comes with two factory calibration codes, representing the cell’s response
to O
3
and NO
2
individually. The O
3
cell must be paired with an NO
2
cell,
and concentrations are calculated as follows:
C
O3
(t) = Ozone concentration at time ‘t’ (ppm)
C
NO2
(t) = Nitrogen dioxide concentration at time ‘t’ (ppm) – from other cell
Code
O3
= Ozone cell’s ozone calibration code (nA/ppm)
Code
NO2
= Ozone cell’s nitrogen dioxide calibration code (nA/ppm)
Gain = 7.3 x10
5
V/A
1.2. Environmental compensation
All air quality instruments are susceptible to environmental fluctuations.
For example, the temperature sensitivity of the aerosol absorption
photometer’s LEDs, photodiodes, and other electronics results in
erroneous or inaccurate BC measurements during rapid environmental
changes, such as may be expected diurnally when the sensor is deployed
outdoors. As a result, traditional instruments must be housed in dedicated
stations maintained at stable operating conditions, which is costly and
cumbersome. In order to overcome these limitations, the ObservAir
incorporates proprietary hardware and software features to minimize the
sensor’s environmental dependence. These unique features enable the
ObservAir to accurately and reliably monitor air pollution concentrations
over extended outdoor deployments, as is needed for practical
networked applications.
Hardware compensation features include the aerosol absorption
photometer’s active reference filter (see Section 1.1.1). Since clean,
particle-free air is drawn through the reference filter, the transmitted light
intensity is largely dependent on the flow’s temperature and humidity
content. By passing the same air through both filters and monitoring each
intensity measurement independently, the ObservAir corrects for the
photometer’s environmental sensitivity and other measurement artifacts
(e.g., water absorption in the filter). The sensor is also outfitted with
temperature control hardware and other proprietary design elements that
preserve measurement accuracy in harsh environments.
Similarly, the gas sensing cells are outfitted with hardware
features to reduce environmental sensitivity. Each cell outputs two signals,
each generated by a separate electrode. The first ‘working’ electrode
exposed to air, and is sensitive to both the analyte gas and operating
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