Casella CEL Airborne particulate monitoring system - Manual (Page 50)

HB3256-03 Page 48
Before commencing the flow calibration, the system will wait for the inlet heater to reach the
correct temperature. Once the heater column has reached 55°C, the purge and main pump will
be switched on for a short period. This brings the flow sensor near to its operating temperature.
Both pumps will then be turned off for a few seconds and a 'Zero flow' signal will be read from
the mass flow sensor.
Both pumps will then start running again. The 'FLOW' adjustment on the power interface
should be adjusted to provide a flow meter indication of 16.7L/min (if the ambient conditions
are close to the 'Standard Conditions') or alternatively the corrected flow rate to suit ambient
temperature and pressure. The P.C. display will indicate the required 'M' and 'C' values for
calibration of the mass flow sensor.
Calibration constants for the flow sensor should be entered into the following line of the
program file ‘COMMAND.CMD’,-
Y12=-5.368,0.0116"l/min."
(Y12=Cvalue,Mvalue"l/min.")
Temperature Corrected Volumetric Flow.
For systems exposed to wide variation in temperature or pressure, or where volumetric flow
control is preferred, a temperature corrected volumetric flow control is available.
The signal from an ambient air temperature sensor is connected to the 'Temp' input of the power
interface unit. The signal should be 0-1V for -40 to +60°C.
The temperature signal is used to modify the pumps mass flow rate to maintain a more stable
volumetric flow. Changes in ambient pressure will result in a small change of volumetric flow.
A change of approximately ±3% in the volumetric flow can be expected as a result of typical
variations in atmospheric pressure (±30mB).