B-16 Model 3936 Scanning Mobility Particle Sizer
TM
(SMPS
TM
) Spectrometer
Operating at Higher Concentrations
Although the CPC is often used with low concentrations of aerosol,
it is capable of detecting high particle concentrations. This may be
advantageous in applications such as filter testing. The limiting
factor is coincidence. See below for a discussion of coincidence
correction calculations.
The CPC counts single particles, such that each particle scatters a
separate pulse of light. At high concentrations, two or more
particles are occasionally in the viewing volume at the same time.
The pulses they generate overlap and are counted as one particle.
The frequency of this event depends on the particle concentration.
Within limits, you can determine coincidence.
The coincidence correction is particularly important at high
concentrations. For most of the range, especially in clean air, the
coincidence effect is insignificant.
Model 3782, 3785, 3786 WCPCs, 3772, 3775, 3776, 3022A, and
3025A CPCs have built-in coincidence correction. The Model 3010
CPC, however, uses the Aerosol Instrument Manager
®
Software for
SMPS
™ spectrometer to compute the actual particle concentration
according to the following equation:
paia
QNNN
τ
exp=
Equation B-10
where N
a
= the actual concentration (particles/cm
3
)
N
i
= the indicated concentration (particles/cm
3
)
Q = 16.67 cm
3
/s (CPC sample flow rate)
τ
p
= 0.4 microsecond is the effective time each particle
resides in the viewing volume
Table B-3 shows the calculated coincidence for several
concentrations. Coincidence is 1-N
a/
N
i
.
Table B-3
Coincidence Levels
Concentration Calculated
(particles/cm
3
) Coincidence (%)
10 >.01
100 .07
1000 .67
5000 3.5
10000 7.4
As shown in Table B-3, the Model 3010 CPC is suitable for
monitoring concentrations up to 10,000 particles/cm
3
with very
little coincidence. For concentrations above those in Table B-3,
contact TSI Incorporated for a more suitable particle counter.
To Next Page
Loading...
Need help?
General
