Dashboard please contact DSTech for further instructions – both the
reference and ObservAir data must be sent manually, and the derived
compensation factors must be uploaded to each sensor over serial
connection.
4.3. Flow rate calibration
The flow rate sensor is calibrated at DSTech prior to shipment, but sensor
output may drift over long periods (weeks or months of operation) and
should be re-calibrated periodically using the instructions below:
1. Turn on the ObservAir and allow it to warm-up for at least 15 minutes.
2. Connect ObservAir to a serial monitor (Section 3.8).
3. Set the ObservAir to the desired flow rate setting using the appropriate
serial command.
4. Connect a primary flow calibrator, such as a Gilian Gilibrator, to the
ObservAir’s inlet such that the sensor’s intake flow is measured.
5. Collect at least 5 flow rate measurements using the calibrator. The
flow rate measurements should not vary by more than ±1 ccm. Take
the average of all calibrator measurements: this is the reference value
(FR
ref
).
6. From the serial output or Cloud Dashboard, collect at least 5 flow rate
measurements from the ObservAir and take the average: this is the
measured value (FR
m
).
7. Calculate the calibration factor: FRcal = FR
ref
/FR
m
8. Update the sensor’s FRcal factor using the serial command or Cloud
Dashboard.
9. Repeat Steps 5 and 6 until the reference and measured values agree
within 2 ccm.
Note: If a primary flow calibrator is not available, another flow rate sensor
may be used, but clearly the calibration is only as good as the reference
measurements collected.
4.4. Black carbon correction: Filter loading
As particulate matter deposits on the filter, the ObservAir uses a simple
mathematical relationship to calculate BC concentrations in the sample
flow as a function of the light attenuation rate through the filter (Section
1.1.1). As the aerosol filter becomes overly saturated with BC deposits,
however, this mathematical relationship degrades and the ObservAir’s BC
concentration measurements are underreported (lower than the true
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