5-6 Model 3775 Condensation Particle Counter
The Model 3775 CPC uses two modes of particle counting: single-
particle counting mode and photometric mode. At concentrations
below 50,000 particles/cm
3
, individual electrical pulses generated
by light scattered from individual droplets are counted, employing a
continuous “live-time” coincidence correction algorithm to improve
counting accuracy. Coincidence occurs when the presence of one
particle obscures the presence of another particle creating an
undercounting error. “
Live-Time Counting” is discussed in
Chapter 6. At concentrations above 50,000 particles/cm
3
, live time
correction becomes less effective and the instrument transitions to
a mode where the instrument measures the total light scattered
from all droplets present in the laser beam. The DC voltage from the
photodetector is calibrated against known concentrations to provide
concentration measurements as high as 10
7
particles/cm
3
.
Figure 5-1
Flow Schematic of the Model 3775 CPC
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