ADM-860C 04/14/0830
6.3.1 DUCT VELOCITY USING AIRFOIL PROBE
The scribed rings on the AirFoil probe shaft are located at one inch increments from the tip orifice, and are
provided to assist in controlling the probe measurement depth during duct velocity readings. It is helpful to apply
a single wrap of electrical tape around the probe shaft at each desired depth increment to mark measurement
points.
Negative air velocities may exist in some areas of a duct traverse due to turbulence or eddy currents. The
AirFoil probe tip is designed to provide equal differential pressure for velocity in either direction across the tip.
Therefore, it is recommended that the negative velocity readings be included in the averages of the readings
taken with the AirFoil probe.
6.3.2 FUME HOODS AND SAFETY CABINETS - AIRFOIL PROBE
The AirFoil probe may be used to test the downflow air pattern and average velocity in the horizontal plane at
the sash height of safety cabinets. This test is usually done at 6" centers on a 6" x 6" traverse pattern and at
8" or 10" above the work opening threshold. This is normally 9" to 11" above the work surface pan.
Position the AirFoil probe horizontally and up against the bottom edge of the sash door. Tape markers on the
AirFoil probe and along the sash door edge at 6" centers will aid in accurate positioning of the AirFoil probe.
The individual readings for the downflow grid should be recorded to establish the uniformity, or zone velocity
profile, and compliance with the standards required. Also the average of the downflow readings may be used
to calculate the downflow cfm if required. The work opening face velocity on total exhaust cabinets may be
tested in a manner similar to procedures for fume exhaust hoods. The velocity sample grid should be a 4"
square grid for 8" sash height and a 5" grid for 10" sash settings. When calculating average velocity or total
flow, the "Kv" factor must be taken into account as discussed in Section 6.1 VELOCITY CORRECTION
FACTORS.
The exhaust filter face velocity may also be tested with the AirFoil probe to determine exhaust air flow. The
cabinet manufacturer's probe position schedule should be used as a guide. The AirFoil probe readings have
been found to be essentially the same as "hotwire" anemometer readings (corrected to local air density) taken
in laboratory and field condition testing of filter discharge face velocity.
NOTE: The exhaust air flow is most accurately determined by direct air flow measurement using the FlowHood.
The 1' x 4' top assembly should be positioned so as to capture all of the intake air at the work opening. This
may require the use of masking tape and materials to block off part of the opening, depending upon the size of
the cabinet.
6.3.3 EXHAUST HOODS - AIRFOIL PROBE
The AirFoil probe provides single point air velocity samples and may be used to collect data for graphing face
velocity profiles at exhaust hoods.
Air flow at the extremely low velocities (50 to 150 fpm) used in chemical exhaust hoods and safety cabinets will
show significant percentage variability at any given point (slight fluctuations in velocity represent a very large
percentage fluctuation at low velocities). Readings should be repeated several times at each sampling point
to obtain an average velocity reading for that point.
The face of the exhaust hood should be divided into a grid with each section of the grid representing an equal
area division of the exhaust hood. The equal area divisions are often set at 6" x 6", and seldom need to be set
at less than 4" x 4". Each velocity sampling location should be at the center of an equal area division of the grid.
All equal area divisions should be tested. The leading edge of the AirFoil probe should be directly in line with
the plane of the sash while taking face velocity sample measurements.
The actual airstream direction is usually at various angles to the plane of the opening around the sash perimeter,
so velocities cannot be reliably measured near the edge of the opening. The tip of the AirFoil probe must be
positioned at least 2 inches from the edge of the sash opening of the exhaust hoods.
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