CFM-88L 07/20/0926
Where: P
b
= local barometric pressure (in Hg)
13.5 DUCT TRAVERSE USING THE AIRDATA MULTIMETER
No corrections for density are necessary when an AirData Multimeter is used for the comparison pitot tube traverse if use
of the TemProbe is consistent during both the air flow measurement using the FlowHood and the velocity measurement
using the AirData Multimeter and pitot tube. This means that, if the TemProbe is used, it must be used for both types of
measurements.
If the TemProbe is not used for either flow or velocity, it must not be used for the other type of measurement. In this case,
both sets of measurements will be calculated using standard 70
/F (or 21.1/C) conditions.
13.6 BACKPRESSURE COMPENSATED COMPARISON READING
Perform a multipoint pitot tube traverse of a supply duct which serves a single supply diffuser. Calculate the air flow with
careful attention to all of the factors discussed in this section. A backpressure compensated reading on this same diffuser
should be within ± 5% of the air flow calculated from this traverse.
13.7 NONBACKPRESSURE COMPENSATED COMPARISON READING
After determining the air delivery by duct traverse, measure and record the duct centerline velocity, both with and without
the FlowHood (flaps open) in place. (A reduction in the duct centerline velocity when the FlowHood is placed over the
diffuser, is caused by the flow resistance (backpressure) of the FlowHood).
A flaps open, nonbackpressure compensated FlowHood reading on this same diffuser should be within ± 3% of FLOW
2
(calculated from the ratio of the two centerline readings).
FLOW
2
is obtained from the following calculation:
Where: Flow
1
= flow calculated from duct traverse
Flow
2
= flow in duct with FlowHood in place (flaps open)
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