representative BC concentration data, or (2) conducting preliminary
testing with the ObservAir at a flow rate setting of 100 ccm. The
logging interval is dictated by the application context and goals. For
example, long-term ambient monitoring may only require hourly
measurements, while mobile platforms require rapid data logging
every 10 seconds or less. In both cases, these parameters may simply
be estimated to set the sensor flow rate initially, and adjusted
thereafter depending on the results. As an example, we will choose
values of 0.4 ug/m
3
and 1 hour to illustrate each step in this
procedure.
2. Calculate requisite BC measurement resolution: As a rule of thumb,
the baseline noise at the desired logging interval should be < 10% of
the expected BC concentration. So for our example, baseline noise
should be < 0.1*0.4 = 0.04 µg/m
3
on an hourly basis.
3. Calculate the minimum allowable flow rate: From Table 3, find the
BC baseline noise at the requisite timebase (2 sec, 15 sec, 1 min, or
1 hour). Since this noise specification is for a flow rate setting of 100
ccm, the minimum required flow rate can be estimated using the
equation below. For our example, Table 3 shows that the baseline BC
noise at 1 hour is 0.01 µg/m
3
, so the minimum flow rate is
(0.01/0.04)*100 = 25 ccm. Note that the results of this calculation
are bounded by the ObservAir’s minimum and maximum flow rate
settings: 25 and 150 ccm, respectively.
FR
min
= Minimum allowable flow rate (ccm)
Noise
spec
= Baseline specification from Table 2 (µg/m
3
)
Noise
req
= Maximum BC noise calculated in Step 2 (µg/m
3
)
4. Calculate the maximum filter life: Using the average BC concentration
from Step 1 and minimum flow rate setting from Step 3, calculate the
filter life according to the equation provided in Section 5.1. In our
example, the maximum filter life is 510/(25*0.4) = ~51 days.
5. Optimize the flow rate to meet your needs: Given these limiting
values, the flow rate setting can be optimized to meet your needs. For
example, if greater BC measurement resolution is desired at the
expense of filter life, the flow rate can be increased past the minimum
value. Conversely, flow rate may be reduced to achieve the opposite
result.
For reference, Table 7 shows the ObservAir’s minimum flow rate setting
and maximum filter life as a function of average BC concentration and
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