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.

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ObservAir Operating Manual
BC compensation publication: Caubel, J.; Cados, T.; Kirchstetter, T. "A
New Black Carbon Sensor for Dense Air Quality Monitoring Networks".
Sensors 18.3 (2018): 738-745.
1.2.2. Gas compensation
The electrochemical cells can also be compensated for environmental
sensitivity, but this is not done at the DSTech factory (like black carbon).
The electrochemical cells’ sensitivity varies depending on operating
conditions, and so compensation factors derived in the lab tend to drift
significantly when the sensor is deployed to the field. As a result, it is best
to derive compensation factors by collocating the ObservAir with a
reference instrument in the intended deployment location. This calibration
should be conducted after the sensors have been allowed to acclimate to
operating conditions for at least a few days. Gas concentration data from
the ObservAir and reference instrument is sent to DSTech, and
compensation factors are derived and uploaded remotely by our staff
using the online portal. The environmental sensitivity of each gas cell is
compensated using a multivariate linear model, derived with machine
learning. Please contact us at [email protected] for more information on this
procedure.
1.3. Base package contents
Each ObservAir comes with the following base set of accessories and
supplies:
•10 replacement filter tabs
•Micro-SD card with 16 GB capacity
•Charger (US Plug) and 3-foot (1 m) micro-USB cable
•1-foot length of conductive sample line
•Zero-calibration sheet
•Quick-start guide

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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 Alphasense. Gas
measurement performance specifications are adapted from data provided
by the manufacturer, and where possible, DSTech 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.alphasense.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
Table 1. ObservAir general specifications

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ObservAir Operating Manual
2.2. Measurement performance
Table 2. Measurement performance specifications

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ObservAir®Operating Manual
Table 3. Baseline measurement noise at various data logging intervals
2.3. Operational limits and warnings
•Environmental limits: Only operate the ObservAir within the
conditions listed in Table 1: Temperature and relative humidity must
remain between 5 to 40ºC and 15 to 80%, respectively. Exceeding
these limits may damage the sensor.
•Moisture/rain: The ObservAir must not be directly exposed to rain or
moisture of any kind without a DSTech enclosure. When fitted with
external sampling lines, these must be fitted with rain covers or water
catches to ensure that water is not aspirated into the sensor. Exposure
or aspiration of moisture may permanently damage the sensor.
•Direct sunlight: Do not operate the ObservAir in direct sunlight for
extended periods without a DSTech enclosure. Even with a DSTech
enclosure, direct sunlight may compromise data integrity. Exposing
the bare sensor to direct sunlight may result in the sensor overheating,
and/or erroneous data generation.
•Filter replacement: Filter tab must be changed regularly for best BC
measurement performance. Failure to replace the filter over extended
periods may result in damage to the pump if the filter becomes
completely saturated.

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ObservAir Operating Manual
•Gas cells: Electrochemical cells must be calibrated regularly and
replaced every 2 to 5 years for best performance. Please contact
DSTech for information on our calibration services.
•Do not pressurize the sensor: Never connect the ObservAir to a
compressed gas cylinder (even when fitted with a regulator), air
compressor, or other pressure source without a purge valve that is
open to the atmosphere. Positive pressure air in the sensor can
damage sensing elements and other hardware.
•Charger: Only charge the sensor with the included charger or
equivalent 2.1A rated USB charger. The included charger is rated for
operation at both 110V/60Hz and 220V/50 Hz –simply adapt to local
plug style if needed. Do not use chargers rated above 2.1A or below
2.0 A. The ObservAir can also be charged from computer ports that
are rated to at least 500mA.
•Accessories: Do not use third party accessories with the ObservAir,
such as external battery packs, solar panels, etc. without first
consulting DSTech Technical Support.
•Replacement parts: Only use replacement parts provided by DSTech
or an authorized DSTech distributor.
•Disassembly: Never disassemble the unit, this voids the warranty and
may result in permanent damage to the unit and/or harm to the user.
Please contact DSTech Technical Support at [email protected] regarding
any problems with your unit that cannot be resolved with the
instructions provided in this manual.
2.4. Data processing overview
For black carbon (BC) and gaseous pollutant monitoring, data goes
through processing in the order summarized below:
1. Raw signal acquisition: Digital voltage readings proportional to light
intensity (for BC) or electrical current (for gases). These raw signals
are always logged to the SD card and Cloud Dashboard unaltered,
such that users can calculate pollutant concentrations from scratch
using their own methods.
2. Environmental compensation: The voltage signals are compensated
for their environmental sensitivity, as outlined in Section 1.2.

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ObservAir®Operating Manual
Compensation factors are derived and uploaded by DSTech, they
cannot be modified by the user.
3. Digital filtering: The compensated voltage signals pass through a low-
pass filter, outlier filter, and other signal processing procedures. This
ensures that the voltage signals are not affected by electromagnetic
interference or other measurement noise.
4. Fundamental equation: Using the processed voltage signals, pollutant
concentrations are calculated using the fundamental equations
provided in Section 1.1.
5. Corrections and calibration: The pollutant concentration data is
corrected for artifacts and calibration factors are applied, as outlined
in Section 4. This provides the final concentration values that are
logged and displayed.
Data from other sensors, such as that used for particulate matter
monitoring, are not processed in any way. Data is simply logged from the
third-party hardware, which often incorporates data processing
procedures similar to that outlined here.
3. Operating Instructions
3.1. Hardware overview

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ObservAir Operating Manual
The ObservAir is shown in Figure 5 below. The sample air inlet and outlet
nozzles are located on the front panel, and are denoted by arrows facing
towards and away from each nozzle, respectively. The SD card slot and
USB port are also located next to one another on the sensor’s front panel.
In Figure 5, the aerosol filter tab protrudes from its dedicated slot through
the front panel, as it does during normal operation. On top of the sensor,
there is a thumbscrew for securing the aerosol filter tab, and the
interactive LED button for sensor control and feedback.
Figure 5. Overview of the ObservAir. Isometric view.
The serial number is displayed on the sensor’s bottom surface. Please
include this serial number when contacting DSTech technical support.
Figure 6. Bottom view of the ObservAir
3.2. Interactive LED button: Sensor display and control
The ObservAir’s interactive LED button is used to control basic operational
settings and display errors, pollutant concentrations, and other messages.
Serial Number

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ObservAir®Operating Manual
3.2.1. Sensor startup
To turn on the sensor, press and hold the button for 5 seconds until the
LED flashes green, and then release the button. The LED will hold yellow
while initializing the bootup sequence then briefly flash red, green, and
blue in sequence to indicate that it is starting normal operating mode. If
no errors or alarms are detected, the LED will begin to indicate current
pollutant concentrations by ‘breathing’ slowly (see Section 3.2.2 below).
The sensor should be allowed to warm up for 30 to 60 minutes before
monitoring pollutants. For every new start up event, a new data file is
created on the SD card (.txt file) and is assigned a filename that contains
the sensor’s ID number and the start-up time/date (Section 3.6.1).
3.2.2. Default LED mode: Pollutant concentration display
When the sensor is operating normally (no errors or warnings), the LED
slowly glows on and off in a ‘breathing’pattern. By default, the color of
the breathing LED ranges from green to red to indicate the current BC
concentration. Green corresponds to a BC concentration of 0 µg/m3, and
red represents concentrations that are higher than or equal to a user
defined maximum setting (factory default is 5µg/m3). LED colors are
scaled according to BC concentration between these two limiting values.
For example, yellow corresponds to a BC concentration of 2.5 µg/m3with
the default settings. The LED can also be configured to display gaseous
pollutant concentrations. By default, the LED turns off automatically after
20 minutes to conserve battery power, but this delay can be set by the
user. The LED can be turned back on by pressing the button briefly, and
it will breathe to indicate pollutant concentrations for another 20 minute
period. This LED timeout period can also be set by the user. For
instructions on configuring the LED display settings, please see Section
3.8.3.
3.2.3. Sensor menu Interface
To interact with the sensor, press and hold the button. The LED will cycle
through flashes of different colors and patterns that correspond to the
menu items listed in Table 4. When holding the button, the LED cycles
through the menu options in the order listed with two seconds in between
each option. After reaching the desired menu item, release the button, and
the LED will flash the menu item color to confirm the selection. To exit the
menu, press and hold the button through the entire menu selection, the
LED will start breathing normally, and you can release the button.
Hold LED button:
Options appear in order.
Release for menu selection.

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ObservAir Operating Manual
LED Flash Pattern
Sensor Function
1x Blue
Menu start
1x Red
Power off
1x blue
Turn Cloud Dashboard on/off
2x Blue
Provision backend service/WiFi
2x Blue
Exit menu –do nothing
Table 4. Index of menu items displayed when holding down the LED
button. The LED display will cycle through the options in the order
listed until the button is released at the desired selection.
3.2.4. Sensor shutdown (Power off)
To turn off the ObservAir, hold the LED button to cycle through the menu
until the first red flash and release. The sensor will flash red two more
times and make a chime to confirm that it is shutting down.
3.2.5. Toggle Cloud Dashboard connection
To turn on or off the connection to the Cloud Dashboard, press and hold
the LED button to cycle through the menu until the first blue flash and
release. An escalating or a descending tune will sound to indicate whether
the service has been turned on or off. Note, the instrument must first be
provisioned before Cloud Dashboard services can be used (see next
Section). Also, note that the instrument must have an accessible internet
connection either via WiFi or LTE. Please consult the ObservAir Cloud
Dashboard Manual for more information.
3.2.6. Provision Cloud Dashboard service
Before the instrument can access Cloud Dashboard services, it must be
provisioned - Network credentials must be provided for the sensor to
connect to the cloud, and the sensor must be assigned to your Cloud
Dashboard account and configured. To activate provisioning, press and
hold the interactive button until the LED flashes blue twice, and release.
The instrument will restart into provisioning mode: The LED will slowly
breathe blue. Please consult the ObservAir Cloud Dashboard Manual for
more information and further instructions.
3.2.7. Sensor alarms and errors
When a sensor error occurs or an alarm is triggered, the LED button
flashes red, orange, or yellow. The speed and color of the flashing LED

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ObservAir®Operating Manual
denotes different error codes. If the ObservAir displays an error, refer to
Section 6 for troubleshooting instructions to diagnose and resolve the
issue.
3.3. Filter tab replacement
To change the filter tab, first release the thumbscrew by turning it
counterclockwise for about four full turns (Figure 7). Remove the filter tab
by pulling it straight out from the front of the sensor. The filter tab should
release from the sensor with little resistance. If there is any resistance,
further loosen and/or push down lightly on the thumbscrew to release the
filter tab. Second, insert a new filter tab with the notch aligned to the solid
white circle on the faceplate, as shown in Figure 8. Third, insert the new
filter as far as possible, such that the tip of the notch is flush to the front
panel, and hand tighten the thumbscrew firmly.
Figure 7. Step 1 of filter tab replacement: Loosen thumbscrew and
remove the filter tab