Definition of Free Area
Free Area is the total area in which air flows through a supply out-
let or a return grille. Free area is the effective area of the outlet or
grille and takes into account any restrictions.
If a grille is not used and there is no restriction on the area in
which air flows then the free area is equal to the actual area. This
is the case when measuring air flow at the middle of a duct, or if
the grille is removed from the return or supply duct.
When measuring airflow with a grille in place or restriction present,
the free area is the total area minus the area covered by the
restriction (grille fins).
Grille manufacturers publish free area specifications and this is the
most accurate information to use when setting up your DC580 to
measure air flow (volume).
Whenever possible it is best to use free area when entering duct
size. This will ensure the most accurate air flow reading.
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