TSI Instruments 3082, Operation And Service Manual

Page: 24 / 225

1-4 Electrostatic Classifier Model 3082 and SMPS Spectrometer Model 3938
Figure 1-2
Electrostatic Classifier Schematic
Impactor
The primary function of the impactor is to remove particles greater than a
known particle size from the sampled aerosol. If not removed, these
particles will contribute to multiply-charged aerosols. The impactor is also
used as a flowmeter, since the pressure drop across the impactor is
proportional to the square of the flow rate. The Model 3082 can be
operated with one of three impactors. The orifice size and flow rate
determine the largest particle size that can be sampled within each SMPS
spectrometer measuring size range.
The aerosol flow is accelerated through a nozzle directed at a flat plate.
The plate deflects the flow to form a 90° bend in the streamlines. Particles
with high inertia cannot follow the streamlines and impact upon the plate.
Smaller particles follow the streamlines, do not impact upon the plate, and
pass through the impactor. The aerodynamic particle size at which the
particles are separated is the cut-point diameter or D
50
.