DST ObservAir User manual

ObservAir
Operating Manual
Distributed Sensing Technologies
Released: April 2023
Version: 2.2
© 2022 Distributed Sensing Technologies,
LLC

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ObservAir®Operating Manual
Table of Contents
1. Introduction ...............................................................................3
1.1. Principle of operation ..........................................................4
1.1.1. Aerosol absorption photometer (Black carbon) ..................4
1.1.2. Electrochemical cells (Gaseous pollutants) ........................6
1.2. Environmental compensation ...............................................8
1.2.1. Black carbon (BC) compensation......................................9
1.2.2. Gas compensation .........................................................10
1.3. Base package contents ......................................................10
2. Technical Specifications........................................................... 11
2.1. General specifications........................................................11
2.2. Measurement performance ................................................12
2.3. Operational limits and warnings .........................................13
2.4. Data processing overview...................................................14
3. Operating Instructions..............................................................15
3.1. Hardware overview ............................................................15
3.2. Interactive LED button: Sensor display and control ..............16
3.2.1. Sensor startup ..............................................................17
3.2.2. Default LED mode: Pollutant concentration display ..........17
3.2.3. Sensor menu Interface ...................................................17
3.2.4. Sensor shutdown (Power off) ..........................................18
3.2.5. Toggle Cloud Dashboard connection ...............................18
3.2.6. Provision Cloud Dashboard service .................................18
3.2.7. Sensor alarms and errors...............................................18
3.3. Filter tab replacement .......................................................19
3.3.1. Attenuation reset ...........................................................21
3.4. Battery charging ...............................................................21
3.4.1. Charging Outside of Enclosure........................................ 21
3.4.2. Charging Inside Enclosure..............................................21
3.5. Active Ventilation Enclosure ...............................................22
3.5.1. Enclosure Overview........................................................ 22

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ObservAir Operating Manual
3.5.2. Enclosure Mounting .......................................................23
3.5.3. Solar Power Kit .............................................................25
3.5.4. Mounting Panels to Bracket............................................26
3.6. Data collection from onboard SD card ................................27
3.6.1. Settings file...................................................................27
3.6.2. Data file........................................................................31
3.7. Cloud Dashboard Interface (WiFi or LTE).............................31
3.8. Computer (serial USB) connection......................................32
3.8.1. Connecting to Arduino Serial Monitor..............................32
3.8.2. Serial data collection .....................................................33
3.8.3. Sensor configuration: Serial commands ..........................34
3.9. WiFi connection.................................................................35
3.10. Firmware Updates.............................................................36
3.11. External sample lines ........................................................36
4. Data Calibration and Correction Procedures..............................36
4.1. Calibration: Black carbon...................................................36
4.2. Calibration: Gaseous pollutants ..........................................37
4.3. Flow rate calibration..........................................................39
4.4. Black carbon correction: Filter loading................................39
5. Best Practices .........................................................................41
5.1. Filter replacement .............................................................41
5.2. Leak check .......................................................................41
5.3. Flow rate setting: Filter life vs. BC resolution .......................42
5.4. Operational settings for common applications.....................44
5.5. Indoor/Outdoor monitoring guidelines ................................46
5.6. Accurate sample flow rate measurements are critical...........47
6. Troubleshooting.......................................................................47
6.1. LED error codes ................................................................47
6.2. Unresponsive sensor .........................................................48

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ObservAir®Operating Manual
1. Introduction
The ObservAir is an air quality sensing platform that provides accurate
pollutant concentration measurements in real time. Key features include:
•Modular: The ObservAir is centered around a black carbon (BC)
sensor, and can optionally monitor up to two of the following seven
gaseous pollutants: CO, NO, NO2, SO2, O3, H2S, and VOC.
•Portable: The lightweight (600g) and compact (120x80x45 mm)
ObservAir is easily deployed in both stationary and mobile monitoring
applications.
•Connected: All ObservAir units support WiFi and USB communication
protocols, and include a 16GB removable SD card for onboard data
storage. Units may also be supplemented with an LTE communication
module and a GPS unit for location logging. An integrated mobile app
and optional data backend services enable real-time air quality
monitoring, sensor diagnostics, and data collection.
•Accuracy anywhere: Using DSTech’s proprietary environmental
compensation algorithms, each ObservAir is individually ‘trained’ to
maintain measurement accuracy even in harsh operating
environments (e.g. outdoors) where existing air quality instruments
typically suffer.
•Network-ready: With up to 24 hours of battery life, flexible wireless
communication options, and environmental compensation, the
ObservAir is ready for networked deployments at a moment’s notice.
•Flexible: Accessories are available to enable a wide range of
monitoring applications. Environmentally controlled outdoor
enclosures and solar panels are available for extended stationary
measurements in harsh conditions. Optional sensors can be outfitted
to measure particulate matter pollution, ambient meteorological
conditions, and other environmental factors. Custom mounting and
packaging solutions for mobile and airborne platforms are also
available on request.
The ObservAir is designed to be easily deployed anywhere, and trusted to
deliver accurate air quality measurements reliably and conveniently. If you

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ObservAir Operating Manual
have any questions about integrating the ObservAir into your air quality
monitoring efforts, please contact us at [email protected]o.
1.1. Principle of operation
Figure 1. Functional diagram of the ObservAir
The ObservAir is centered around an aerosol absorption photometer
configured to measure concentrations of black carbon (BC). BC is a type
of particulate matter (PM) pollution generated by the incomplete
combustion of fossil fuels or biomass. For most purposes, BC is
functionally defined as the light-absorbing component of PM pollution. The
ObservAir's micropump first draws air into the inlet and through a fibrous
aerosol filter. that is mounted on black supporting material (the
disposable filter tab). As light absorbing PM collects on the fibrous filter,
BC concentrations are calculated in real time. Downstream of the
photometer, a relative humidity and temperature sensor records
environmental sampling conditions, and optional electrochemical cells
measure up to two gaseous pollutants. Air then passes through the flow
rate sensor and is exhausted by the pump.
1.1.1. Aerosol absorption photometer (Black carbon)
A schematic of the ObservAir's aerosol absorption photometer is provided
below. Photodiodes continuously monitor the intensity of 880 nm light
transmitted from an LED source through two aerosol filters. As polluted
air is drawn through the photometer, light absorbing BC accumulates on
the first ‘signal’ filter and the transmitted light intensity attenuates
predictably over time. The filter collection area is 3 mm in diameter. After
the first filter, the air flow passes through a second ‘reference’ filter

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ObservAir®Operating Manual
assembly that is identical to the first. Since the air is filtered (devoid of
PM), the intensity of light transmitted through the reference filter is
unaffected by BC concentrations. By comparing the reference light
intensity to that measured at the signal filter, it is possible to isolate the
light attenuation resulting from BC absorption alone, while largely
eliminating other factors.
Figure 2. Schematic of the aerosol absorption photometer
Optical attenuation (ATN) is defined in terms of the two light intensity
measurements, as shown below. In the ObservAir, both measurements are
reported as the bit count from the photodiodes’ Analog to Digital
Converter (ADC), ranging from 0 to 8388607 (full scale 23-bit output).
󰇧
󰇨󰇛󰇜
Iref = Light intensity through reference filter (ADC count)
Isig = Light intensity through signal filter (ADC count)
Using the ATN measurements, BC concentrations are calculated using the
fundamental equation:
󰇛󰇜 
󰇛󰇜   󰇛󰇜󰇛󰇜󰇛󰇜  󰇛󰇜
BC(ti)= Black carbon at time ti(µg/m3)
A = Filter collection area (D = 3mm) = 7.07x10-7 m2
MAC = Mass absorption coefficient of BC at 880 nm = 7.8x10-6 m2/µg

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ObservAir Operating Manual
Q(ti)= Flow rate at time ti(m3/sec)
∆ATN = Difference of two ATN measurements = ATN(ti)- ATN(ti-1)
∆t = Measurement interval (seconds) = ti- ti-1
The Mass Absorption Coefficient (MAC) is a calibration factor that relates
the time differential of ATN to BC concentrations in the flow. The MAC
varies depending on the air pollution source, PM composition and other
factors. By default, the ObservAir uses a MAC of 7.8 m2/g, but this value
may be adjusted following cross-calibration with a reference instrument.
For the default factors specific to the ObservAir, Equation (2) simplifies to:
󰇛󰇜 
󰇛󰇜  󰇛󰇜
K = ObservAir default constant = 5438461.5 µg ∙ccm ∙sec/m3
Q(ti)= Flow rate at time ti(ccm)
Note: Flow rate (Q) is input in units of cubic centimeters per minute (ccm).
1.1.2. Electrochemical cells (Gaseous pollutants)
The ObservAir uses interchangeable electrochemical cells to monitor up
to two toxic gases: Carbon monoxide (CO), nitric oxide (NO), nitrogen
dioxide (NO2), ozone (O3), sulfur dioxide (SO2), hydrogen sulfide (H2S), and
volatile organic compounds (VOC). O3 cells must always be paired with
NO2.
Air diffuses through a membrane and comes into a contact with
the cells working electrode, as shown in Figure 3. The analyte gas oxidizes
or reduces the working electrode, and generates a small electrical current
that is proportional to the analyte gas concentration in the air sample. The
reference electrode does not contact air and generates a baseline current.
By comparing the electrical currents generated by the working and
reference electrodes, gas concentrations are logged in real time. 

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ObservAir®Operating Manual
Figure 3. Schematic of electrochemical cell for monitoring gaseous
pollutant concentrations
The electrical currents from the working and reference electrodes
are amplified and converted to a voltage signal for digital acquisition.
Using these two voltage signals, gas concentrations are calculated as
follows:
󰇛󰇜󰇧󰇛󰇜󰇛󰇜
 󰇨󰇛󰇜
Cgas(t) = Gas concentration at time ‘t’ (ppm)
Vworking(t) = Voltage from working electrode at time ‘t’ (V)
Vreference(t) = Voltage from reference electrode at time ‘t’ (V)
Code = Calibration code (nA/ppm).
Gain = Voltage gain (V/A)
span = Span calibration factor
zero = Zero calibration factor (ppm)
Code is the factory calibration factor, specific to each individual
cell. The code for each cell is logged on the SD card’s Settings file, as
outlined in Section 3.6.1. Gain is gas specific and constant across cells –
it is set in the gas sensing circuitry. Gain is 8x105V/A for all gas species
except for NO2and O3, where it is equal to 7.3 x105V/A. The span and
zero calibration factors (span and zero, respectively) are determined
experimentally for each cell, as outlined in Section 4. By default, the span
and zero are set to 1 and 0, respectively, and are stored to the SD card.
The NO2and O3gas cells are identical, except that the NO2cell is
outfitted with a chemical filter to remove O3from the sample. The O3 gas
cell has no filter and outputs a signal that is proportional to the
concentration of both O3 AND NO2 in the sample. As a result, each O3cell

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ObservAir Operating Manual
comes with two factory calibration codes, representing the cell’s response
to O3 and NO2 individually. The O3cell must be paired with an NO2cell,
and concentrations are calculated as follows:
󰇛󰇜󰇧󰇛󰇜󰇛󰇜󰇛󰇜
  󰇨󰇛󰇜
CO3(t) = Ozone concentration at time ‘t’ (ppm)
CNO2(t) = Nitrogen dioxide concentration at time ‘t’ (ppm) – from other cell
CodeO3 = Ozone cell’s ozone calibration code (nA/ppm)
CodeNO2 = Ozone cell’s nitrogen dioxide calibration code (nA/ppm)
Gain = 7.3 x105V/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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ObservAir®Operating Manual
conditions. The second ‘reference’ electrode is not exposed to air, and is
therefore only sensitive to the cell’s operating conditions. By comparing
these two signals, the response from gas analyte concentrations can be
isolated.
1.2.1. Black carbon (BC) compensation
While hardware features contribute significantly to correcting the
ObservAir’s environmental dependence, some BC measurement artifacts
remain that must be corrected by software. This software compensation
centers on DSTech’s proprietary environmental training approach. Prior
to delivery, all ObservAir units sample filtered air for at least 24 hours
while being subjected to fluctuating environmental conditions. Using the
data collected during this training period, the unique environmental
dependence of each ObservAir’s absorption photometer is modeled
mathematically. The models are uploaded to each unit, and used to
correct BC concentration measurements in real-time. Each ObservAir is
delivered with its own unique zero-calibration sheet, as shown in Figure 4
below. The calibration sheets show the sensor’s baseline environmental
dependence and black carbon measurement performance both before and
after compensation. The sensor also carries out regular calibration and
diagnostic checks of the underlying electronics, and includes other
software features to maintain and validate photometer performance.
Figure 4. Each ObservAir comes with its own zero-calibration sheet
Details regarding the onboard BC compensation algorithm and it’s
derivation can found in the publication below. Please note that this
publication describes the operation of the Aerosol Black Carbon Detector
(ABCD), an academic prototype of the ObservAir. The underlying
compensation methods and principles remain the same as that shown in
the publication, although experimental procedures and data processing
have been significantly improved in the ObservAir.