Casella CEL Airborne particulate monitoring system - Manual (Page 47)

HB3256-03 Page 45
A record should be kept of the most recent correction values. An average of these should be
taken and applied to the reference filter value.
Example:- Reference filter reading = 82
Gravimetric correction factor = 1.32
Instrument adjusted to read (82x1.32) = 108
Note:- it is recommended that this new optical filter value is carefully recorded for future
reference.
Pump Flow Rates.
Purge Pump Flow Rate.
The purge pump supplies a source of clean dry air to the optical sensing head. This is used to
protect optical components from contamination and is also used to establish a zero dust
condition.
The purge flow rate can be measured by connecting a flow meter to the purge filter inlet. The
purge pump is set for a nominal flow rate of 1.1 L/min using the ‘PURGE’ adjustment on the
‘Power interface’ unit. A change in purge flow beyond ±0.2L/min may indicate that
maintenance of the purge pump is required.
During the measurement cycle the purge air is supplied from the outlet of the main flow pump.
Calibration of Inlet Flow Rate.
The U.S. E.P.A. currently reference mass concentrations to cubic metres of air based on a
standard temperature and pressure of 25°C and 1 atmosphere 760mmHg / 1013.2mB.
Because the APM uses a mass flow sensor, the inlet mass flow is unaffected by temperature and
pressure, however the volumetric flow rate and hence velocity through the inlet head will vary
with atmospheric conditions.
When work is carried out in conditions other than E.P.A. standard condition, the corrected flow
rate is the volumetric flow rate indicated by the calibration device under the prevailing
conditions.
The inlet flow should be measured using preferably a bubble meter or calibrated rotameter
corrected to establish the actual volumetric flow at the ambient conditions.