ISO 15900-2020 pdf free download – Determination of particle size distribution — Differential electrical mobility analysis for aerosol particles.
5 General principle
5.1 Particle size classification with the DEMC
The measurement of particle size distributions with a DMAS is based on particle classification by electrical mobility in a DEMC. The DEMC may be designed in many different ways; for example, coaxial cylindrical DEMC, radial DEMC, parallel plate DEMC, etc. The coaxial cylindrical DEMC shown in Figure 1 is an example of a widely used design. It consists of two coaxial, cylindrical electrodes with two inlets. One inlet (marked q 1 in Figure 1) is for filtered clean sheath air. The other inlet (marked q 2 ) is for the sample aerosol. The sample aerosol, some of whose particles are electrically charged, enters the DEMC as a thin annular cylinder around a core of filtered, particle-free sheath air. By applying a voltage, an electric field is created between the inner and outer electrodes. A charged particle in the presence of an electric field will migrate within the field and reach a terminal migration velocity when the fluid dynamic drag on the particle balances the driving force of the electric field. Charged particles of the correct polarity within the sample aerosol begin to drift across the sheath air flow towards the inner electrode. At the same time, the clean sheath air flow carries the charged airborne particles downward. A small fraction of the charged particles enters the thin circumferential slit near the bottom of the centre electrode and is carried by the air flow to the detector (in the direction marked q 3 ). By varying the voltage, particles of different electrical mobility are selected. The remaining (not extracted) air flow leaves the DEMC as excess flow (q 4 ). When used within a DMAS, measurements of relevant parameters such as voltage, flow and their timings need to be combined with other measurements such as the output from the particle detector. These parameters are usually controlled using a system controller as shown in Figure 5.
The transfer function, Ω, of a DEMC is defined as the probability that an aerosol particle which enters the DEMC at the aerosol inlet will leave via the detector outlet. It can, mathematically, easily be described in the mobility regime. Therefore, this approach is taken here. The transfer function depends on the particle’s electrical mobility, Z, on the four volumetric flow rates, on the geometry of the DEMC and on the electrical field strength. The influence of the geometry and the electrical field strength on the transfer function is expressed by ΔΦ, which is a function of the geometry and the variable supply voltage, U, of the DEMC. For a given supply voltage, ΔΦ is constant. If particle inertia, gravimetric sedimentation, Brownian motion, space charge and its image forces are neglected, and if the sheath flow in the DEMC is recirculated (q 1 = q 4 , also resulting in q 2 = q 3 ), the transfer function of a DEMC can be described as an isosceles triangle with the half-width, ΔZ, centred around the electrical mobility, Z*, as in Figure 2.
When particle diffusion due to Brownian motion is significant, the resolution of the DEMC classification is reduced, which corresponds to a broader and shorter transfer function.
A detailed discussion of the transfer function for the example of a coaxial cylindrical DEMC can be found in Annex E.ISO 15900 pdf download.