6-2 Model 3775 Condensation Particle Counter
Total Count Accuracy
At very low concentrations, the accuracy of the measurement in the
single-particle-counting mode is limited by statistical error. If the
total number of particles counted in each time interval is very
small, the uncertainty in the count is large. The relative statistical
error of the count
σ
r
is related to the total count n by
.
n
n
r
=
σ
In totalizer mode, the accuracy of the concentration is increased by
sampling for a longer period and counting more particles. The
concentration is displayed on the front panel in totalizer mode and
is calculated by:
tQ
n
×
=
sensor in the flow aerosol of volume
counts total
=ion concentrat
where
Q = Aerosol flow rate displayed on the Status Screen; this value was
entered at the factory and is very close to its nominal value of
0.3 L/min (5 cm
3
/sec).
t = total sample time in sec.
Live-Time Counting
Coincidence occurs when more than one particle occupies the
optical sensing region simultaneously. The optical detector cannot
discriminate between the particles and multiple particles are
counted as a single particle. At higher particle concentrations,
particle coincidence begins to significantly impact the measured
concentration.
The CPC corrects for coincidence continuously with the instrument
electronics performing a “live-time” correction. Live-time refers to
the time between electrical pulses. This is the total measurement
time interval minus the time during which the counter is disabled
with one or multiple particles in the optical sensing volume (the
Dead Time). The dead time should not be included in the sample
time since particles can’t be counted during this time interval
except the ones that are already in the viewing volume. The actual
particle concentration therefore equals the number of counted
particles divided by the live time and the aerosol flow rate.
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