Determining Cross-Section Areas; Measuring Duct Velocity - TSI Instruments Alnor Velometer 6000 Series Operation And Service Manual

Centers of Equal
Concentric Areas
0.316R
0.548R
0.707R
0.837R
0.949R
Round Duct Traverse Points

Determining Cross-Section Areas

The cross-section of the duct is based on the actual inside duct area
through which air flows. If the duct is internally lined with insulation, the
thickness of that insulation must be taken into consideration when
calculating the duct area.
Example: 24" wide x 12" high duct with no internal insulation has a
cross-section area of 2 square feet.
38" wide x 14" high duct with 1" internal insulation has a
cross-section area of 3 square feet.

Measuring Duct Velocity

The velocity of an air stream in a duct is not uniform throughout the
cross-section; air near the walls moves more slowly due to friction.
Elbows, transitions and obstructions also cause variations in the velocity
at any one cross-section.
To obtain the average velocity in ducts of 4" diameter or larger, drill a ½"
diameter or larger hole in the duct and take a series of duct velocity
readings (commonly referred to as a traverse) with the Pitot Probe at
points of equal area across the duct. A formal pattern of sensing points is
recommended and these points are referred to as traverse point
readings. Shown are recommended velocity reading point locations for
traversing round and square (or rectangular) ducts.
Note: Pitot Probe Stations Indicated By .
Centers of Equal Areas
Square and Rectangular Duct Traverse Points
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