32
ObservAir
Operating Manual
1.
Determine BC
avg
and logging interval
: Both of these parameters
depend on the sensor application and must be determined by the user.
The average BC concentration (BC
avg
) during the campaign can be
estimated by (1) searching the literature or regulatory databases for
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
µ
g/m
3
on an hourly basis.
3.
Calculate the minimum allowable flow rate:
From Table 4, 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 4 shows that the baseline BC noise at 1
hour is 0.01
µ
g/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 200
ccm respectively.
𝐹𝑅
.%/
=
𝑁𝑜𝑖𝑠𝑒
$0"1
𝑁𝑜𝑖𝑠𝑒
!"2
× 100ccm (6)
FR
min
= Minimum allowable flow rate (ccm)
Noise
spec
= Baseline specification from Table 3 (
µ
g/m
3
)
Noise
req
= Maximum BC noise calculated in Step 2 (
µ
g/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 4.1. In our
example, the maximum filter life is 313.5/(25*0.4) = ~31 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.
33
ObservAir
®
Operating Manual
For reference, Table 7 shows the ObservAir’s minimum flow rate
setting and maximum filter life as a function of average BC concentration
and data logging period. The values in the table are calculated according
the procedure presented above. Two illustrative scenarios are also provided.
Data logging period
2 sec
15 sec
1 min
1 hour
Av
g
.
BC
0.5
µ
g/m
3
N/A
200/3.1
100/6.3
25/25.0
1
µ
g/m
3
200*/1.6ª
100/3.1
50/6.3
25/12.5
5
µ
g/m
3
60/1.0
25/2.5
25/2.5
25/2.5
10
µ
g/m
3
30/1.0
25/1.3
25/1.3
25/1.3
Minimum Flow Rate (ccm)*/ Maximum filter life (days)ª
Table 8.
ObservAir’s minimum flow rate setting and maximum filter life as
a function of average BC concentration and data logging period. The
ObservAir cannot provide BC concentration data with a baseline noise <
0.05
µ
g/m
3
on 2-second basis, so the top left cell is empty.
•
Scenario 1:
BC
avg
= 7 ug/m
3
, Logging interval = 2 seconds
For this scenario, the BC baseline noise should be
≤
0.7
µ
g/m
3
at 2-second
logging. Using the calculation procedure above, the minimum flow rate
setting is (0.3/0.7)*100 = 43 ccm. For these settings, the maximum filter
life is ~1 day. Alternatively, refer to Table 8 and see that for 2-second
measurements of 5
µ
g/m
3
average concentrations, the flow rate setting is
60 ccm. By interpolation, the flow rate can be set to 60*7/5 = 43 ccm.
•
Scenario 2:
BC
avg
= 0.35 ug/m
3
, Logging interval = 1 minute
The BC baseline noise should be from
≤
0.035
µ
g/m
3
at 1-minute logging.
From Table 1, the noise is 0.05 ug/m
3
at 1-minute and 100 ccm, so the
minimum flow rate is (0.035/0.05)*100 = 70 ccm. In Table 8, a flow rate
of 100 ccm is recommended for 1-minute measurements at 5
µ
g/m
3
. By
interpolation, the flow rate can be set to 100*0.35/0.5 = 70 ccm. The filter
life can be calculated as 313.5/(0.35*70) = 12.8 days.
IMPORTANT NOTE:
The above considerations do NOT apply to gas
sensing. The gas sensors’ measurement performance remains largely
constant with flow rate. Only the gas sensors’ time response is affected by
flow rate, simply because of the residence time of sample air through the
sensor’s closed volume naturally varies. At higher flow rates, the residence
time diminishes and the gas cells respond more rapidly.