ADM-860C 04/14/08
39
9.0 AIR FLOW MEASUREMENT
9.1 FLOWHOOD FUNCTION
The AirData Multimeter 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 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 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.
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 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 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
.
Summary of Contents for ADM-860C
Page 38: ...ADM 860C 04 14 08 33 FIGURE 6 3 VELGRID ASSEMBLY...
Page 47: ...ADM 860C 04 14 08 42 FIGURE 10 1 FRAME STORAGE FIGURE 10 2 FLOWHOOD IN CASE...
Page 48: ...ADM 860C 04 14 08 43 FIGURE 10 3 FLOWHOOD ASSEMBLY...
Page 50: ...ADM 860C 04 14 08 45 FIGURE 10 7 1X5 FRAME ASSEMBLY FIGURE 10 8 3X3 FRAME ASSEMBLY...
