CFM-88L 07/20/0913
8.0 AIR FLOW MEASUREMENT
8.1 FLOWHOOD FUNCTION
The AirData FlowMeter utilizes the Series 8400 FlowHood Kit for backpressure compensated measurement of air flow. The
FlowHood unit captures and directs the air flow from an outlet, or inlet, across the highly sensitive flow sensing manifold
within the FlowHood base. This manifold simultaneously senses the total pressure, and the lee side “static” pressure, at
sixteen precision orifices spaced at the correct representative measurement points for the known cross-sectional area of
the FlowHood base. The sensed total pressure and “static” pressure are combined to a single differential pressure, which
is transmitted to the meter for conversion to direct air flow readout.
Air flow readings taken using the AirData FlowMeter are automatically corrected for the density effect of barometric
pressure. If the temperature probe has been installed during the measurement, the air flow reading is further corrected for
the density effect of the temperature of the airstream, and the result is corrected for local air density. If flow measurements
are initiated without the temperature probe, STD 70
/F or STD 21.1/C will be flashed on the display immediately prior to the
display of the air flow reading. The flow will be calculated using this assumed standard temperature.
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.
8.2 BACKPRESSURE COMPENSATION
The air flow delivery of a supply or return outlet will be reduced to some variable degree whenever any capture hood device
is placed over the outlet. The degree of flow reduction is a function of the capture hood resistance combined with the outlet
resistance for a given air flow. A duct velocity traverse is often used as a reference air delivery test, to determine the
"average" backpressure compensation factor for a particular system. This "average" correction factor does not specifically
apply to each outlet, but only to the average outlet for that system. This method, commonly used in the air balance industry,
may result in significant inherent errors, particularly in flow readings taken at low resistance outlets, and also on the larger,
more efficient, low resistance, ducted or "extended plenum" types of air delivery systems.
The FlowHood air balance system provides backpressure compensated air flow measurement. This capability allows the
operator to determine the flow that is passing through an air terminal without the added pressure loss caused by the
FlowHood System. The backpressure compensated flow value is obtained through a two part measurement performed at
each duct terminal, using the flaps feature of the FlowHood unit. The backpressure compensated measurement is always
performed following a required preliminary nonbackpressure compensated measurement taken with the flaps open.
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