072-0300 Alphasense User Manual
OPC-N2 Optical Particle Counter Issue 5
Alphasense Ltd Page 5 of 34 December 2015
Sensor Technology House, 300 Avenue West, Skyline 120, Great Notley. Essex.CM77 7AA. UK
Tel: +44 (0) 1376 556700 - Fax: +44 (0) 1376 335899
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.
