1. Differential Mobility Analizer
By Sinelk Berhane

2. 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

3. Overview
• Nano DMA
• Long DMA
• Process
– Charging particles
– Mobility
– Sizing particles
• Application

4. Nano DMA
• 2nm<=Dp<=150nm
• Used to optimize performance
• Outer electrode = 0.937cm

5. Long DMA
• TSI Long DMA
– 10nm <=Dp<=1000nm
– Inner electrode (1V-10KV)

6. Process
1. Particles get charged in the Neutralizer
•Positively charged are sorted
•All the others get lost
2. Separated by the Sheath Air flow
Passing down the annulus
3. 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

7. 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.

8. 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

9. Charging particles cont’d
Po-210 source gives nearly eight times the saturation current as Kr-85 source

11. Midpoint mobility, Diameter &
concentration

12. Mobility
Fe=qE = qV/d
r1 – radii of outer electrode =0.937cm
E= V r2 – radii of inner electrode = 1.961 cm
r ln(r2/r1) q – charge
Q - flow rate in cm3/s
Fdrag = 3πμDp*u μ – gas viscosity
C – cunningham slip correction
C (2λ/Dp)
λ – gas mean free path
Z = ieC(Kn) L – electrode length
3πηDp V - average voltage
E – Electric field
Zp = (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)

13. Sizing
Transfer function for tsi-long DMA operated at flow rate of beta = 0.1

14. 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

15. Thank you