Alphasense OPC-N2 User manual

072-0300 Alphasense User Manual
OPC-N2 Optical Particle Counter Issue 5
Alphasense Ltd Page 1 of 34 December 2015
Sensor Technology House, 300 Avenue West, Skyline 120, Great Notley. Essex.CM77 7AA. UK
Tel: [phone] - Fax: [phone]
Email: [email protected] - Web: www.alphasense.com
Contents
1 Quick Start
2 Product Specification
3 Health and Safety and Regulatory Information
4 How the OPC N2 works
5 PM Measurement
6 Sampling the environment
Connecting and running the OPC
7 Connecting Power and Taking Readings
8 Running the OPC N2 using the Alphasense software
•Installation
•Data Display and taking measurements
•Other software functions
9 Standalone mode (Recording to onboard SD card)
10 Running the OPC N2 using direct API control
11 Revision Control
12 Appendices (order may change!)
A. FAQs
B. Installing the device driver
Windows XP
Windows 7
Windows 8, 8.1 and 10
C. Updating Firmware (TBC)
D. Summary of firmware commands

072-0300 Alphasense User Manual
OPC-N2 Optical Particle Counter Issue 5
Alphasense Ltd Page 2 of 34 December 2015
Sensor Technology House, 300 Avenue West, Skyline 120, Great Notley. Essex.CM77 7AA. UK
Tel: [phone] - Fax: [phone]
Email: [email protected] - Web: www.alphasense.com
1 Quick Start
The purpose of this manual is to explain how to set up, install and use the Alphasense Optical
Particle Counter OPC-N2 for measuring PM1, PM2.5 and PM10, as well as measuring particle size
distributions in real time.
When using this OPC for the first time we recommend that you use the Alphasense SPI adapter,
which is available from Alphasense. This will enable you to very quickly use the OPC at your desk
by running it off a PC or laptop using the Alphasense software. Apple compatible software is not
available.
A full deployment package will have been provided with the OPC-N2. This is supplied either on a
CD or on the SD card on the unit. The SD card can be accessed simply by attaching the OPC to
your computer using a micro USB cable.
Full instructions for carrying out the installation, (specifically a driver must be installed for the SPI
adapter) and for the use of the software can be found later in this document, Appendix B.
You can view the data options along with the data resolution and range when the OPC-N2 is
connected to a PC running the Alphasense-supplied software. This also provides a useful reference
to those customers who wish to develop their own software.
The OPC-N2 can work in standalone mode, where the data is stored directly to the onboard SD
card. To make full use of these features by changing parameters it is necessary to use the supplied
software or be able to send SPI commands through your own interface.
Please note that the on-board temperature and pressure sensor is now an optional extra on the
OPC-N2.

072-0300 Alphasense User Manual
OPC-N2 Optical Particle Counter Issue 5
Alphasense Ltd Page 3 of 34 December 2015
Sensor Technology House, 300 Avenue West, Skyline 120, Great Notley. Essex.CM77 7AA. UK
Tel: [phone] - Fax: [phone]
Email: [email protected] - Web: www.alphasense.com
2 OPC-N2 Specification
All dimensions in millimetres (± 0.15 mm)
MEASUREMENT
Particle range (μm) Spherical equivalent size (based on RI of
1.5+i0)
0.38 to 17
Size categorisation (standard) Number of software bins 16
Sampling interval (seconds) Histogram period (recommended) 1 to 10
Total Flow rate (typical)
Sample flow rate (typical)
L/ min
mL/ min
1.2
220
Max particle count rate Particles/ second 10,000
Detection limits (PM10) Minimum
Maximum
0.01 μg/m3
1 500 mg/m3
Coincidence probability % at 106particles/ L 0.84
POWER
Measurement mode mA (typical) 175
Non-measurement mode mA (typical) Laser at minimum power; fan
off
95
Transient power on start-up mW for 1 ms <5000
Voltage range V DC 4.8 to 5.2
KEY SPECIFICATIONS
Digital Interface SPI (Mode 1), USB 2.0
Data storage micro SD 16 GB
USB VID Ox04D8
USB PID OxF3D5
Laser classification as enclosed housing Class 1
Temperature range °C -20 to 50
Humidity range % rh (continuous) 0 to 95 (non-condensing)
Weight g < 105
Table 1. Power and environmental performance limits
Ricardo AEA will certify performance and calibration, following ISO 17025 standard, this is ongoing.

072-0300 Alphasense User Manual
OPC-N2 Optical Particle Counter Issue 5
Alphasense Ltd Page 4 of 34 December 2015
Sensor Technology House, 300 Avenue West, Skyline 120, Great Notley. Essex.CM77 7AA. UK
Tel: [phone] - Fax: [phone]
Email: [email protected] - Web: www.alphasense.com
3 Health and Safety
The OPC-N2 uses an embedded diode laser light source that operates at typically 5-8 mW (max. 25
mW) at a wavelength of 658 nm. The OPC-N2 is a Class 1 laser product, since the user does not
have access to the laser source. The OPC-N2 is designed for OEM use, normally mounted in a
secondary housing. The user must not open or adjust any parts of the OPC-N2. It is the user’s
responsibility to ensure that the unit is used safely and complies with any local regulations. Do not
remove any safety stickers or warnings.
DO NOT remove the external housing: this not only ensures the required airflow but also protects
the user from laser light. Removal of the casing may expose the user to Class 3B laser radiation.
You must avoid exposure to the laser beam. Do not use if the outer casing is damaged- return to
Alphasense. Removal of the external housing exposes the OPC circuitry which contains
components that are sensitive to damage by static discharge.
4 How it Works
Like conventional optical particle counters, the OPC-N2 measures the light scattered by individual
particles carried in a sample air stream through a laser beam. These measurements are used to
determine the particle size (related to the intensity of light scattered via a calibration based on Mie
scattering theory) and particle number concentration. Particle mass loadings- PM1, PM2.5 and PM10,
are then calculated from the particle size spectra and concentration data, assuming a particle
density and refractive index (RI). Default settings are: density 1.65 g/ml, RI 1.5+i0. Respiratory
profiles are included in the PM calculations.
An additional weighting is applied on units with Firmware 18 or higher to account for under counting
at low particle sizes and the effect of carbon particles in urban air so that the output matches co-
located reference detectors. This correction can be retrofitted to units with older firmware, contact
Alphasense for additional information.
Most conventional OPCs employ a narrow inlet to physically constrain the airborne particles to pass
through a uniform central part of the illuminating laser beam and ensure accurate sizing. Such
instruments incorporate both an air-pump sufficiently powerful to draw the sample aerosol through
the narrow inlet tube and a particle filter upstream of the pump to avoid pump contamination and
ultimate blockage. The result is an OPC with high current consumption and a regular maintenance
requirement to replace the pump protection filter (which can be frequent in dirty atmospheres).
The Alphasense OPC-N2 uses a different design: the pump and replaceable particle filter have
been removed. Instead, the patented OPC-N2 uses an elliptical mirror and dual-element
photodetector to create a ‘virtual sensing zone’ in free space at the centre of an open scattering
chamber. The airflow through the OPC is produced by a low power microfan.

072-0300 Alphasense User Manual
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Alphasense Ltd Page 5 of 34 December 2015
Sensor Technology House, 300 Avenue West, Skyline 120, Great Notley. Essex.CM77 7AA. UK
Tel: [phone] - Fax: [phone]
Email: [email protected] - Web: www.alphasense.com
The OPC-N2 classifies each particle size, at rates up to ~10,000 particle per second, recording the
particle size to one of 16 “bins” covering the size range from 0.38 to 17 μm. The resulting particle
size histograms can be evaluated over user-defined sampling times from 1 to 10 second duration,
when driven using the software or by the customers own system. Longer periods can be used but
the OPC will reset if no communication is received for 65s; also, longer times in dirty environments
can result in the bins “over filling”. If longer periods are needed it is recommended that you carry
out averaging of shorter measurement periods. This histogram data is transmitted via an SPI
interface to a host computer. The patented OPC-N2 design results in virtually all sampled airborne
particles passing straight through the sensor without being deposited, allowing the OPC-N2 to
operate for very long periods (>1 year) without the requirement for regular maintenance or cleaning.
Consistent with most commercial Optical Particle Counters (OPCs), all particles, regardless of
shape are assumed to be spherical and are therefore assigned a ‘spherical equivalent size’. This
size is related to the measurement of light scattered by the particle as defined by Mie theory, an
exact theory to predict scattering by spheres of known size and refractive index (RI). The OPC-N2
is calibrated using Polystyrene Spherical Latex Particles (PSLs) of a known diameter and known RI.
Correction factors can be applied for errors resulting from particles of different density.
5 PM measurements
The particle size histogram data recorded by the OPC-N2 sensor can be used to calculate the mass
of airborne particles per unit volume of air, normally expressed as μg/m3.
The accepted international standard definitions of particle mass loadings in the air are PM2.5 and
PM10, as not all standards groups have defined PM1yet. These definitions relate to the mass and
size of particles that would be inhaled by a typical adult. So, for example, PM2.5 is defined as
‘particles which pass through a size-selective inlet with a 50% efficiency cut-off at 2.5 μm
aerodynamic diameter’. The 50% cut-off indicates that a proportion of particles larger than 2.5 μm
will be included in PM2.5, the proportion decreasing with increasing particle size, in this case out to
approximately 10 μm particles.
The OPC-N2 calculates the respective PM values according to the method defined by European
Standard EN 481. Conversion from the ‘optical size’ of each particle as recorded by the OPC-N2
and the mass of that particle requires knowledge of both particle density and its RI at the
wavelength of the illuminating laser beam, 658 nm. The latter is required because both the intensity
and angular distribution of scattered light from the particle are dependent on RI. The OPC-N2
assumes an average RI value of 1.5 + i0. The OPC-N2 allows a different value to be set for each
size bin to correct for particle density variation with particle size. The default setting for each size
bin is a Particle Density value of 1.65 g/ml, a figure that equates to a typical value found in many
environments. However, where it is known that different size fractions in the ambient aerosol have
different densities (for example, the smallest carbon particles will have a higher density than larger
aggregates of the same particles); different Particle Density values may be set for different bins to
achieve a more accurate determination of PM. Contact Alphasense for further discussion and
instructions on how to to modify the particle bin density.
The OPC-N2 also has a Sample Volume Weighting factor for each size bin that is applied to the
total mass of particles in that bin. The default values are those defined by European Standard EN
481 for PM1, PM2.5 and PM10 with an additional multiplier being used such that the OPC-N2 matches
better to standard reference instruments when used in the field and to correct for some of the
missing mass below the OPC detection limit of 0.38 μm.
It is also possible for end users to set their own multipliers to align the OPC-N2 to their own co-
located reference instruments.

072-0300 Alphasense User Manual
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Alphasense Ltd Page 6 of 34 December 2015
Sensor Technology House, 300 Avenue West, Skyline 120, Great Notley. Essex.CM77 7AA. UK
Tel: [phone] - Fax: [phone]
Email: [email protected] - Web: www.alphasense.com
Note
•The EN 481 standard definition for PM10 extends to particle sizes beyond the upper
measurable size limit of the OPC-N2. In some cases, this can result in the reported PM10
value being underestimated by up to ~10%.
6 Sampling the environment
The sample air flow rate through the unit is determined by both the fan speed and any obstruction
that affects the inlet or outflow of the OPC. Considering this problem- tubing, valves, baffles or
obstructions that will restrict air flow into or out of the OPC should be avoided. Particle distribution
can also be affected by sharp turns and narrow sample pipes. Maximum pressure drop through the
entire flow system must be less than 75 Pa and ideally less than 40Pa.
However, because fan speed can vary and external factors such as wind direction in the vicinity of
the OPC may affect the sample flow rate through the OPC, such variations are monitored and
corrected dynamically by the OPC so that the particle concentrations and derived PM values are
unaffected by moderate flow variations.
The OPC N2 unit will operate adequately on its own on the bench; however it will need to be placed
in a secondary housing for use in the field.
Alphasense recommends that the OPC-N2 inlet is exposed directly to the target sample volume and
that the fan exhaust is left free to exhaust into an unconstrained space. The OPC-N2 can be
positioned in any orientation. However, to mitigate the effects of wind direction on sampling it is
best for the inlet to be pointing upwards. When mounted pointing upwards care should be taken in
this case to avoid very large droplets or soot and grit from entering the unit due to gravity. Some
form of “umbrella” or mushroom should be used which protects the unit but does not interfere with
particle flow. Coarse gauze filters can also be used to prevent the ingress of large particles or
insects without interfering with the fine particles being measured. All electrical connections must be
protected from moisture and temperatures outside of the operational range. Please note at
temperatures between –20 and –10°C the OPC is reliant on its own generated heat to ensure the
laser is operating correctly, thermal lagging of the secondary housing or heating may be required.

072-0300 Alphasense User Manual
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Alphasense Ltd Page 7 of 34 December 2015
Sensor Technology House, 300 Avenue West, Skyline 120, Great Notley. Essex.CM77 7AA. UK
Tel: [phone] - Fax: [phone]
Email: [email protected] - Web: www.alphasense.com
Connecting and Operating The OPC N2
7 Connecting power and taking readings
The OPC-N2 is shipped pre-calibrated. There are no user serviceable parts. Power and data
communications are provided via the SPI socket. Firmware uploading and SD card downloading
are via the micro-USB socket. Use of the SD card will require internal firmware updating if the
firmware version is prior to version 0015d.
Connection to the OPC-N2 for real-time data transfer can be made via the SPI direct to your own
circuit’s internal bus using your own or the SPI interface provided by Alphasense. The SPI interface
supplied by Alphasense requires a USB A-B lead to connect to the USB port of a computer. The
green LED shows that power is supplied to the OPC-N2 and the red LED flashes when the PC and
OPC-N2 are communicating. The micro-USB socket can be used for updating the internal firmware
and downloading the SD card measured data, it is not possible to download real time data via this
socket.
SPI Connection
The SPI socket is a Molex 'Pico Clasp' 6 way Housing, Part Number 501330-0600. Pins are
assigned in table 2. Pin 1 on the OPC-N2 is closest to the USB micro connector.
Pin Function
1Vcc
2SCK
3SDO
4SDI
5/SS
6GND
Table 2. SPI pin assignments
Please note that it is not possible to put the MISO (SDO) line into a high impedance mode (tri-state).
This means that it is not possible to use the OPC on a shared SPI bus.
OPC power requirement
The OPC-N2 requires a 4.8 to 5.2 Volt DC supply with minimum electrical noise, this is then stepped
down to a 3v3 supply (via the SPI-ISS adapter) for the SPI logic lines. The logic lines are NOT 5
volt tolerant.
The OPC N2 requires 175 mA continuous (95 mA continuous with fan off and laser at low power)
with a short one amp current surge at switch-on. Care must be taken that the current limit is not
exceeded if multiple units are operated from of a single computer
It is recommended to allow 1 second for the OPC to respond to the first SPI command after power-
on and >0.5s after any control system code. (Note this is longer than on previous versions)
Software interface configuration
The following interface rules will help you to make a reliable connection with the OPC.
1. The SPI mode is a Mode 1 device, where the idle state for the clock is low and data is
transmitted on the leading edge (transition from idle to active clock) and is received on the
trailing edge of the signal.
2. SPI clock acceptable frequency range: 300-750kHz. (Other frequencies may also be acceptable,
but are untested).

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3. SPI word length is 8 bits
4. The OPC is an SPI slave device.
5. No CRCs or checksums are used on the data stream.
6. Generally, an 8 bit command is sent to the OPC, which then responds after a short period with a
sequence of one or more 8 bit values, depending on the command.
7. SPI sequence timing is given in 072-0325 Supplemental SPI information for OPC N2
OPC SPI Commands
A full list of current SPI commands given in 072-0325 Supplemental SPI information for OPC N2, a
summary is listed in Appendix D. This can also be found on the supplied software CD, on the OPC
N2 unit’s SD card for recent releases or is available from Alphasense directly, as are command lists
for earlier versions of the firmware.
The file name is of the form: OPC N2-Firmware command list for “Firmware Version”
8 Using the Alphasense Software
The software and necessary drivers to run the OPC from a computer (PC only, a version for Apple
Computers is not available) is supplied either on the CD supplied with the OPC or can be
downloaded from the SD card on board the instrument (firmware 18 and above)
Full guided examples for installation using Microsoft Windows XP, Microsoft Windows 7, 8, 8.1 and
10 are given in Appendices C. It is recommended that the Windows PC is running .NET version 3.5
or above.
Connecting the device and running the software
Connect the USB-SPI interface lead and OPC device to the PC. If you are prompted for device
drivers refer to the previous section of the user manual. Double click the OPC-N2.exe icon to start
the software application. When the application is first started the main form will be in “start-up
mode” as shown in the next section.
Run the software as with previous versions and connect to the OPC-N2 by choosing the virtual
COM port it is assigned to. Some text should appear in the software’s main text box indicating
some details of the OPC on successful connection. At this stage the OPC electronics are on but the
laser is running at reduced power and the fan is switched off. Press Ctrl+R to read out all the
configuration variables stored the OPC.
There is no uninstall function for the software interface. The interface is stored as a set of files in a
single folder (to be kept intact) and will run as a normal Windows application. The entire folder can
be deleted or archived when redundant.
Data Display Screens and taking Measurements
Default start-up screen
Select the allocated COM port: A list of COM ports available on the PC/Laptop is displayed in the
drop-down menu at the bottom-left of the screen. Be sure to select the correct port number; the
software will not respond unless the port with the attached OPC-N2 device is selected.

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•Device information (Right hand side text window): This shows hardware serial number and
firmware versions currently installed on the OPC-N2.
•Start sampling: Starts data collection with the fan and laser running continuously. The
button will then show ‘Stop sampling’ to allow termination of the sampling process.
•Repeat interval ms: Sets the duration (in milliseconds) over which a particle size histogram
is acquired. The default is 1,000 ms. We recommend a maximum of 10,000 ms to avoid the
risk of an individual size bin becoming full (65,536 counts). Longer intervals can be set in
very clean environments, but the unit will reset if no communication is received for 65s.
•Y max: This sets the maximum y-axis value of the histogram screen display. Note: If the
displayed data reaches the top of the display, enter a larger Y max value.
•Histogram y-axis ‘Counts/ s’: This displays the recorded counts per second in each size bin,
regardless of the setting of the ‘Repeat interval ms’. For example, if a 10,000 ms sampling
interval is set, the ‘Counts/ s’ figure will represent the average counts per second over that
period. This average figure is also recorded in the CSV file.
Histogram Counts/s vs Particle Size display mode
When the Start sampling button is pressed, the OPC-N2 will first ask if the data are to be saved.
Once answered, it will begin to display particle size histogram data and if selected, store data to a
file.

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•Data relating to each acquired histogram, including PM values, is given in the right-hand window
of the display. The RollMean_PM10, etc., are the current rolling mean values for PM evaluated
over the previous 5 minutes or to the beginning of sampling if that is less time.
•The total particle count rate per second across all size bins (Counts/ s) is given in the bottom-left
of the screen.
•Clicking on the graph and pressing l will display the values for each bin, pressing l again will
toggle it back.
Histogram Number Concentration vs Particle Size display mode
Click the ‘Histogram #/ml’ button on the left-hand side of the screen to show the recorded data in
particle number concentration (particles per millilitre of sampled air) format, as below:
History Plot of Counts/s display mode
Click the ‘Histogram plot #/s’ button on the left-hand side of the screen to show a temporal record of
the particle count rate since the start of the sampling session. The plot scale will automatically
change to show longer recording time periods and/or increasing Counts/ s.

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History Plot of Mass Concentration display mode
Click the ‘Histogram plot mass conc.’ Button on the left-hand side of the screen to show a temporal
record of PM1, PM2.5 and PM10 values (in µg/m3) since the start of the sampling session. The plot
scale will automatically change to show longer recording time periods and/or increasing PM values.
Data relating to each acquired histogram, including PM values, is given in the right-hand window
of the display.
Other Software Functions
Shutting down the software
It is recommended that the software application be closed before removing the USB to SPI interface
to avoid USB communication errors.
Log file
The application will also create a log file from all of the output bins into a single .CSV file. You will
be asked if you require a log file after selecting the “start” button. If you select “yes” you will be
prompted for a file name and location to store the file. An example of the log file is included in the
deployment package and the CSV out put explained in more detail later in the document.
Firmware information
The information window on the right of the main form shows the firmware version installed as the
software first loads up. The OPC-N2 also has the ability to install firmware upgrades via the USB
port. This is achieved using a Bootloader tool see Appendix C for more details. To check to see if
your device can upgrade firmware via the USB port, check the information string printed as the
device is first connected. To upgrade firmware via USB you need to have firmware version 0015d
or above.
Example: “OPC-N2 FirmwareVer=OPC-015d…………………………..BD”
As the firmware first loads it follows the procedure below:
Check for SD memory card. All versions of the OPC-N2 will come with a 16GB micro SD card fitted.
This card is internal and is not accessible from outside the case, please note it may not be fitted on
early release OPC-N2 units.

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1. If a SD card is found, the OPC will check for a USB connection. The OPC-N2 comes equipped
with a micro USB port.
2. If a USB connection is found the device will switch to “USB mode”. This makes the OPC-N2
behave like an external storage device (pen-drive) until the USB is disconnected.
3. If a USB connection is not found then it will check for the SPI master connection (SCLK). The
OPC will wait up to 60 seconds for an SPI connection to be established.
4. If an SPI connection is not found the OPC-N2 will switch to standalone “SD card mode”. When
in SD card mode the SPI is still active, if SPI command “SPIShutDownDAC” (0x03) is sent, the
OPC will close the current SD card log file and exit SD card mode.
Please note that if the unit has entered standalone mode before you have connected the
software the following error will be returned:
Found USB-ISS Version 6
USBISS SerialNo = XXXXXXXX
Device interface = USBISS
InfoStr not Found
SPI Busy
This can be cleared by connecting a second time or if this fails unplugging and replugging in the unit
More firmware commands (via software)
There are four commands that can be used to edit factory settings through the OPC software:
1. Ctrl + R: Read and display all configuration variables. This will display the variables available to
the user for edit. The data is displayed in the information window to the right of the software
window.
2. Ctrl + W: Write all values to configuration memory. This command will write the current
configuration values to volatile memory, this means that the user can run the OPC with their
desired configuration but the changes will be reset once the power is disconnected.
3. Ctrl + S: Save all values to configuration memory. This command will write the current
configuration values to non-volatile memory, this means that the user can permanently save
their configuration values.
4. Ctrl + B: Enter Bootloader mode. This allows the OPC to interact with the Bootloader application
so that new firmware can be installed via USB port.
5. These commands are also needed to make full use of the stand alone mode

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Explanation of the CSVs
Please note sheets have been truncated to save space
Comment
Software ver: OPC-N2
Alphasense 1.0.5779.33206
Size LUT file OPC-N2_06-02-
2015.LUT
Device SerNo SerNo: 176390116 Unit serial number
InfoString OPC-N2 FirmwareVer=OPC-
018.2..............................BD
Gain scaling coeff 1 Scaling coefficient that can be
applied to all bins
Laser digital pot
setting
200 Laser setting
Fan digital pot
setting
255 Fan setting
ToF to SFR factor 74 Needed for dynamic fan speed
correction
Bins Bin00 Bin01 Bin02 Bin03 ….. Bin15
Bin low boundary
(ADC o/p)
12 17 27 41 ….. 3685
Bin high boundary
(ADC o/p)
17 27 41 60 ….. 4095
Bin low boundary
(particle diameter
[um])
0.38 0.54 0.78 1.05 ….. 16
Bin high boundary
(particle diameter
[um])
0.54 0.78 1.05 1.34 ….. 17.5
Bin centre
(particle diameter
[um])
0.46 0.66 0.915 1.195 ….. 16.75 Diameter at the centre is used for
PM cals
Vol of a particle in
bin (um3)
(calculated by
software)
0.051 0.150
5
0.401
1
0.8935 ….. 2460.6
12
Vol of a particle in
bin (um3) (values
stored on OPC)
0.051 0.150
5
0.401
1
0.8935 ….. 2460.6
12
Density of a
particle in bin
(g/ml)
1.65 1.65 1.65 1.65 ….. 1.65 Default 1.65
Sample volume
weighting for bin
1 1 1 1 ….. 1 Individual additional bin weighting
Annotated CSV produced by an OPC-N2 run by the Alphasense Software top section

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Data:
OADate
Time
Bin00 … Bin15 MeanToF
Bin1(us)
… MeanToF
Bin7(us)
Count/s Sampling
Period(s)
SFR
(ml/s)
Temperature
(C) or
Pressure(Pa)
PM1
(ug/m3)
…PM10
(ug/m3)
RollMean
_PM1
… RollMean
_PM10
42341.
6282
8.001 … 0 6 … 0 20.003 1 3.2 x 1.38 … 6.85 1.38 … 6.85
42341.
6282
5.946 … 1.982 9.33 … 0 18.83 1.009 3.1 x 1.09 … 198.75 1.24 … 102.8
42341.
6282
8.003 … 0 7 … 0 32.014 1 3.21 x 2.36 … 108 1.61 … 104.53
42341.
6282
7.925 … 0.991 5.67 … 0 28.727 1.01 3.44 x 1.88 … 149.15 1.68 … 115.69
42341.
6282
8.008 … 0 6 … 0 23.023 0.999 3.55 x 1.46 … 87.34 1.63 … 110.02
42341.
6282
11.99
7
… 0 6.67 … 0 25.993 1 3.56 x 1.74 … 3.56 1.65 … 92.27
42341.
6282
4.954 … 0 7.67 … 23 20.806 1.009 3.52 x 1.52 … 12.79 1.63 … 80.92
Counts per s in
each bin
Used for dynamic fan
compensation
Total
counts
per s
across
all bins
Sample
flow
through the
OPC
x if no sensor
fitted
Instantaneous PM1,
2.5, 10
5 minute averaged
PM1, 2.5, 10
Annotated CSV produced by an OPC-N2 run by the Alphasense Software bottom section
For CSVs generated in standalone mode the main difference is that the bin count is the total count
in the sampling period and is not a count/s

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9 Standalone mode (Recording to onboard SD card)
If the OPC-N2 unit is powered but receives no SPI communication for 60s it starts running and logs
data to the on board SD card. The default settings are such that it will run with the fan and laser on
continuously and record a histogram every ~1.4s. Every 24 hours a new data file is recorded and
the files are numbered sequentially.
Please note the histogram records total counts in the 1.4s time period and does not quote an
average figure in counts/s. The unit does not have a real time clock so any break and resumption in
power will result in the next numbered file being created. Only the default option was offered on
previous versions of firmware (16b and 17b), the latest version firmware 18 now offers other
variables.
Autonomous Mode Variables
The easiest way to access these is to use the supplied Alphasense Software. The variables can be
changed by SPI commands using your own system if preferred (See the commands list summary in
Appendix D for details).
Connect the OPC-N2 to the software in the standard manner. Type control R and scroll to the
bottom of the text list. Listed here are the Autonomous Mode variables. The variables listed in table
3.
Command Description Default
AMSamplingInterval count The number of 1.4s samples
taken for each measurement
1
AMIdleInterval count The number of 1.4s periods
between each measurement
0
AMMaxDataArraysInFile The number of saved
histograms per CSV (default is
24 hours of continuous 1.4s
measurements)
61798
AMFanOnInIdle Selects if the fan is on or off
during the idle phase
0
AMLaserOnInIdle Selects if the fan is on or off
during the idle phase
0
AMOnlySavePMDataToSD Selects full data or PM data
only to be saved
0
Table 3. Autonomous Mode Variables
When running with an Idle Interval the unit automatically runs the fan and laser for a period of 7
~1.4s cycles before starting the measurement. This allows the laser and fan to come up to full
power. As a result in the CSV file the Data RequestInterval(s) is the total time of the cycle, so
includes the Idle Interval, the warm up time and the measurement time, the SamplingPeriod(s) is the
time taken for the active measurement.
To alter these variables type the new value in the window, then press control W, and confirm that
you want to change the current OPC configuration then type Ctrl + S: Save all values to
configuration memory, and again confirm it. The OPC should then be disconnected from the
software and unplugged and will start logging when powered up again.

072-0300 Alphasense User Manual
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10 Running the OPC N2 using direct SPI control
Full details of the SPI commands and connections are given for the OPC-N2. This is should be
sufficient for the user to be able to design their own SPI system to control the device and gather
data.
A coding example is given in given in 072-0325 Supplemental SPI information for OPC N2
If, to save power the OPC is to be run periodically with the fan off and laser at low power, it is
recommended that the fan and laser are powered up for a minimum of 10 s before taking a
measurement.
Alphasense can offer only limited support for programming/coding and customers may need to seek
third part assistance if they are unfamiliar with SPI protocols.
11 Revision Control
Version Comment Release Date Released by
5 Update to version 18 firmware, and combine
documents into one manual
December 2015 Mark Giles

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Tel: [phone] - Fax: [phone]
Email: [email protected] - Web: www.alphasense.com
Appendix A: FAQs
•What are the differences between the different firmware versions
Firmware 16 and 17 including 16B and 17B
•Firmware 18 builds on the previous firmware release 16 and 17. Please note Firmware 16 and
17 are basically equivalent, different versions were needed to support the use of two different
PCBs, issue 2 with Firmware 16 and issue 3 with 17. Similarly for version 18, 18.1 is for issue 2
and 18.2 for issue 3. In operation the only significant difference is that units with Issue 2 boards
use more power in the standby mode as the standby laser power is not as low as in issue 3.
Units with issue 2 boards must only be updated with the correct firmware as irreversible damage
to the laser may occur otherwise.
•General running and features of 16 and 17 are the same in issue 18. All have dynamic
correction for sample flow-rate caused by changes to the of the fan speed and also apply the
respiratory profiles to the PM values
•Issue 18 has more advanced standalone recording to the onboard SD card and some additional
commands to make it easier for end users to write their own software. The serial number of the
OPC is stored on the unit and is recorded on any CSV files.
•Issue 18 units have, in addition a scaling factor applied to each bin to align the unit more closely
to reference detectors following co-location studies. This scaling factor can be retrofitted to units
in the field; details can be obtained by request from Alphasense.
•It is recommended that you continue to use the initial software release in deployment versions
9.7 and 9.8 for units with firmware 16 and 17.
•If you are running 16, 17, 16B or 17B using your own software developed for an earlier version
such as 15D please allow more time at start up before issuing the first command, ~1 s. The
main commands are the same across most releases. Data positions in the call histograms and
read functions may have changed; see the full list of commands for details.
Firmware 15D
•There are differences in firmware commands and responses between units with 15D and later
firmware.
•In terms of performance these units to not offer the dynamic correction of sample flow-rate for
variations in fan speed and have a simplified method of calculating the PM values
•If you continue to use these units is recommended that you use the initial software released in
deployment 9.5.
•It is possible to update the majority of these units in the field to firmware 18.1.
Older Units, other Firmware 15 variants and 14
Substantial improvements have been made since these units have been issued. It is not possible to
update these units in the field. Please Contact Alphasense to discuss potential upgrade options.
•Can the OPC be connected to a gas flow at 500 SCCM (or similar)?
•The OPC is designed to sample ambient air using its own fan. Connecting to a pressurised
system will alter the calibration and may also lead to deposition of particles on the inside of
the unit. The OPC is also designed to have air pulled through it rather than blown into it.
•What is the effect of low pressure (altitude)
•Fundamentally, fans are constant volume devices and so, at altitude where the density of air
is lower, the mass transported through the fan will be reduced but the volume stays the
same (assuming the fan speed remains the same).

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•The unit should therefore operate normally at altitude with particle size and number
concentrations being accurate. However, in situations at altitude where ambient
temperatures are expected to fall to -20°C, care should be taken to ensure the OPC itself
has some thermal insulation to allow the heating effect of the laser and electronics to
maintain the internal temperature to approximately -10°C or above. If the ambient
temperature falls to lower than -20°C, additional heating should be provided to the OPC to
ensure correct operation..
•How are the units calibrated
•The units are calibrated for sizing using controlled aerosols of polystyrene latex
microspheres, taking into account the small response difference caused by the higher
refractive index (1.58) of these particles) compared to the refractive index (1.5) used in the
OPC-N2 calibration curve. Aerosol number concentration is assessed by comparison to an
OPC N2 ‘gold standard’, previously calibrated against a certified TSI 3330 OPC instrument.
•What maintenance can be carried out on the OPC N2
•The OPC-N2 does not have any user-serviceable parts. The fan and laser are both chosen
to give good lifetimes. The flow path is designed to minimise particle deposition on any
internal surfaces of the OPC. The unit must not be opened for cleaning as this may expose
the worker to class 3B-laser radiation and could effect the calibration.
•Raspberry Pi and Arduino
•The OPC-N2 is ideally suited to be operated by devices such as Raspberry Pi or Arduino via
its SPI interface. While Alphasense does not yet distribute example OPC-N2 control
programs to be used on these devices, many of its customers have successfully
implemented such control programs following the OPC-N2 SPI commands list. Further
information may be released in a separate annex in 2016

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Appendix B: Installing the device driver
Windows XP
Copy the folder “OPC Interface Software” to the
PC desktop. Connect the USB interface lead to
the PC. If the USB interface lead (USB to SPI
converter) is connected to the PC for the first
time, Windows will need a device driver and this
will start the “Found New Hardware” wizard.
Select the “Yes, this time only” option and click
next.
The following window will give you an option as
to whether to use a CD to install the device
driver or to use another location. Select “Install
from a list or a specific location (Advanced)”.
Navigate to the OPC folder containing the folder
named “USB Driver”, this contains the driver file
devtech2.inf which Windows needs to drive the
OPC device.
Click OK to allow Windows to locate and install
the device driver. This process is automatic but
you will be prompted by the form below to
confirm the installation.
Click “Continue Anyway” to finish the installation.
Once the device driver is installed correctly, the
OPC device should appear in the Device
Manager window as a “Communication Port”
with an assigned COM port number. Make a
note of this assigned port number, as you will
need it when starting the software.
The Driver installation is now complete.

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Sensor Technology House, 300 Avenue West, Skyline 120, Great Notley. Essex.CM77 7AA. UK
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Windows 7
Copy the folder “OPC
Interface Software” to the PC
desktop. Connect the USB
interface lead to the PC. If the
USB interface lead (USB to
SPI converter) is connected to
the PC for the first time,
Windows will need a device
driver.
Open the system properties
and locate the device
manager.
Windows 7 will show the new
device as “USB-ISS” with a
yellow exclamation mark
indicating a driver problem.