6-1
C H A P T E R 6
Particle Counting
This chapter discusses specific aspects of particle counting and particle
count measurements performed using the Model 3772/3771 Condensation
Particle Counter (CPC).
The Model 3772 CPC has two modes for particle counting:
Concentration mode, where data is presented as particle concentration
in p/cc, updated each second on the display (the maximum time
resolution is tenth of a second).
Total Count Mode (Totalizer Mode), where total particle counts are
accumulated and presented each second.
The Model 3771 CPC has the Concentration mode and is capable of Total
Count Mode using the Aerosol Instrument Manager
®
software.
Concentration mode is commonly used for most applications. Total Count
Mode is used at very low particle concentrations. Particles can be
accumulated until a desired statistical accuracy is achieved. Refer to the
section below discussing total count accuracy.
In the concentration mode, the CPC operates in the single count mode with
continuous, live-time coincidence correction over the range between 0 and
10
4
particles per cubic centimeter.
Optic a l D etection
Submicrometer particles are drawn into the counter and enlarged by
condensation of a supersaturated vapor into droplets that measure several
micrometers in diameter. The droplets pass through a lighted viewing
volume where they scatter light. The scattered-light pulses are collected by
a photodetector and converted into electrical pulses. The electrical pulses
are then counted and their rate (live-time corrected) is a measure of
particle concentration.
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