B-12 Model 3936 Scanning Mobility Particle Sizer
TM
(SMPS
TM
) Spectrometer
Equation B-7 allows calculation of the particle diameters that pass
through the exit slit of the Electrostatic Classifier, if the number of
charges on the particle is known. Table B-1 shows that the
majority of the aerosol in charge equilibrium exists as singly
charged particles. However, a fraction of the particles exist as
multiply charged particles.
A particle with certain mobility may exist as a small particle with a
single charge or as a larger particle with multiple charges. Either
has the same mobility and is removed by the system with the
monodisperse airflow. The effect of multiply charged particles on
the data analysis is discussed in the “Data Reduction” section in
this chapter. Figure B-6 shows the relationship between the
diameter of particles with a single charge and center rod voltage for
a Model 3081 Long DMA.
The Equations listed above do not account for diffusion
broadening. This is an important factor in particles below 50 nm.
For the more comprehensive formulas, refer to work by
Stoltzenburg [1988].
Once the particles are classified according to electrical mobility,
their concentration is measured by a CPC or Electrometer.
Particle Diameter, Dp, µm
Collector Rod Voltage V, Volts
Figure B-6
Center Rod Voltage as a Function of Particle Diameter for Normal Operating
Conditions of the Long DMA [Agarwal and Sem, 1978]
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