072-0300
Alphasense User Manual
OPC-N2 Optical Particle Counter
Issue 5
Alphasense Ltd Page 6 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
Note
•
The EN 481 standard definition for PM
10
extends to particle sizes beyond the upper
measurable size limit of the OPC-N2. In some cases, this can result in the reported PM
10
value being underestimated by up to ~10%.
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. Considering this problem- tubing, valves, baffles or
obstructions that will restrict air flow into or out of the OPC should be avoided. Particle distribution
can also be affected by sharp turns and narrow sample pipes. Maximum pressure drop through the
entire flow system must be less than 75 Pa and ideally less than 40Pa.
However, because fan speed can vary and external factors such as wind direction in the vicinity of
the OPC may affect the sample flow rate through the OPC, 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.
The OPC N2 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-N2 inlet is exposed directly to the target sample volume and
that the fan exhaust is left free to exhaust into an unconstrained space. The OPC-N2 can be
positioned in any orientation. However, 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 in
this case to avoid very large droplets or soot and grit from entering the unit due to gravity. Some
form of “umbrella” or mushroom should be used which protects the unit but does not interfere with
particle flow. Coarse gauze filters can also be used to prevent the ingress of large particles or
insects without interfering with the fine particles being measured. All electrical connections must be
protected from moisture and temperatures outside of the operational range. Please note at
temperatures between –20 and –10°C the OPC is reliant on its own generated heat to ensure the
laser is operating correctly, thermal lagging of the secondary housing or heating may be required.
