The Research And Realization Of Photoionization Gases Alarm Instrument

In the test field of indoor and emergency pollution accident there are many volatileorganic compounds and other toxic and harmful gases, which have characteristic of varioustype and low concentration and so on. However, the domestic large-scale equipments can notmeet real-time and continuous detection and personal protection of the field staff. Thus, a newportable harmful gases alarm Instrument based on photoionization detection technology wasdesigned in this paper. The instrument has the advantages of a small size, low powerconsumption, fast response, a variety of gases detection, and no carrier gas and so on.Firstly, the photoionization detection principle and features were introduced in the paper.According to the composition and structure of ionization light source, we described andanalyzed the characteristics and drive mode of light source in detail. On this basis, we selectedthe 10.6eV electrodeless ultraviolet lamp as the light source of the instrument, and designedthe capacitance coupling driver circuit of the light source. By analyzing the structural designtheory, we designed a one-dimensional porous structure ionization chamber, and used PCBprinted circuit technology to achieve the processing and production of the ionization chamber.Secondly, the basic structure of the system hardware based on C8051F040 MicroControl Unit (MCU) was introduced in this paper. And the circuit design schematic of eachmodule was given, which included interface circuit, signal conditioning circuit, data accesscircuit, key-press alarm display circuit and power management circuit. These work focused onthe signal conditioning module design, including the PID weak signal extraction,programmable amplification and second-order Butterworth low-pass filter. And the design ofdynamic power management power module which was used for reducing the system powerconsumption was also described in detail. And the ideas and processes of the system controlunit software design were given, which focused on the sampling and digital filtering module.The technology of combining the over-sampling and the moving average filtering was used, which reduced system quantization noise and random noise, and improved the signal to noiseratio (SNR) of the system.Finally, we carried out the actual test and analysis to the PID alarm instrument, andtested and studied PID output, the sensitivity of PID alarm instrument to different gases andthe four factors that could affect the PID output response which include through optical areaof ionization chamber, the vertical length of ionization chamber, UV lamp driving power andthe ionization chamber bias voltage.