Particle Counting 6-3
where C
a
= actual or true concentration in particles/cm
3
C
i
= indicated or measured concentration in
particles/cm
3
Q = flowrate at 5.0 cm
3
/s
t = time the counter is disabled for a single particle
(approximately 4 µs in the CPC)
QtC
a
= the average number of particles that occur during
the disabled period t.
Since C
a
cannot be solved directly and is a double-valued function
of C
i
, the equation can be closely approximated by using C
i
in place
of C
a
in the exponent.
For low concentrations—where the average time between the arrival
of particles is much larger than the disable time (QtC
a
< 0.01)—
coincidence is low and no correction of the measured concentration
is necessary. At higher concentrations, correction of coincidence is
required and the internal microprocessor does not automatically
correct for coincidence.
The effect of coincidence in the CPC up to 1000 particles/cm
3
is
small, as can be seen in Table 6-1.
Table 6-1
Coincidence Correction for Concentrations up to 1000 Particles/cm
3
Indicated
Concentration
Correction Factor Correct
Concentration
(particles/cm
3
)
exp (QtC
i
)
(particles/cm
3
)
9.99e2 1.020 1.02e3
9.00e2 1.018 9.16e2
5.00e2 1.010 5.05e2
1.00e2 1.002 1.00e2
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 (total mode) in each interval is
very small, the uncertainty in the count is large. The statistical
error of the count
σ
r
is related to the total count n by
σ
r
n= .
To Next Page
Need help?
General
