072-0500 Alphasense User Manual
OPC-R1 Optical Particle Counter Issue 1
Alphasense Ltd Page 5 of 33 February 2019
Sensor Technology House, 300 Avenue West, Skyline 120, Great Notley. Essex.CM77 7AA. UK
Tel: +44 (0) 1376 556700 - Fax: +44 (0) 1376 335899
Email: sensors@alphasense.com - Web: www.alphasense.com
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 or refractive
index.
5 PM measurements
The particle size histogram data recorded by the OPC-R1 sensor can be used to calculate the mass
of airborne particles per unit volume of air, expressed as μg/m3.
The accepted international standard definitions of particle mass loadings in the air are PM2.5 and
PM10, PM1 is not yet an international standard. These definitions relate to the mass and size of
particles that would be inhaled by a typical adult.
The OPC-R1 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-R1
and the mass of that particle requires knowledge of both particle density and its RI at the
wavelength of the illuminating laser beam, 639 nm. The OPC-R1 assumes an average RI value of
1.5 + i0. The OPC-R1 allows a different value to be set for each size bin to correct for particle
density variation with particle size. The OPC-R1 has 10 bin weighting indexes, one is adjustable by
the end user and 9 are factory set. Please note that not all of these indexes will be defined in early
released units.
The default setting for each size bin assumes a Particle Density value of 1.65 g/ml, bin weighting
index 2, a figure that equates to a typical value found in many environments. Bin weighting index 0
can be modified by the end user, default setting is currently 1 for all bins.
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. The other indexes will be preset with values adjusted for common ambient
environments and to ensure that the OPC-R1 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.
Contact Alphasense on how to modify standard settings.
Note
·The EN 481 standard definition for PM10 extends to particle sizes beyond the upper
measurable size limit of the OPC-R1. In some cases, this can result in the reported PM10
value being underestimated.
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. Therefore tubing, valves, baffles or obstructions that will
restrict air flow into or out of the OPC should be avoided. Particle distributions can also be affected
by sharp turns and narrow sample pipes. The maximum pressure drop through the entire flow
system must be less than 40 Pa.
However, because fan speed can vary, the sample flow rate through the OPC may vary also. 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 (patent pending).
The OPC R1 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-R1 inlet is exposed directly to the target sample volume and
that the fan exhausts into an unconstrained space. An exhaust extension is provided to facilitate
this. The OPC-R1 can be positioned in any orientation, but 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 to avoid very large droplets or soot and grit from entering the unit due to gravity.
We recommend an “umbrella”or mushroom to protect the inlet but that does not interfere with
