CFM-88L 07/20/0920
10.0 FLOWHOOD OPERATING PROCEDURE
The meter is inserted into the recess in the FlowHood base, using the captive fastener inside the base to secure it. The two
pneumatic tubes from the flow sensing grid attach to the pneumatic inlets on the meter. The tube from the top side of the
grid connects to the positive (+) pneumatic inlet on the meter. It is very important that these connections are not reversed,
since this will cause false flow measurements. The FlowHood flaps plug inserts into the flaps receptacle on the back of the
meter.
The thumbswitch plug connected to the handgrip is inserted into the external read jack on the left side of the meter. The
TemProbe sensor plugs into the temperature input jack on the back of the meter through the cutout designed into the
FlowHood base.
CAUTION: Avoid possible damage by removing all three electrical connections and both pneumatic connections before
removing the meter from the FlowHood base.
Turn the meter on. The display will then read FLO-HOOD, followed by CF READ. After the operator presses the READ key,
either CF UNCORR or CF CORR will be displayed during the measurement period if the TemProbe is being used. If the
TemProbe is not being used, the standard temperature (70
/F or 21.1/C) will be displayed following LOCAL DENS. Each
subsequent air flow measurement is triggered by pressing the READ key.
Place the FlowHood over the outlet (or inlet) diffuser or grille, so as to capture and direct the air flow through the FlowHood
base. The foam gasket around the edge of the top assembly should be firmly pressed against the ceiling or the edges of
the diffuser. Air flow through the unit should not be blocked or influenced by any object within one foot of the FlowHood base.
The unit should be supported with both hands, one on the handle, and the other at any convenient point on the outside of
the base. Set the flaps control knob to the open position.
Press the READ key or the thumbswitch on the FlowHood handle and the reading will be displayed directly in cfm or
liters/sec. A negative number indicates that the air flow is being exhausted through a return outlet. A second reading with
the flaps closed will be necessary to determine the actual air flow.
NOTE: The FlowHood may require the development and use of correction factors when used on swirl diffusers, or on other
types of diffusers with uneven air throw. The FlowHood may not be appropriate for use on small supply outlets at high jet
velocities or “nozzle” type outlets. These outlets cause an extreme concentration of air velocity on portions of the flow
sensing grid. The FlowHood readings may be inaccurate under such conditions.
Consideration must be given to other system components, such as may be encountered on some single supply air outlet
applications, where the FlowHood's slight backpressure may directly affect fan performance.
SAFETY NOTICE: When using the FlowHood to test air flow at ceiling outlets, be sure that you can safely raise and support
the unit during the measurement. This is especially important when working on a ladder.
10.1 AIR FLOW - NONBACKPRESSURE COMPENSATED READINGS
See Section 8.2 BACKPRESSURE COMPENSATION for a general discussion of backpressure and its effect on any capture
hood device before using the FlowHood for nonbackpressure compensated readings. This technique measures air flow
without compensating for FlowHood induced backpressure. This method may be used for both supply and return outlets.
After the meter and all of its connections have been attached to the FlowHood, as described previously, set the flaps control
knob to the open position. Place the FlowHood over the outlet (or inlet) diffuser or grille. Wait two or three seconds to allow
the air flow to stabilize, and then press the READ key or the thumbswitch on the FlowHood handle. The displayed result will
represent the flow value of the outlet while the FlowHood is in place. This flow reading will generally be somewhat less than
the actual flow, due to the backpressure effect of the FlowHood.
10.2 AIR FLOW - BACKPRESSURE COMPENSATED READINGS
Air flow through an outlet (or inlet) is reduced to some degree by any capture hood device placed over the opening.
Correction curves had traditionally been used to provide an average correction factor for the backpressure effects at different
air flows.
This method measures air flow with compensation for the FlowHood induced backpressure automatically calculated into the
displayed result. Backpressure compensation requires two readings in immediate sequence. The first reading is the flaps
open reading discussed above. The second reading is done with the flaps closed. The meter automatically determines the
required backpressure compensation calculated from the relationship between the two readings, and displays the
compensated result. A backpressure compensated reading must always be immediately preceded by a nonbackpressure
compensated (flaps open) reading at the same location. The meter will register incorrect readings if the CF UNCORR
reading is taken at another time or any other location. Any adjustments or changes to the air flow system between the two
measurements will result in inaccurate results.
This technique may be used for either supply or return outlets. Backpressure compensated readings may be performed at
any flow rate, but are usually unnecessary at flows less than 100 cfm.
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