072-0500
Alphasense User Manual
OPC-R1 Optical Particle Counter
Issue 1
Alphasense Ltd
Page 4 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: [email protected] - Web: www.alphasense.com
3
Health and Safety
The OPC-R1 uses an embedded diode laser operating at 4-8 mW (max. 11 mW) at a wavelength of
639 nm. The OPC-R1 is a Class 1 laser product, since the user does not have access to the laser
source. The OPC-R1 is designed for OEM use, normally mounted in a secondary housing. The
user must not open or adjust any parts of the OPC-R1. 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-R1 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- PM
1
, PM
2.5
and PM
10
,
are then calculated from the particle size spectra and concentration data, assuming a particle
density and refractive index (RI). Default settings, weighting index 2 are: density 1.65 g/ml, RI
1.5+i0. Respiratory profiles are included in the PM calculations. It is also possible to select PM
4.25
instead of one of the other PM values.
The OPC-R1 contains 10 weighting index sets, each comprising a weighting value for each of the
16 size bins. Index Set 0 can be adjusted by the end user; the other 9 are factory set (See later for
more information).
The OPC-R1 classifies each particle size, at rates up to ~10,000 particles per second, recording the
particle size to one of 16
“bins” covering the size range from 0.35 to 12.4 μm. Please note the
maximum particles per second and the maximum mass detectable are based on the potential
performance of the electronics and will be reduced in actual measurements. The resulting particle
size histograms can be evaluated with user-defined sampling times from 1 to 30 second duration.
Longer times in dirty environments with high particle concentrations can result in the bins
“over
filling
”. If longer periods are needed it is recommended to undertake the averaging of shorter
measurement periods. This histogram data is transmitted via an SPI interface to a host computer.
The OPC-R1 is designed to minimise particle deposition within the unit and thus allow for prolonged
unattended operation in dusty environments. We would recommend the use of the OPC-N3 in very
dusty environments.
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-R1