11
diameter) are used for ducts with diameters of 10 inches or smaller.
Four or five concentric circles (eight or ten measuring points per
diameter) are used for ducts with diameters of 10 inches or more.
The preferred method is to drill three holes in the duct at 60½ angles
from one another as shown in Figure 1. Three traverses are taken
across the duct, and the velocities obtained are averaged at each
measuring point. The average velocity is multiplied by the duct area
to get the flow rate. (A different method uses two holes at 90½ angles
from one another, decreasing the number of traverses with the probe
by one.)
Figure 1: Location of measuring points when traversing a round duct
using the log-Tchebycheff method
Number of measuring
points per diameter
Position relative to inner wall
6
0.32, 0.135, 0.321, 0.679, 0.865, 0.968
8
0.021, 0.117, 0.184, 0.345, 0.655, 0.816, 0.883, 0.981
10
0.019, 0.077, 0.153, 0.217, 0.361, 0.639, 0.783,
0.847, 0.923, 0.981
