Airflow Instruments ProHood PH731 - Traversing a Square Duct

Appendix B: Traversing a Duct to Determine Average Air Velocity or Volume 65
Traversing a Square Duct
Using the log-Tchebycheff method, the duct is divided into rectangular areas, which are further adjusted
in size to account for effects of the duct wall on the airflow. A minimum of 25 points must be measured in
order to get a good average. The number of data points to be taken along each side of the duct depends
on how wide that side of the duct is. For duct sides less than 30 inches, 5 traversal points must be taken
along that side. For duct sides of 30 to 36 inches, 6 points must be taken. For duct sides greater than 36
inches, 7 points must be taken. Multiply the numbers in the table times the duct dimension to get insertion
depth for the probe.
0.061 H
0.235 H
0.437 H
0.563 H
0.765 H
0.939 H
H
0.074 V
0.288 V
0.500 V
0.712 V
0.926 V
V
Figure 32: Location of Measuring Points for Traversing a
Rectangular Duct using Log-Tchebycheff Method
For this duct, a 30-36 horizontal dimension requires 6 points (or 6 traverse lines). For this duct, a vertical
dimension less than 30 requires 5 points (or 5 traverse lines).
# of Points or
Traverse Lines
per Side
Position Relative to Inner Wall
5
0.074, 0.288, 0.500, 0.712, 0.926
6
0.061, 0.235, 0.437, 0.563, 0.765, 0.939
7
0.053, 0.203, 0.366, 0.500, 0.634, 0.797, 0.947