6-4 Model 3022A Condensation Particle Counter
The accuracy of the concentration is increased by sampling for a
longer period. The concentration is calculated by:
concentration = total counts/volume of air sampled
where volume = Q × time
= 5 (cm
3
/s) × time (s).
Single-Particle, Live-Time Counting
In the concentration range of 1000 to 10,000 particles/cm
3
, the
CPC uses a different method of counting single particles that is
independent of t. The coincidence correction is applied directly by
measuring particle counts over the live-time of the counter.
Since the time t during which the particle passes through the
viewing volume effectively disables the counter, the value should
not be included in the sample time. The actual particle rate equals
the number of counted particles divided by the time no pulses were
present during the measurement time. A high-speed clock and
accumulator measure the live-time or the time in the spaces
between particle pulses. At the start of the measurement, the
accumulator is cleared and the clock started. The accumulator
adds up the live-time and the counter adds up pulse counts until
the accumulator reaches a value that triggers the counter and clock
to stop. The particle concentration is then calculated by
number of counted particles 1
C
a
=
__________________________
×
________
accumulated live-time flowrate
Photometric Calibration
For concentrations greater than 10,000 particles/cm
3
, the overall
amount of light scattered from the droplets is measured as a DC
voltage from the photodetector and then calibrated to a known
concentration. The method of calibration is explained in Chapter 7.
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