Casella CEL Airborne particulate monitoring system - Manual (Page 49)

HB3256-03 Page 47
Example;
Average Temperature = 5°C, Average Pressure = 1020mB
Conditions at time of calibration = 8°C, 1018mB
Under the measurement condition, if the mass flow sensor maintains 16.7L/min (referenced to
25°C and 1013.2mB) then the actual measured inlet volumetric flow would be:-
QI = 3.3977 x 16.7 x ( (273.2+8)/1018) = 15.67 L/min
(i.e. Inlet flow is 6.1% Low).
In this situation it is best to assume that the flow rate of 16.7 L/min occurs at standard
conditions of 5°C and 1020mB. To do this we must re calibrate the mass flow sensor to provide
an output of 16.7 L/min at these ‘new standard’ conditions.
If the conditions at the time of calibration are now 8°C and 990mB, then the flow control should
be adjusted such that the calibration device measures;-
QI = 16.7 (1020 / 1018) x ( (8+273.2) / (5+273.2) ) = 16.9 L/min
(i.e. 1.2% High).
When the temperature and pressure return to the standard (average) conditions, then the inlet
flow will be exactly 16.7L/min.
Calibrating the Mass Flow Sensor.
If the ambient temperature and pressure conditions are significantly different to the standard
values, then the 'Special' 'Test' utility should be used to calibrate the systems mass flow sensor.
The calibration routine is accessed using the 'Test' option from the 'Special' menu.
The inlet flow rate should be checked typically every 6 months. The use of a bubble meter is
recommended although a rotameter may be used if lower accuracy is acceptable.
Remove the sample head and connect a flow measurement device to the sample inlet tube.
Select and activate the 'Test' option from the 'Special' menu.