Product Overview 1-5
Concentration Calculations
The UWCPC can report particle concentration values in three
different ways. These are: the primary concentration display, the
totalizer display, and the concentration values reported using the
data communications ports.
In operation, the UWCPC internally measures two parameters
related to the particle concentration. These are the number of
particle pulses counted and the sample time. Another important
parameter relating to concentration is the aerosol flow rate which is
always assumed to be 0.3 L/min. The flow rate is actually the
difference between the total or inlet flow rate which is 0.6 L/min
and the sheath flow rate of 0.3 L/min. The basic calculation for the
number of particles per volume of air is:
QT
N
C
P
P
×
=
where:
C
P
is the particle concentration in particles/cm
3
N
P
is the number of particles counted
T is the sample time
Q is the aerosol flow rate in cm
3
/second
The Totalizer uses the elapse time for the sample time in the above
calculation to provide a useful way to acquire low particle
concentration information on the display. The number of particles
in the measured sample is one of the limiting factors of how low a
particle concentration can be precisely determined. The formula for
this statistical precision is:
%100×=
N
N
σ
where: σ
N
is the relative standard deviation in percent
N is the number of particle counts in the sample
For a sample of 10,000 particles the statistical precision is 1%
which is much better than the accuracy of the instrument. At 100
particles the statistical uncertainty increases to 10% and becomes
a significant factor in determining the aerosol concentration. The
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