072-0300
Alphasense User Manual
OPC-N2 Optical Particle Counter
Issue 5
Alphasense Ltd Page 4 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
Email: [email protected] - Web: www.alphasense.com
3
Health and Safety
The OPC-N2 uses an embedded diode laser light source that operates at typically 5-8 mW (max. 25
mW) 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 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-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- 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 are: density 1.65 g/ml, RI 1.5+i0. Respiratory
profiles are included in the PM calculations.
An additional weighting is applied on units with Firmware 18 or higher to account for under counting
at low particle sizes and the effect of carbon particles in urban air so that the output matches co-
located reference detectors. This correction can be retrofitted to units with older firmware, contact
Alphasense for additional information.
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 (which can be frequent in dirty atmospheres).
The Alphasense OPC-N2 uses a different design: the pump and replaceable particle filter have
been removed. 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 airflow through the OPC is produced by a low power microfan.
