5-2 Model 3321 Aerodynamic Particle Sizer
Spectrometer
Figure 5-1
Aerosol Flow Through the Model 3321 APS™ Spectrometer
After passing through the orifice, the sheath flow is reunited with the
sample flow at the accelerating orifice nozzle. This flow confines the
sample particles to the center stream and accelerates the airflow around
the particles. In this way, small particles (which can accelerate with the
flow) reach a higher velocity than larger particles (which, due to inertia, lag
behind the flow of the air stream).
Particle velocity is then measured in the optics chamber (refer to “Optics
Path and Signal Processing Path,” below).
After measurement, the particle stream exits the optics chamber, drawn by
the total flow pump. The combined flow is controlled by the total flow pump
and the pressure drop across the accelerating orifice nozzle.
Sample flow is filtered before and after each of the two pumps. The filter
upstream of the pump protects the pump from contamination. The filter
downstream of the pump prevents contamination of the flow as the pump
vanes wear.
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