Alphasense User Manual OPC-N2 Optical Particle Counter

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OPC-N2 Optical Particle Counter
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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.
1
OPC-N2 Specification
All dimensions in millimetres (± 0.15 mm)
MEASUREMENT
Particle range (μm)
Size categorisation (standard)
Sampling interval (seconds)
Total Flow rate (typical)
Sample flow rate (typical)
Max particle count rate
Coincidence probability
Spherical equivalent size (based on RI of 1.5)
Number of software bins
Histogram period
L/ min
mL/ min
Particles/ second
% at 106 particles/ L
0.38 to 17
16
1.4 to 10
1.2
220
10,000
0.84
mA (typical)
mA (typical) Laser at minimum power; fan off
mW for 1 ms
V DC
175
95
<5000
4.8 to 5.2
POWER
Measurement mode
Non-measurement mode
Transient power on start-up
Voltage range
KEY SPECIFICATIONS
Digital Interface
Data storage
USB VID
USB PID
Laser classification
Temperature range
Humidity range
Weight
micro SD
as enclosed housing
°C
% rh (continuous)
g
SPI (Mode 1), USB 2.0
16 GB
Ox04D8
OxF3D5
Class 1
-20 to 50
0 to 95 (non-condensing)
< 105
Table 1 Power and environmental performance limits
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OPC-N2 Optical Particle Counter
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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 or 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.
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 (frequent in dirty atmospheres).
The Alphasense OPC-N2 uses a different approach: we removed the pump and replaceable particle
filter. 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 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.
This histogram data and air temperature are 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 of a known diameter and known RI.
Correction factors can be applied for errors resulting from particles of different density.
3
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 PM1, PM2.5
and PM10. 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 of 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.
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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. For particle density, the OPC-N2 allows a different value
to be set for each size bin. 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 clay particles may 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 instructions 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.
Notes
•
The OPC-N2 calculations of particle mass assume a negligible contribution from particles
below approximately 0.38 μm, the lower limit of particle detection of the OPC-N2 sensor.
• 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%.
4
Health and Safety
The OPC-N2 uses an embedded diode laser light source that operates at typically 5-8 mW (max.
25mW) 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 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.
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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-provided SPI interface. The micro-USB socket can be used for
updating the internal firmware and downloading the SD card measured data. See page 15 for
further firmware information.
When first using this OPC, we recommend that you use the SPI adapter. You can then view the
data options along with the data resolution and range when the OPC-N2 is connected to your PC,
that has the Alphasense-supplied software installed. Apple compatible software is not yet available.
Pin assignments for the SPI adapter are shown in Table 2 below. 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 OPC-N2 requires a 4.8 to 5.2 Volt DC supply with minimum noise. On-board regulators supply
power to the laser and fan. Required current is 175 mA when operating/ measuring and 95 mA
when the fan is disabled and laser is operating at its lowest power setting. There is a very short one
amp current surge at switch-on.
SPI Connection
When connecting direct to your own SPI connector, the SPI pinouts are:
Pin
Function
1
Vcc
2
SCK
3
SDO
4
SDI
5
/SS
6
GND
Table 2. SPI pin assignments
Use the micro-USB socket to either download the SD data card or to reprogram the OPC-N2, using
programs supplied by Alphasense. You cannot download real time data via this socket.
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. Because of this, tubing, valves, baffles or obstructions
that will restrict air flow into or out of the OPC should be avoided. 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 flow variations.
Alphasense recommends that the OPC-N2 inlet is exposed directly to the target sample volume and
the fan exhaust blowing into an unconstrained space. The OPC-N2 can be positioned in any
orientation but if the inlet is pointing upwards, care should be taken to avoid very large droplets or
grit from entering the unit due to gravity.
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Software installation
The examples below are for installation using Microsoft Windows XP and Microsoft Windows 7.
Please contact Alphasense directly if you are using Windows 8. It is recommended that the
Windows PC is running .NET version 3.5 or above.
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.
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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.
Installing the device driver (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.
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Windows 7 will show the new device as “USB-ISS” with a yellow exclamation mark indicating a
driver problem.
Right click the icon and select “Update Driver Software…
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Select the “Browse my computer for driver software” option.
Navigate to the folder “OPC Interface Software” copied to your desktop and locate the folder named
“USB Driver”, Click Next.
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Windows 7 will then issue a security warning. This is due to a licence issue and not a concern to the
operating system. Select the “Install this driver software anyway” option.
Windows will then install the driver files for the device.
Once the device driver software has been installed the form below will be displayed. Make a note of
the allocated COM port number (COM4 in the example below)
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Incorrect installation of the device driver
If the device driver is installed incorrectly the Device Manager will indicate this with a yellow
exclamation mark symbol shown below.
If this should happen remove the USB lead and uninstall the device by right clicking the symbol and
selecting “Uninstall”. Return to the beginning of the “Installing the device driver” (Windows XP or
Windows 7) section.
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 when redundant.
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 section 8.
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 is on, but the laser is running at
reduced power. Press Ctrl+R to read out all the configuration variables stored the OPC. See page
15 for more information on firmware variables.
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Data Display Screens
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.
•
If particle density of ambient particles is known to be different from the default setting (1.65
g/ml), please contact Alphasense for details on how to change this value.
•
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.
•
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.
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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.
•
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.
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:
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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.
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.
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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.
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 install firmware upgrades via the USB port.
This is achieved using a Bootloader tool, contact Alphasense to obtain access to the Bootloader tool
User Manual. 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 or OPC-N1.
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 “SD card mode”, This will create a
new file on the SD card (.csv format) and continue to log data. A new data file will be created
every 24 hrs. 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.
More firmware commands (via software)
There are four commands that can be used 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.
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OPC-N2 Factory settings
The OPC firmware retains the factory settings and calibrations.
These settings should not be modified as this will affect the OPC calibration and its accuracy.
If you wish to modify any of these settings, then contact Alphasense at (+44) 1376 556700.
The following parameters are factory set and stored in the firmware:
Bin boundaries
The upper and lower particle size limits defining each of
the 16 size bins.
Bin particle volumes (um3)
The volume ascribed to each particle in that bin in
firmware.
Bin particle volumes by software (um3) The volume ascribed to each particle in that bin in
software (parameter present to confirm firmware values,
which should be the same).
Bin particle densities (g/ml)
The density ascribed to each particle in that bin. The
default setting is 1.65 g/ml for all bins.
Bin sample volume weightings
Correction for size dependent sampling efficiency.
Normally set to 1.0.
Gain scaling coefficient
A global factor to normalise between units. Normally
1.0.
Laser digital pot setting
A parameter to determine laser beam power.
Fan digital pot setting
A parameter to determine fan speed.
NOTE: Changing either the fan speed or laser power will change calibration and the OPC-N2 will
require recalibration. When the OPC-N2 is not sampling, both the laser and fan are switched
automatically to low-power settings.
End of User Manual
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