamber,UV lamp drive circuit,signal detection circuit,and USB charging.The circuit and telecommunication module were designed,and their performance had been tested.The detector used a vacuum ultraviolet lamp with an ionization energy of 10.6 e V as the ultraviolet light source,and a push-pull self-excited circuit was designed to drive the ultraviolet lamp to emit light normally.Through the simulation analysis of the ionization chamber,various parameters of the ionization chamber were determined.The structure of the ionization chamber was designed as an axial flow type,the distance between the plates was 6 mm and the depth of the ionization chamber was 10 mm.In the design of the ionization chamber,a bipolar plate structure was proposed.The signal intensity of the bipolar plate structure was measured through experiments to fit the calibration curve better,and the signal sensitivity in the low concentration range was better.An air intake system was designed at the front end of the ionization chamber to limit the UV irradiation area and protect the window surface of the UV lamp.The materials of the ionization chamber and the air intake system were made of PTFE.The hardware circuit of the detector was divided into signal detection circuit,power management module and multi-function control module.In order to detect the weak current generated by ionized VOCs,a logarithmic amplifier circuit and filter circuit were designed to compress a wide range of input current into a small range of voltage signals.In order to meet the requirements of portable charging of the detector,a USB charging circuit and a remaining power monitoring circuit were designed.The microcontroller used STM32-F103RCT6 to realize the collection and displayed of the concentration of the gas to be measured.The electronic clock,temperature and humidity monitoring,digital display,buzzer alarm,suction pump control and Wi-Fi communication were designed in the system to enable the detector to have The function of remote communication.When the area to be tested were large,multiple devices can be placed in different areas to achieve centralized network monitoring.The detector completed the prototype assembly and set up an experimental platform.The detector was calibrated and verified with 0～200 ppm isobutylene gas.The experiment proved that the system has good repeatability and the calibration curve fit reached 0.99762.The experiment uses 30 ppm isobutylene to measure the stability of the detector to be 2.72%.The response time of the detector is approximately 8 seconds.The calibration curve of isobutylene gas concentration and output signal intensity is divided into four sections(0-20 ppm,20-50 ppm,50-100 ppm,100-200 ppm)for linear fitting.The four-section curve has a good linear fit.When the isobutylene gas concentration is 0-20 ppm,the linear fit is0.96509,and the output signal sensitivity is 24.80 m V/ppm.