73
Appendix D
Calculating Particle Concentration
Particle concentration is calculated by counting the number of particles
passing through the sensor in a given period of time. Since the flow rate is
known (5.83 cm
3
/sec), the particle concentration can be determined.
In 1-Second Count Mode the equation is:
sec
/
83
.
5
sec)
1
(
ion
Concentrat
Particle
3
1
cm
N
=
where
N
1
is the number of particles counted in a 1-second period
Note that the total flow rate of air through the P
ORTA
C
OUNT
®
P
RO
is a
nominal 16.7 cm
3
/sec. The reason that we use 5.83 cm
3
/sec. in the
calculations above is because the flow path inside the P
ORTA
C
OUNT
®
P
RO
is
divided into two branches, the sensor flow and the bypass flow. The sensor
flow is set at
precisely
5.83 cm
3
/sec and the bypass flow is
approximately
10.8 cm
3
/sec.
Using the equation above, you can see that the minimum measurable
concentration is 0.17 particles per cm
3
in 1-Second Count Mode.
The equations above can also be used to determine the minimum measurable
mask concentration that can be measured in Fit Test Mode given the mask
sample time that is used. For example, with a 40 second mask sample
(factory setting), the minimum measurable concentration is 0.004 particles
per cm
3
.
