CHAPTER 6
Particle Counting
6-1
The Model 3022A Condensation Particle Counter (CPC) uses three
methods of particle counting for concentration calculations: single-
particle, realtime counting; single-particle live-time counting; and
photometric calibration. This chapter focuses on counting the
individual particles up to a concentration of 10,000/cm
3
.
Note: Photometric calibration is described in greater detail in
Chapter 7.
Optical Detection
Submicrometer particles are drawn into the CPC 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
(calculated) is a measure of particle concentration. The optical
detector used in this process is described in Chapter 5.
Single-Particle, Realtime Counting
At particle concentrations below 1000/cm
3
, the concentration is
calculated by counting the individual pulses from the photodetector
and by counting the real time. The particle pulses are typically
1.4-volts high, 5-microseconds wide at the base, and 3.3-
microseconds wide at the trigger-threshold point
*
(see Figure 6-1).
The counting trigger-threshold voltage is about 750 millivolts. The
threshold has a hysteresis of about 100 millivolts so that another
particle cannot be detected until the voltage first drops below 650
millivolts. This prevents false counting of noise pulses riding on top
of the particle pulse.
While the pulse exists, the counter is disabled, that is, it cannot
count another pulse until the present pulse is completed. A second
particle entering the viewing volume while the first remains can
*
Actual pulse heights for your CPC are listed on the data sheets in Appendix E.
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