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07506C DCN6727 61
CHAPTER 7
Particle Counting
This chapter discusses particle counting and particle count
measurements performed using the Ultrafine Particle Monitor -
Model 651.
The Model 651 has two modes for particle counting:
Concentration mode, where data is presented as particle
concentration in particles/cm
3
, updated each second on the
display (the maximum time resolution is one second).
Totalizer mode, where total particle counts are accumulated and
presented each second.
Concentration mode is commonly used for most applications and
for averaging over a period of time. Totalizer mode is used at very
low particle concentrations and includes live-time corrections.
Particles can be accumulated until a desired statistical accuracy is
achieved.
In the concentration mode, the Model 651 operates in the single
count mode with continuous, live-time correction over the range
between 0 and 1 10
6
particles/cm
3
.
The instrument can display up to 10
6
particles/cm
3
. The Model 651
must be calibrated against a concentration reference (e.g., an
aerosol electrometer or another Ultrafine Particle Monitor with a
dilution bridge with a known dilution ratio) in the range from 3
10
5
to 10
6
particles/cm
3
in order to provide a single dead-time
correction calibration (DTC) factor.
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
r
is related to the total count n by
.
n
n
r