8
Control of Airflow
A qualitative indication of airflow can be monitored with the Magnehelic gauge. It is a good
practice to indicate the original reading, or the reading immediately following certification, on the
gauge with a marker or grease pencil. This will indicate the pressure that relates to airflows within
the acceptable range to provide containment and product protection. Since the change of flow is
related to the increased particulate load on the HEPA filters, which will be apparent by an
increased pressure reading on the Magnehelic gauge, the flow can be monitored. Since the
particulate loading of a HEPA filters happens slowly, except in dusty unconditioned environments,
a large change in the gauge reading in a short period of time will indicate cause for concern. Any
time a concern about the cabinet airflow is present you should call the GERMFREE’s customer
service department or your local certifier for advice prior to resuming operation in the unit. The
exact reading of the Magnehelic gauge on each unit may vary depends on several factors unique to
the individual unit such as resistance of a particular set of HEPA filters, electrical supply voltage,
motor-blower characteristics, etc.
As the filters become “loaded” with particulate matter, the motor blower will overcome a
considerable increase in resistance while maintaining a constant airflow. This automatic
compensation by the blower will cause only minimal changes in the reading of the Magnehelic
gauge. Eventually the resistance across the filters will increase to the point where, at a given setting
of the variable speed control the reading of the Magnehelic gauge increases. At this point it is
necessary to turn the speed control counter-clockwise to increase the motor speed to reset the
airflows (This operation should only be performed by a licensed certifier). This will again change the initial
reading of the gauge and change the location of the “mark” for qualitative airflow monitoring.
Depending on the cleanliness of the air of the room in which the BENCHTOP BIOFLOW
CHAMBER is located, the initial change may not occur for a year or considerably longer. Thus,
with new filters it is only necessary to examine the Magnehelic gauge approximately once a month.
Over time as the HEPA filters continue to load with particulates, mostly from the ambient air,
changes of the variable speed control will be required at increasingly shorter intervals to provide an
adequate airflow. To be sure your unit is functioning properly the velocity should be measured
directly by a qualified, third party, certification professional. Readings of air velocity, measured in
feet per minute, can be made with a velometer or thermoanemometer to determine if the unit is
within specification and when the filters should be changed. This certification should be
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