Appendix A. Back Pressure
The quantity of air flowing through a supply diffuser or a return grille is reduced to some extent whenever
a capture hood is placed over the flow opening. The amount of flow reduction will vary depending on the
combined effects of the diffuser/grille resistance, the capture hood resistance, system pressure and the
flow rate.
The PH731 capture hood features a built-in capability to make flow measurements which are
compensated for the effects of the capture hood-induced back pressure. This is accomplished by taking
OPEN
CLOSED. The resulting Backpressure Compensated Flow displayed by the
meter represents the air flow through the diffuser/grille in its original state, that is, without the presence of
the capture hood.
Verifying Flow Measurements
It is always the recommended practice to verify the flow measurements obtained with a capture hood by
performing appropriate
*
multi-point, in-duct velocity traverses using a Pitot-static tube or a thermal
anemometer.
We recommend the use of the Log-linear method when traversing a round duct, and the Log-Tchebycheff
method when traversing a rectangular duct.
*
We recommend that you refer to the most up-to-date copy of the duct traverse specification you require from an
approved regulatory or professional organization.
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