should be checked for leaks (see Section 5.2). If the problem persists after
a successful leak check, contact DSTech for assistance. The ObservAir
should provide a robust zero reading during calibration. Zero-calibration
is straightforward and can be carried out regularly to verify the sensor’s
proper operation.
In order to span-calibrate the BC data, the ObservAir must be
collocated with a reference instrument. If calibrating relative to another
absorption photometer, ensure that the reference instrument is operating
at a wavelength of 880 nm, like the ObservAir. Derive a linear regression
between the ObservAir and reference BC data, as shown in Equation 6,
and note the slope. Generally, this calibration should be done using hourly-
average data collected over at least 1 week of collocated sampling.
BC
ref
(t) = Reference BC measurement at time ‘t’ (µg/m
3
)
BC
OA
(t)
= ObservAir BC measurement at time ‘t’ (µg/m
3
)
m = Linear regression slope
b = Linear regression intercept (µg/m
3
)
Using the slope value, adjust the ObservAir’s MAC factor to reconcile the
two datasets, as shown in Equation 7. For example, if the ObservAir’s BC
measurements are consistently 10% lower than the reference (slope of
1.1), the MAC should be decreased by 10% to calibrate the ObservAir data
relative to the reference.
MAC
cal
= Calibrated MAC (m
2
/g)
MAC
default
= Default MAC (m
2
/g) = 7.8 m
2
/g from factory
Note: BC measurements are also proportional to flow rate, so it is crucial
that the ObservAir’s flow rate sensor be calibrated prior to any BC span
calibrations (see Section 4.3).
4.2. Calibration: Gaseous pollutants
The most accurate and practical method for gas sensor calibration is
collocation with a reference instrument in the intended deployment
setting. As discussed in Section 1.2.2, the electrochemical cells’
operation varies significantly depending on environmental conditions, so
To Next Page
Loading...