Research And Design On Photo Ionization Detector

Photo Ionization Detector (PID) mainly used in workplace of oil and chemicalindustry, for the sudden VOC leaks, PID is able to track and locate the leak source,monitor air quality of the workplace and ensure the workers’ safety.It has theadvantage of high sensitivity, strong distinguishable ability, Wide detection range, fastresponse and portable type. It is a kind of simple, easy, fast and practical instrument.The following work was made in this paper:Firstly, the fundamental of ionization was elaborated, and based on thedevelopment situation and the future development trends at home and abroad on PID,combined with the advantages and disadvantages of the existing technologies in thefield, the design objectives of PID was made.Secondly, the energy source of photo ionization－ultraviolet lamp was analyzed.Synthetically considering from the external structure, filling gas, the lamp windowmaterial and other aspects, this design determined to select the ultraviolet lampproduced by RAE, its output power was up to10.6eV. In this design, PID uses12Vbattery power, and uses dual transistor push-pull self-oscillating circuit to powerultraviolet lamp. From a practical point of view, ionization chamber uses axial flowstructure, and uses DC~DC converter centering on the integrated switching powersupply controller MAX641to obtain150V voltage to power polar plate. Thenintroduced the principle and characteristic of diaphragm pump, this design determinedto select NMP05M diaphragm pump produced by KNF to spur gas into ionizationchamber, the rotate speed was0.5lmin, and designed power supply circuit centeringon W7805to power the diaphragm pump.Thirdly, the principle of weak signal detection was elaborated, and thenconfirmed methods of noise suppression and shielding. The integral structure ofpreamplifier without distortion was designed. First converted the weak current sensoroutput to a voltage signal through the I/V conversion circuit, then amplified theconverted voltage signal, and finally filtered out harmful noise mixed in the usefulsignal. Power circuit mainly constituted by the LM117, AD580and MAX660 integrated component. Realized I/V conversion and amplifier with LMC6082as thecore device, and designed second amplifier and filter circuit centering on AD627.Finally, the related functional experiments are made on PID, and the experimentresults meet the desired requirements.