ADM-870 05/23/0827
9.0 AIR FLOW MEASUREMENT
9.1 FLOWHOOD FUNCTION
The AirData Multimeter uses 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
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.
The FlowHood is a much more convenient alternative to time consuming multiple velocity readings across air
diffusers, as this instrument virtually eliminates the use of A
k
factors, and the calculations necessary to convert
the average velocity into air flow.
Air flow readings taken using the AirData Multimeter are automatically corrected for the density effect due to
barometric pressure. If the temperature probe has been installed during the measurement, the air flow reading
is further corrected for the density effect due to the temperature of the air stream, and the result is corrected for
local air density. If flow measurements are initiated without the temperature probe, STD 70
/ F will be flashed
on the display immediately prior to the display of the air flow reading. The flow is calculated using an assumed
standard temperature of 70
/ F.
9.2 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 correction 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 is the only capture hood system which provides backpressure compensated
air flow measurement. This previously unavailable 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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