ISO 15900:2020 pdf download – Determination of particle size distribution — Differential electrical mobility analysis for aerosol particles.
6.2.3 DEMC The DEMC is the core component of a DMAS. The basic operating principle is electrical mobility discrimination by particle migration perpendicular to a laminar sheath flow. The migration is determined by an external electrical force and the counteracting particle drag forces in the laminar sheath flow. DEMC can be designed in a variety of geometries. The classifying characteristic of a DEMC is described by its transfer function. Whatever the geometrical design of the DEMC, the transfer function is defined by the critical mobilities, which are determined by the geometrical dimensions of the device, by the flow rates and by the voltage potential between the electrodes. Details on the transfer function of the DEMC can be found in 5.4. For the example of a coaxial cylindrical DEMC, details on the critical mobilities as well as the transfer function can be found in Annex E. Measuring a particle size distribution is usually achieved by changing the voltage. For transient measurements, DMASs with multiple particle detectors have been designed. These systems typically operate with fixed voltages. 6.2.4 Aerosol particle detector The aerosol outlet from the DEMC shall be connected to a well-characterized particle detector. This instrument shall be able to detect particles exiting the DEMC with known efficiency across the entire size range to be reported. The lower end of the size range limits the aerosol particle detector that can be used. In general, either a continuous flow condensation particle counter (CPC) or an aerosol electrometer is used. In a CPC, aerosol particles are exposed to condensable supersaturated vapour. Vapour condensing on the particles grows them into droplets that can be counted by optical means. Aerosol electrometers are typically designed as Faraday cup aerosol electrometers (FCAEs). The aerosol particles deposit on a filter inside the Faraday cup.
6.2.5 System controller, data acquisition and analysis The system controller with data acquisition and data inversion should cover several tasks. Before the measurement, the system control of an automated measuring system should set all operating parameters and flag the operator if any of these parameters cannot be set correctly. During the measurement, the system control should acquire and monitor all critical parameters (for example, the flow rates) and flag a warning to the operator if any of these parameters are outside pre-established tolerances. At the same time, the control system should set the electrode voltage through the variable high voltage supply and read the particle number data from the aerosol particle detector. Either during or after the measurement, data inversion takes place and the measured particle size distribution is presented, stored, exported, etc.