A Submicron Electrical Aerosol Detection System with a Faraday Cup Electrometer
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Submicron-sized aerosol particles, defined as aerosols with particle diameters less than 1 µm, suspended in air have significant effects on the human health, global climate, air quality and processes in various industries such as food, pharmaceutical and medical, electronic and semiconductor industries. Automotive engines have long been recognized as a major source of submicron-sized aerosol particles. Development of aerosol detection and size distribution measurement methods has been primarily motivated by the need to find better means of monitoring and controlling indoor and outdoor aerosols for pollution and process control industry. In this study, a submicron electrical aerosol detection system for measuring particle number concentration in the size range between 1 nm – 1 µm using electrostatic charge measurement technique was developed. It consists of a size selective inlet, a particle charging system, an ion trap, a Faraday cup electrometer, a signal conditioning and processing system, and an I/O control and human-computer interface. In this system, an aerosol sample first passes through the size selective inlet to remove particles outside the measurement size range based on their aerodynamic diameter, and then pass through the unipolar corona charger that sets a charge on the particles and enter the ion trap to remove the free ions. After the ion trap, the charged particles then enter the Faraday cup electrometer for measuring ultra low current about 10-12 A induced by charged particles collected on the filter in Faraday cup corresponding to the number concentration of particles. Signal current is then recorded and processed by a data acquisition system. A detailed description of the operating principle of the system as well as main components was presented. The performance of the prototype electrometer circuit used in this work was evaluated and compared with a commercial electrometer and good agreement was found from the comparison. Finally, the preliminary experimental testing results were also shown and discussed.
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