B-4 Model 3936 Scanning Mobility Particle Sizer
TM
(SMPS
TM
) Spectrometer
Equation B-1 is used to calculate the cut-point diameter.
Equation B-1
where:
D
50
= particle cut-point diameter centimeter
Stk = Stokes number = 0.23
p
= particle density (g/cm
3
)
Q = volumetric flow rate (cm
3
/s)
C = Cunningham slip correction
= 1 + 2.492 λ/D
50
+ 0.84 λ/D
50
exp(-0.435 D
50
/λ)
λ
= gas mean free path
µ = gas viscosity (dyne•s/cm
2
)
W = nozzle diameter (cm)
The Stokes number is a dimensionless parameter that
characterizes impaction.
The impactor is used in the SMPS
™ spectrometer to remove
particles larger than a certain aerodynamic diameter due to their
contribution to multiply charged aerosols.
Electrostatic Classifier
The purpose of the Electrostatic Classifier is to extract a known
size fraction of submicrometer particles from the incoming
polydisperse aerosol.
In the Electrostatic Classifier, the aerosol enters a Kr-85 Bipolar
Charger (or neutralizer), which exposes the aerosol particles to high
concentrations of bipolar ions. The particles and ions undergo
frequent collisions due to the random thermal motion of the ions.
The particles quickly reach a state of equilibrium, in which the
particles carry a bipolar charge distribution.
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