Casella CEL Airborne particulate monitoring system - Manual (Page 48)

HB3256-03 Page 46
The following equation is used to determine the corrected flow rate (volumetric) which must be
measured by the calibration device to suit conditions other than the standard of 25°C and
1013.2mB.
QI = QS (PS / PI) x (TI / TS)
Where;
QS= Flow under standard conditions (16.7L/min flow)
QI= Calibrator flow reading (Volumetric L/min) at given temperature and pressure.
PS= Standard Condition Pressure (1013.2 mb / 760mmHg)
PI= Indicated pressure at time of measurement.
TS= Standard condition temperature in °K (25°C + 273.2)
TI= Indicated temperature at time of calibrator flow reading in °K
Therefore, for standard conditions (equivalent mass of air as 16.7L/min at 25°C and 1013.2mB),
the measured volumetric flow at the prevailing temperature and pressure should be:-
QI = 3.3977 x 16.7 x (TI/PI)
i.e. If the ambient conditions at calibration are 19°C and 1017mB then the calibration device
will indicate a flow of ;
3.3977 x 16.7 x (292.2/1017)=16.3L/M
Flow Sensor Calibration Utility.
The 'Special' menu of the P.C based software incorporates a 'Test' function which may be used
to calibrate the systems mass flow sensor.
The correction of flow is not necessary if prevailing conditions are close to the standard values.
Correction may be required the ambient temperature or pressure is significantly different to the
standard value due to say climatic conditions, geographic location or altitude.
Special Cases / Extreme Conditions.
When a system is located at a site with average conditions very different from the standard,
whilst the mass flow will remain the same, the inlet flow (and hence inlet velocity) will be
different. The size selective characteristics of the head will be affected by this velocity variation.
In these circumstances it is best to adjust the flow rate to provide the correct volumetric flow
based on standard conditions reflecting the sites seasonal average temperature and pressure.