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Differential Mobility Analizer By Sinelk Berhane
Introduction History of electrical mobility studies• Erikson (1922) and Zeleny (1929) studied measure of ions in gases• Rohmann (1929) atmospheric airborne particles• Hewitt (1957) two inlet and two outlet flow• Whitby and Clack (1966) Unipolar charging by.• Liu and Pui (1974) improved Hewitt’s design• Knutson & Whitby (1975) designed the modern DMA
Overview• Nano DMA• Long DMA• Process – Charging particles – Mobility – Sizing particles• Application
Nano DMA• 2nm<=Dp<=150nm• Used to optimize performance• Outer electrode = 0.937cm
Long DMA• TSI Long DMA – 10nm <=Dp<=1000nm – Inner electrode (1V-10KV)
Process1. Particles get charged in the Neutralizer •Positively charged are sorted •All the others get lost2. Separated by the Sheath Air flowPassing down the annulus3. Influenced by Electric field• positively charged attracted• Electrode contact with particles depends on the particle mobility (Zp)4. Electrical mobility depends on •Size •Charge Note: if particles have the same charge, their mobility will be of the same size
Charging particles• Uni-polar charging: occurs when only ions of one polarity are present.• Bipolar charging : occurs when a mixture of positive and negative ions is present in a particle.
Charging particles cont’d• Radio active ionizers are used to neutralizer aerosols.• Neutralizers are bipolar chargers that use unstable elements such as Kr-85, Am-241 and Po-210 to charge particles Experimental setup to measure ionization current
Charging particles cont’dPo-210 source gives nearly eight times the saturation current as Kr-85 source
Midpoint mobility, Diameter & concentration
MobilityFe=qE = qV/d r1 – radii of outer electrode =0.937cmE= V r2 – radii of inner electrode = 1.961 cm r ln(r2/r1) q – charge Q - flow rate in cm3/sFdrag = 3πμDp*u μ – gas viscosity C – cunningham slip correction C (2λ/Dp) λ – gas mean free pathZ = ieC(Kn) L – electrode length 3πηDp V - average voltage E – Electric fieldZp = (Qsh+ Qa) ln(r2/r1) r – radius of the electric field u – particle velocity 2πLV Kn – Knudsen number (2λ/Dp)Dp = ___2neVL___ 3μ Qsh ln(r2/r1)
SizingTransfer function for tsi-long DMA operated at flow rate of beta = 0.1
Application• To Measure Aerosole particle size distribution – Determining the concentration of particles in the flow entering the CNC (condensation nuclei counter)• To make Aerosol particles equal size – Reagent in an experiment – See a change in conditions to see how they can be charged – Chemical analysis to find their composition – Determine the concentration of particles as a function of size