Alphasense Ltd Page 1 of 15 September 2014The purpose of this manual is to explain how to set up, install and use the Alphasense Optical Particle Counter OPC-N1 for measuring PM1, PM2.5 and PM10, as well as measuring the particle size distribution in real time.1 OPC-N1 Provisional SpecificationTable 1 below specifies power and environmental performance and limits.MEASUREMENTParticle range (um)Spherical equivalent size 0.38 to 17Size categorisation (standard)Number of software bins 16Sampling interval (seconds)Histogram period 1 to 10Flow rateL/ min 1Particle count rateParticles/ second 10,000Coincidence probability% concentration at 106 particles/L 0.24POWERMeasurement mode (mA)Continuous measurement 260Laser power (mW)Typical, at 658 nm 5-8Standby mode (mA)Fan off 186KEY SPECIFICATIONSDigital InterfaceSPI (Mode 1)Laser classification3B Temperature range°C -10 to 50Humidity range% rh (continuous)0 to 99 (non-condensing)Weight g < 70Table 1. Specifications for OPC-N12How it WorksLike conventional optical particle counters, the OPC-N1 measures the light scattered by individual particles carried in a sample air stream through a laser beam. These measurements can then be used to estimate particle size (proportional to the amount of light scattered) and particle number concentration. Estimates of particle mass loading, eg: PM 2.5 or PM 10, can then be determined from the particle size spectra and concentration data.Most conventional OPCs must employ narrow inlet tubes (and often an additional ‘sheath-flow’ of particle-free air) to physically constrain the airborne particles to the more uniform central part of the laser beam and ensure accurate sizing. Such instruments must therefore 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 a unit with relatively high current consumption and a periodic requirement (frequent in dirty atmospheres) to replace the pump protection filter.However, the OPC-N1 uses a fundamentally different approach that removes the need for both the pump and a replaceable particle filter. Instead, it uses a combination of custom-designed elliptical mirror and a dual-element photodetector to create a ‘virtual sensing zone’ in free space at the centre of an open scattering chamber. The sensing zone and its surrounding space is then illuminated by a thin ribbon-shaped laser beam. Particle-laden air passing unconstrained though the scattering chamber can traverse the laser beam both within and outside the sensing zone, but only those particles traversing within the sensing zone are ‘validated’ by the dual-element detector and counted/sized by the OPC electronics.The result is a new sensor that can use a miniature low-power fan instead of a pump to generate the required particle flow through the sensing volume and does not require any form of particle filter. This obviates the need for periodic manual intervention to replace the conventional pump protection filter and instead allows long periods of unattended field operation.In the OPC-N1, each particle, at rates up to ~10,000 particle per second, is classified as to its particle size and added 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 typically 1 to 10 second duration, the histogram data being transmitted along with other diagnostic and environmental data (chamber temperature and air pressure) via an SPI interface to a host computer.Notes1 The patented OPC-N1 design results in virtually all sampled airborne particles passingstraight through the sensor without being deposited. This allows the OPC-N1 to operate for potentially very long (>1 year) periods without the requirement for regular maintenance or cleaning. If you are concerned about particulate/ contaminate build-up and loss of performance, then contact Alphasense.2 Particle coincidences (two particles within the sensing zone at the same time) may occur,though software algorithms partially correct for such effects. The probability of particle coincidence is approximately 0.24% for particle concentrations of 1000 per ml.3 As with most commercial Optical Particle Counters, all particles, regardless of shape, areassumed to be spherical and are therefore assigned a ‘spherical equivalent size’. This size is related to the measurement of light scattered by the particle by so-called Mie theory, an exact theory to predict scattering by spheres of known size and refractive index. The OPC-N1 is calibrated using Polystyrene Latex Spherical Particles of a known diameter and known refractive index. Correction factors may be applied in future versions of the PC software to reduce errors resulting from airborne particles of different refractive index. Alphasense Ltd Page 2 of 15 September 20143PM measurementsThe particle size histogram data recorded by the OPC-N1 sensor can be used to estimate the mass of airborne particles per unit volume of air, normally expressed in μg/m3.There are accepted International Standards Organisation (ISO) definitions of particle mass loadings in the air, for example: PM1, PM2.5 and PM10. These 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 out to approximately 10μm particles.The current version of OPC-N1 software does not calculate the ISO definition of the PM masses (although later versions will do this). Instead, the OPC-N1 calculates and plots a set of continually updated time-series graphs which show:- <=PM1 – ie: the mass (in μg/m3) of all particles below 1 μm in size.- <=PM2.5 – ie: the mass (in μg/m3) of all particles below 2.5 μm in size.- <=PM10 – ie: the mass (in μg/m3) of all particles below 10 μm in size.The conversion from the ‘optical size’ of each particle recorded by the OPC-N1 and the mass of that particle requires a knowledge of both the density of the particle material and its refractive index at the wavelength of the illuminating laser beam (658 nm). Since in most field measurement situations this information is not known accurately (and indeed there may be a variety of particles present with different densities and refractive indices), it is usual to apply a ‘correction factor’ to the particle mass as recorded by the OPC. This OPC-N1 software has a default setting for the correction factor which may be altered by the user if required to allow the measured mass to be aligned as closely as possible with the particle mass concentration in the air as measured by a direct method (eg: weighing particles collected on a filter).Note: The OPC-N1 calculations of particle mass assume a negligible contribution from particles below 0.38 μm, the lower limit of particle detection of the OPC-N1 sensor.4 Health and SafetyThe OPC-N1 uses a diode laser light source. This diode laser operates at typically 5-8 mW at a wavelength of 658 nm. This laser is classed as a 3B type laser; 3B rating includes laser light up to 500 mW power. This product is designed for OEM use and will normally be mounted in a secondary housing. We do not recommend that the user open or adjust any parts of the OPC-N1. It is the user’s responsibility to ensure that the unit is used safely.DO NOT remove the black cover: this protective cover not only ensures the required airflow but also protects the user from the laser light.Some of the components are static sensitive, so only touch the circuit when properly grounded and use correct procedures for handling electrostatically sensitive devices.There are no user-adjustable components.DO NOT adjust the laser power potentiometer: this will alter the calibration and may increase the laser power above typical 5-8mW.Alphasense Ltd Page 3 of 15 September 20145 Connecting power and taking readingsThe OPC-N1 is shipped pre-calibrated, with no user adjustments. Power and communications areprovided via the SPI socket.Connection can be either via the SPI/ USB connector (Alphasense part number 000-0SPI-00)direct to a PC or to your own circuit’s internal bus using your own-provided SPI interface.that you use the SPI/ USB cable. You canthen view the data options and data resolutionand range when the OPC-N1 is connected toyour PC, when the Alphasense-suppliedsoftware is installed (Apple compatiblesoftware is not yet available).The SPI/ USBconnection cable is shown in figure 1 below.The green LED shows that power is suppliedto the OPC-N1, and the red LED flashes whenthe PC and OPC-N1 are communicatingFigure 1. SPI/ USB cable. USB socket connects to USB port on PCs via USB/ USB cable.SPI ConnectionWhen connecting direct to your own SPI connector, the SPI pinouts are:Pin Function1Vcc2SCK3SDO4SDI5/SS6GNDPin 1 is closest to the mounting pillar and R29.6 Sampling the environmentAlthough the OPC-N1 controls the fan speed to ensure correct flow rate, do not include anytubing, valves, baffles or obstructions that will restrict air flow- the entire OPC is designed for lowpressure drops. The OPC-N1 works best when the inlet is exposed direct to the sampling volumeand the fan is blowing the exhaust into a large space.Alphasense Ltd Page 4 of 15 September 2014Alphasense Ltd Page 5 of 15 September 20147 Software installationThe examples below are for installation using Microsoft Windows XP and Microsoft Windows 7 It is recommended that the Windows PC is running .NET version 4.0 or above.Installing the device driver (Windows XP)Copy the folder “CD Version OPC Interface V7” 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.infwhich Windows needs to drive the OPC device.Alphasense Ltd Page 6 of 15 September 2014Click 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.Alphasense Ltd Page 7 of 15 September 2014Installing the device driver (Windows 7)Copy the folder “CD Version OPC Interface V7” 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 adriver problem.Alphasense Ltd Page 8 of 15 September 2014Right click the icon and select “Update Driver Software…Select the “Browse my computer for driver software” option.Navigate to the folder “CD Version OPC Interface V7” copied to your desktop and locate the folder named “USB Driver”, Click Next.Windows 7 will then issue a security warning. This is due to a licence issue and not a concern tothe operating system. Select the “Install this driver software anyway” option.Alphasense Ltd Page 9 of 15 September 2014Windows 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 noteof the allocated COM port number (COM4 in the example below)Alphasense Ltd Page 10 of 15 September 2014Incorrect installation of the device driverIf 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.single folder (to be kept intact) and will run as a normal Windows application. The entire folder canbe deleted when redundant.Alphasense Ltd Page 11 of 15 September 2014Alphasense Ltd Page 12 of 15 September 2014Connecting the device and running the softwareConnect the USB interface lead and OPC device to the PC, If you are promoted for device drivers refer to the previous section of the User Manual. Double click the COPCv7.exe icon to start the software application.When the application is first started the main form will be in “start-up mode” as shown below.Select the allocated COM port to begin.1. This shows a list of COM ports available on the PC, be sure to select the correct port number. The software will not respond unless a port with an OPC device attached is selected.2.This shows that the software application is waiting for a device to be attached.12Alphasense Ltd Page 13 of 15 September 2014Setting up and using the software• Set particle density (Default 1.65 g/ml): Adjust this to suit the environment of the device.•Device information (Right hand side text window): This shows the current information relating to the attached device.• Start sampling: Starts the data output with the fan and laser running continuously• Start sequence: Starts the data output with the fan and laser running cyclically• Repeat interval ms: This sets the interval at which the data is read from the device in milliseconds.• Y max: This sets the maximum count size of the histogram bins (Histogram functions only)•Histogram #/s: This shows the particle count per particle size.Alphasense Ltd Page 14 of 15 September 2014• Histogram #/ml: This shows the particle count per millilitre of sample.•History plot #/s: This shows the particle count per particle size Vs the time passed.Alphasense Ltd Page 15 of 15 September 2014• History plot mass conc: This shows the particle count of all three bins Vs the time passed.Log fileThe application will also create a log file from all of the output bins into a single .CSV file. Once either of the “start” buttons is selected you will be asked if you require a log file. If you select “yes” to this 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 versionsThere are two versions of the firmware currently available for the OPC-N1 device and each firmware version requires a different version of the SPI command list. The information window on the right of the main form shows the firmware version installed as the software first loads.1. V07e – This version of the firmware does NOT perform the calculation for PM1, PM2.5 and PM10values. Instead the software application calculates these values at run-time and displays the values as text (e.g “PM1 = 0.14 ug/m³”)2. V009 – This version of the firmware calculates the PM1, PM2.5 and PM10 values in addition to the values calculated by the software application. The firmware values for PM1, PM2.5 PM10 are displayed with an “i” suffix (e.g “PM1i = 0.14 ug/m³”). The PM values can be called via SPI command in version 009 of the firmware.3. V009a – This version of the firmware is identical to V009 but the SPI chip select signal is utilised.*The orginal version of the OPC interface will NOT support V009 of the firmware. Please use the new version of the application as this will support both V07e and V009 firmware versions.Shutting down the softwareIt is recommended that the software application be closed before removing the USB interface lead so as to avoid USB communication errors.End of User Manual。
