ADM-870 05/23/0819
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 or safety cabinet. 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 can not 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.
The standard AirFoil probe is a straight probe. It is often difficult to position the standard probe across an
exhaust hood opening if the hood opening frame has some relief depth on the sides and at the threshold.
Special pattern AirFoil probes are available that have the end of the probe at 90° to the shaft. These probes are
more easily positioned in such hood openings and are designed to fit in the AirData Multimeter accessory kit
carrying case.
6.3.4 LAMINAR FLOW WORKSTATIONS - AIRFOIL PROBE
The AirFoil probe may also be used to measure face velocities and work zone velocities for very small sample
areas. The average of several readings must be used to represent small sample areas, due to the variability
of air flow at low velocities. Readings that vary ± 50% are not unusual when taking single point velocity readings.
The more variable the readings, the more readings must be included in the average obtained at each location.
Ten readings per sample point is usually adequate.
6.4 VELGRID AIR VELOCITY
IMPORTANT: See Section 6.1 VELOCITY CORRECTION FACTORS
The VelGrid accessory is designed especially for use in the measurement of general "face velocity" conditions,
such as HEPA clean room filter outlets; laminar flow work benches; exhaust hoods; terminal air face velocities;
and large filter bank and coil face velocity measurements. The measurement range using the VelGrid is 25 to
2500 fpm. Each reading represents 16 velocity points over a 14"x14" area (1.36 sq ft).
The VelGrid unit is assembled by attaching the pushbutton handle to one of the captive knob screws of the
handle bracket, and attaching one or more of the extension rods to the other end of the handle bracket as shown
in Figure 6.4. The VelGrid swivel bracket is attached to the extension rod end. The two hoses are connected
to the VelGrid hose connectors and to the ports on the meter. The pushbutton handle cord plugs into the
external read jack on the left side of the meter. A neckstrap is provided with the VelGrid to support the meter
and allow hands free operation. The TemProbe must also be used as discussed if full density correction for
temperature is required. If the TemProbe is not used, the velocity will be calculated using STD 70
/ F.
After turning the meter on, press the MODE key until VelGrid is displayed, followed by FP: READ. Press the
READ key to initiate the actual measurement. (Press the STORE key if the memory function is desired). STOR
RDY will be displayed.
Place the VelGrid directly on the face of the filter or coil, with the standoff spacers of the grid against the outlet
or inlet face. When placing the VelGrid near the edges of the filter, grille, coil face, or other opening, the
perimeter standoff spacers should be at least 1.5" from the edge of the active face area. This 1.5" margin
maintains the proper "traverse" spacing of the velocity sample points at 3.5" centers on a 14"x14" area.
Overlapping of reading positions is better than getting too close to the face area edges. If the dimensions of the
outlet are smaller than the VelGrid, the orifices of the grid that are not directly exposed to the air flow must be
covered with tape. All unused orifice positions on both sides of the grid manifold must be covered. Note: The
VelGrid temperature exposure limit is 0
/ F to 140/ F.
The VelGrid is bidirectional in function. A negative sign will be displayed if the hose connections are reversed
or if the air flow direction is reversed in relation to the higher pressure side of the VelGrid. If you know that the
hoses are reversed, you may disregard the negative sign.
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