It is best practice to periodically check the integrity of the closed flow path
using the instructions below. If the instrument leaks, BC measurements
will be compromised, but gas sensor data should remain largely
unaffected.
1. Replace the filter while the ObservAir is operating.
2. Plug the ObservAir’s inlet port. A simple plug may be fashioned by
tying a knot in a length of sample line (soft rubber tubing).
3. Monitor the flow rate measurement using the Cloud Dashboard or
serial monitor.
4. After ~1 minute, the reported flow rate should be less than 3 ccm.
This indicates that the closed flow path is well sealed, as no air can be
sampled. If the sensor meets this criterium, it has passed the leak
check and can be deployed.
5. If the sensor fails the leak check, first tighten the thumbscrew by 1 full
turn and repeat steps 3 and 4.
6. If the sensor still fails, replace the filter and repeat steps 5 to 7.
7. If the sensor fails after these attempts, contact DSTech Technical
support at info@dstech.io.
5.3. Flow rate setting: Filter life vs. BC resolution
The filter’s operational life decreases at higher flow rates, so it may be
tempting to set flow rate at the lowest possible option (25 ccm) and
minimize sensor maintenance. However, effective BC measurement
resolution also depends on flow rate: Since the filter loads up with BC
more rapidly at higher flows, the time rate of light attenuation is more
readily detectable, and BC baseline noise decreases. BC baseline noise
represents the sensor’s effective measurement resolution and is shown for
various sampling intervals in Table 3 for a flow rate of 100 ccm.
BC baseline noise is inversely proportional to the sample flow rate.
For example, 1-minute BC noise is around 0.025 µg/m
3
at 200 ccm (half
that shown in Table 3 for 100 ccm). In this way, higher flow rates provide
measurements with higher temporal resolution, but at the expense of filter
life. The optimum flow rate setting maintains adequate BC measurement
resolution for the monitoring application while maximizing filter life such
that sensor maintenance remains convenient and practical. The procedure
below outlines the calculation and selection of an appropriate ObservAir
flow rate setting.
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
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