ADM-860C 04/14/0849
controller responds to the additional backpressure and maintains the preset air flow. If the controller is slow
acting, the FlowHood must be held in place long enough for the controller to re-attain the set point.
Master and slave, volume controlled outlets often do not interact favorably with the FlowHood backpressure
compensation function. Air delivery measurement for this type system should be performed using a
nonbackpressure compensated measurement.
12.5 LINEAR SLOT DIFFUSERS
Linear slot diffusers deliver supply air in a sheet or air curtain that tends to follow the side of the cloth transition
as it flows to the FlowHood base. This can result in an extremely uneven velocity distribution across the flow
sensing grid when the flaps are open. The air flow is more evenly distributed across the flow sensing grid when
the flaps are closed.
Linear slot diffuser readings at less than 100 cfm per lineal foot of diffuser should be taken with the FlowHood
flaps closed. The AirData Multimeter will permit flaps closed, nonbackpressure compensated readings up to
500 cfm. In most cases, the FlowHood backpressure effect on linear slot diffusers is not significant. Accurate
readings can normally be obtained with flaps open readings on linear diffusers sized for more than 100 cfm per
lineal foot.
The tendency of a sheet of air to cling to, or follow the side of the FlowHood top, can be reduced by using a top
width dimension that is as close as possible to the outlet width. Special transition tops can be constructed of
corrugated cardboard and duct tape. Custom dimension, cloth and frame top sizes may be ordered from our
factory. A commonly used size is 5.25" x 47" with a 4.25" x 46" inside frame dimension.
12.6 SIDEWALL REGISTERS
The FlowHood may be used to measure air delivery of supply sidewall registers using the standard 30" length,
2'x2' top or larger top sizes. It is best if the top dimensions closely match the outlet size to assure even air flow
across the flow sensing grid. Exact matching of hood size to the outlet size is not practical in most cases.
The standard 30" length top sizes may be used with equal accuracy on supply or return air outlets. However,
small supply outlets at high jet velocity, "Punca" diffusers or nozzle type outlets may cause an extreme
concentration of air velocity on portions of the flow sensing grid. The FlowHood readings will be inaccurate under
such conditions.
Reading accuracy is improved by adjusting the register deflection blades to a four-way spreading pattern before
starting the measuring and balancing process. The deflection can then be readjusted for the desired deflection
pattern after the final balancing and readings are completed. Resetting deflection patterns after air balance
usually has little effect on the air delivery. Special top sizes may be ordered from our factory. The most popular
size for sidewall outlets is 1' x 2' (13"x 25" outside dimension).
12.7 14"x14"x14" SHORT TOP SET
This short top design is intended for use on return or exhaust air outlets. The use of this top size on deflection
type supply outlets such as multiblade diffusers or registers with the blades set for a spreading pattern is not
recommended. The 14"x14"x14" top is too short to provide the attenuation of the jet velocities necessary to even-
out the air flow over the flow sensing grid and readings will not be reliable.
Reasonable accuracy can only be obtained on supply outlets when using the short top set if the outlet is at least
12" x 12" with no air deflection, such as a perforated plate, bar grille, egg crate grille, or unfinished opening.
12.8 SYSTEM PROBLEMS
If the measured air delivery is less than expected, check the meter functions first, as described in Section 14.0
METER ACCURACY FIELD TESTING. If the meter is functioning properly, check significant sources of error
such as: duct leakage; loose outlet connections; missing or loose end caps; belt slippage; backward fan rotation;
closed or partially closed dampers of any type; dirty filters or coils; foreign material caught in turning vanes or fan
wheels or other locations where obstructions can occur; square elbows without turning vanes; radical duct
transitions; and any other situations where faulty system design or installation errors can cause inadequate air
delivery.
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