Basic Documentation

Page 4 of 6 Siemens Industry, Inc.
Document No. 149-989
Establishing the Proper Average
Face Velocity
Once a fume hood is installed and set up in an
actual laboratory room, it is recommended that
tracer gas containment tests
11
be conducted to
determine the appropriate face velocity necessary to
maintain a desirable level of containment. Once the
average face velocity that will provide the desired
containment level is determined, that face velocity
can serve as a reasonable benchmark for safe fume
hood operation. This assumes that the room
conditions essentially remain as they were when the
fume hood was tested and that the fume hood users
follow safe fume hood working practices.
Note that since a fume hood is not an airtight
enclosure, perfect or 100% containment is not
generally attainable or even necessary. The level of
containment that is necessary should be established
by facility personnel who are responsible for worker
safety in consideration of the hazard level posed by
the chemicals. If the chemicals or substances that
will be present are so toxic or hazardous that 100%
or near 100% containment is required, than an air
tight glove box should be utilized rather than a fume
hood.
Although it can be time consuming and costly to
conduct fume hood containment tests, the actual
quantity of individual tests may be reduced in
consideration of the following factors:
 Fume hoods that are of the same size and type
and in the exact same location in multiple
identical laboratory rooms need not all be tested
if the laboratory rooms have the same airflow
characteristics. This would include ventilation
system configuration, airflow quantities and
airflow components including the air diffusers
and diffuser locations. Thus, if a facility has
many laboratory rooms that are duplicates of
each other and the room airflows have been
closely set to the same parameters or
specifications, a sampling of fume hoods may be
tested (termed a sample group) rather than
testing each and every fume hood.
11. ASHRAE 110 defines two types of on-site containment tests:
AI (as installed) and AU (as used). The AI test conditions
include the room parameters (air currents, etc.) while the AU
tests also include the actual fume hood contents. AIHA Z9.5
lists ASHRAE 100 tracer gas pass criteria of no greater than
0.10 ppm for AI tests.
 To qualify as part of a “sample group” the fume
hoods must all be in the exact same location in
each of the “identical” laboratory rooms. If a
room has multiple fume hoods, each different
fume hood location then establishes a separate
sample group. (Thus, if a room has four fume
hoods, each of the four fume hoods would be in
one of four different sample groups.)
 All room ventilation parameters must be
adjusted and balanced before the fume hood
containment tests so there will be no
appreciable airflow difference from room to
room. This especially applies to the supply
make-up air, room general exhaust, and any
other room airflow elements such as specialized
exhausts. If any room must be set up different
than the other rooms, its fume hoods cannot be
considered part of the other room’s sample
groups.
 It is recommended that 10% (with a minimum of
3) of the fume hoods from each sample group
be individually tested. More fume hoods should
be tested if the test results of a sample group
are not appreciably similar. Perhaps a variation
of more than 25% in test results would justify
additional testing.
 The safety standards advise that periodic re-
testing to verify fume hood containment should
still be done on an annual basis, or whenever
changes are made to the ventilation system.
Using the above sampling approach could reduce
the time and expense of testing in new facilities.
12
For example, if a facility had 100 laboratory rooms
and each room had two fume hoods of the same
size and type, the containment testing could possibly
be conducted on only two sample groups having 10
fume hoods per group. Then rather than 200
separate fume hood containment tests, perhaps only
20 fume hood tests would suffice.
12. Due to the inevitable changes that occur over time in
laboratory rooms and their ventilation systems the sample
group fume hood testing approach is not always applicable
in existing facilities.