DST ObservAir User manual

ObservAir
Operating Manual
Distributed Sensing Technologies
Released: November 1, 2020
Version: 1.0
© 2020 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 ....................................................................7
1.3. Base package contents .................................................................................8
2. Technical Specifications .........................................................................9
2.1. General specifications ...................................................................................9
2.2. Measurement performance ..................................................................... 10
2.3. Operational limits and warnings ............................................................ 11
3. Operating Instructions ..........................................................................13
3.1. Hardware overview....................................................................................... 13
3.2. Interactive LED button: Sensor display and control...................... 14
3.2.1. Sensor startup .......................................................................................... 15
3.2.2. Default LED mode: Pollutant concentration display ................ 15
3.2.3. Sensor menu Interface ......................................................................... 15
3.2.4. Attenuation reset ..................................................................................... 16
3.2.5. Sensor shutdown..................................................................................... 16
3.2.6. Sensor alarms and errors.................................................................... 16
3.3. Filter tab replacement ................................................................................ 17
3.4. Battery charging ........................................................................................... 18
3.5. Data collection from onboard SD card ............................................... 19
3.5.1. Settings file ................................................................................................ 19
3.5.2. Data file ....................................................................................................... 21
3.6. Computer (serial USB) connection ....................................................... 22
3.6.1. Connecting to Arduino Serial Monitor............................................ 22
3.6.2. Serial data collection ............................................................................. 23
3.6.3. Sensor configuration: Serial commands....................................... 23
3.7. WiFi connection............................................................................................. 25
3.8. Firmware updates ........................................................................................ 26

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ObservAir Operating Manual
3.9. External sample lines.................................................................................. 26
4. Maintenance and Calibration Procedures ..........................................27
4.1. Zero calibration of pollution sensors ................................................... 27
4.2. Span calibration of pollution sensors .................................................. 27
4.3. Flow rate calibration.................................................................................... 28
4.4. Leak check....................................................................................................... 28
5. Best Practices ........................................................................................30
5.1. Filter replacement ........................................................................................ 30
5.2. Filter loading correction............................................................................. 30
5.3. Flow rate setting: Filter life vs. BC resolution ................................... 31
5.4. Operational settings for common applications ............................... 34
5.5. Indoor/Outdoor monitoring guidelines ............................................... 36
5.6. Accurate sample flow rate measurements are critical ................. 36
6. Troubleshooting .....................................................................................37
6.1. LED error codes ............................................................................................ 37
6.2. Unresponsive sensor................................................................................... 37
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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 six gaseous
pollutants: CO, NO2, SO2, O3, H2S, and ethanol.
• 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, LoRa, or SigFox
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 DST’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 a 24-hour 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: Solar panels for extended stationary measurements,
mounting and packaging solutions for mobile platforms,
environmentally controlled enclosures for harsh environments, etc.
The ObservAir is designed to be easily deployed anywhere, and trusted to
deliver accurate air quality measurements reliably and conveniently. If you
have any questions about integrating the ObservAir into your air quality
monitoring efforts, please contact us at [e-mail].

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ObservAir Operating Manual
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, calculates BC
concentrations in real time. Downstream of the photometer, a relative
humidity and temperature sensor records environmental 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 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.
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ObservAir®Operating Manual
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).
𝐴𝑇𝑁=100×ln*𝐼!"#
𝐼$%& ,---------(1)-
Iref = Light intensity through reference filter (ADC count)
Isig = Light intensity through signal filter (ADC count)
In this way, the optical ATN through the photometer is monitored in real
time (note that ATN is unitless) and BC concentrations are calculated using
the fundamental equation:
𝐵𝐶(𝑡%)=- 𝐴
𝑀𝐴𝐶∙𝑄(𝑡%)∙Δ𝐴𝑇𝑁
Δ𝑡 =𝐴 𝑀𝐴𝐶∙𝑄(𝑡)∙𝐴𝑇𝑁(𝑡%)−𝐴𝑇𝑁(𝑡%'()
𝑡%−𝑡%'( ---------(2)
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 = 12.5x10-6 m2/µg
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
Photodiode
(Signal)
Photodiode
(Reference)
LED (880 nm)
Inlet
Exhaust
(to pump etc.)
Glass
Glass
BC
deposit Aerosol
filter
Filter Support
Material (tab)
:Sample flow :Filtered flow

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ObservAir Operating Manual
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 12.5 m2/g, but this value may be
adjusted following cross-calibration with a reference instrument. For the
factors specific to the ObservAir, Equation (2) simplifies to:
𝐵𝐶(𝑡%)=- 𝐾
𝑄(𝑡%)∙Δ𝐴𝑇𝑁
Δ𝑡 ---------(3)
K = ObservAir constant = 339,292 µg · ccm · sec/m3
Q(ti)= Flow rate at time ti(ccm)
Note: In Equation (3) above, the flow rate (Q) is input in units of cubic
centimeters per minute (ccm), as reported by the ObservAir.
1.1.2. Electrochemical cells (Gaseous pollutants)
The ObservAir can be optionally outfitted with electrochemical cells to
measure gaseous pollutants. Six electrochemical cells are available, to
monitor concentrations of carbon monoxide (CO), nitrogen dioxide (NO2),
ozone (O3), sulfur dioxide (SO2), hydrogen sulfide (H2S), and ethanol. The
ObservAir can be configured to measure up to two (2) of these six species.
Electrochemical cells contain a chemical reagent that creates a
small electrical current when exposed to the gaseous analyte of interest
(e.g., CO). This electrical current is amplified and converted to a voltage
signal (the ‘gas voltage’) for digital acquisition. Each electrochemical cell
also outputs a reference voltage to compensate for drift and environmental
sensitivity. Using these two voltage signals, the gas concentration is
calculated as follows:
𝐶&)$(𝑡)=𝑉&)$(𝑡)−𝑉!"#(𝑡)−𝑉*"!+
𝐶𝑜𝑑𝑒∙𝐺𝑎𝑖𝑛∙10', ---------(4)
Cgas(t) = Gas concentration at time ‘t’ (ppm)
Vgas(t) = Gas voltage at time ‘t’ (V)
Vref(t) = Reference voltage at time ‘t’ (V)
Vzero = Offset voltage differential = Vgas – Vref when Cgas = 0 ppm (V)
Code = Calibration code (nA/ppm). See Section 3.5.1.
Gain = Voltage gain (kV/nA). See Table 1.
An offset voltage differential (Vzero)value is determined for each ObservAir
during DST’s zero-calibration procedure, but this value should also be
measured and logged prior to your monitoring application. Operate the
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ObservAir®Operating Manual
ObservAir for 12 to 24 hours in an environment that is free of the analyte
gas(es) and average the voltage differential data collected over the entire
zero calibration period. Code is the factory calibration factor and is specific
to each individual cell. The code for each cell is automatically logged in the
SD card’s Settings file, as outlined in Section 3.5.1.
Gas
Gain (kV/nA)
Carbon Monoxide (CO)
100
Hydrogen Sulfide (H2S)
49.9
Nitrogen Dioxide (NO2)
499
Sulfur Dioxide (SO2)
100
Ozone (O3)
299
Ethanol
249
Table 1. Gain settings for each type of electrochemical cell
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 electronic components results in erroneous
or inaccurate BC measurements during rapid environmental changes, such
as may be expected diurnally when the sensor is deployed outdoors. The
ObservAir incorporates proprietary hardware and software features to
minimize the sensor’s environmental dependence.
Hardware compensation features include the aerosol absorption
photometer’s active reference filter. Since the sampled air flow is actively
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). Similarly, the electrochemical cells’ reference voltage outputs are
logged independently and used to compensate gaseous concentration
measurements. The sensor is also outfitted with temperature control
hardware and other proprietary design elements that preserve
measurement accuracy in harsh environments.
While hardware features contribute significantly to correcting the
ObservAir’s environmental dependence, some sensitivity remains that must
be corrected by software. This software compensation centers on DST’s

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ObservAir Operating Manual
proprietary environmental training approach. Prior to delivery, all ObservAir
units sample clean (‘zero’) 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
unit is modeled mathematically. The models are uploaded to each unit, and
used to correct air pollution concentration measurements in real-time. Each
ObservAir is delivered with its own unique zero-calibration sheet, as shown
in Figure 3 below. The calibration sheets show the sensor’s baseline
environmental dependence and 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 sensor performance.
Figure 3. Each ObservAir comes with its own zero-calibration sheet
1.3. Base package contents
Each ObservAir comes with the following base set of accessories and
supplies:
• Magnetic cover
• 10 replacement filter tabs
• Micro-SD card with 16 GB capacity
• Charger (US Plug) and 3-foot (1 m) micro-USB cable
• 3-foot (1 m) length of conductive sample line
• Zero-calibration sheet (hardcopy)
• Quick-start guide (hardcopy)
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ObservAir®Operating Manual
2. Technical Specifications
The ObservAir’s technical specifications are summarized in the tables
below. All electrochemical cells are sourced from Spec Sensors. Gas
measurement performance specifications are adapted from data provided
by the manufacturer, and where possible, DST validation of the
electrochemical cells in the ObservAir platform. All baseline measurement
noise specifications are derived from data collected while the ObservAir is
sampling clean (‘zero’) air. More information on the electrochemical cells
can be found at www.spec-sensors.com.
All measurement performance specifications are derived from ObservAir
data collected at a sample flow rate of 100 ccm near standard
atmospheric conditions: Temperature and relative humidity (RH) ranging
from 15 to 30ºC and 25 to 40%, respectively.
2.1. General specifications
Air pollution
measurement species
Standard: Black carbon (BC) aerosol
Optional: CO, NO2, SO2, H2S, O3, ethanol (up to 2)
Principle of operation
Black carbon: Filter-based light absorption (880 nm)
Gases: Electrochemical cells
Communications
Standard: Wi-Fi, Bluetooth, USB
Optional: LTE, LoRa, SigFox (choose one)
Sample air flow rate
50 to 200 ccm
Sample interval
2 to 60 seconds
Power consumption
1.2 W (at 100 ccm flow rate)
Battery life
≥24 hours (at 100 ccm flow rate)
Filter life
(BCavg = 1μg/m3)
Flow rate (ccm)
50
125
200
Filter life (days)
6.3
2.5
1.6
Data storage
Removable SD card (16Gb card provided)
Operating conditions
Temperature: 5 to 40ºC; RH: 15 to 80%
Dimensions/Weight
120 x 80 x 45 mm / 600 grams
Charging
5V DC at 2.1A max (microUSB and charger provided)
Table 2. ObservAir general specifications