Particle Counting 6-3
Coincidence Correction for Pulse Output
Live-time coincidence correction is not available if you are using the pulse
output from the CPC which only provides raw counts of the particles.
Concentration can be calculated using raw counts and the aerosol flow rate
of the CPC. This concentration is only accurate for low particle
concentrations when coincidence level is low, e.g., in clean air or after a
filter. If the pulse output is used for higher concentration up to 10
4
particles/cm
3
, the following calculation improves the accuracy of the
particle concentration obtained from pulse output:
N
a
= N
i
exp (N
a
Q
p
)
where N
a
= the actual concentration (particles/cm
3
)
N
i
= the indicated concentration (particles/cm
3
)
Q = 16.67 cm
3
/s
p
= 0.35 microsecond is the nominal effective time each
particle resides in the viewing volume
The N
a
in the exponent can be approximated by Ni.
Table 6-1 shows the calculated coincidence for several concentrations.
Coincidence is 1-Na/Ni.
Table 6-1
Coincidence Levels Based on 0.35 µsec Pulse Width
Concentration Calculated Coincidence
(particles/cm
3
) (%)
10 >.01
100 .06
1000 .59
5000 3.05
10000 6.4
Concentrations obtained from pulse width and coincidence-corrected with
the above equation are slightly different from the live-time corrected
concentrations on the front panel display. The former concentration is
corrected based on nominal pulse width but the latter is corrected based on
the actual pulse widths for particles. For concentrations above those in
Table 6-1, contact TSI Incorporated for a more suitable particle counter.
C a u t i o n
At concentrations above 10,000, two exclamation marks appear at the
sides of the concentration reading on the front panel of the 3772. If this
occurs, the number of particles shown on the display could be lower than
the actual concentration.
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