64 ProHood Capture Hoods PH730/PH731
Traversing a Round Duct
Using the log-Tchebycheff method, the duct is divided into concentric circles, each containing equal area.
An equal number of readings is taken from each circular area, thus obtaining the best average.
Commonly, three concentric circles (6 measuring points per diameter) are used for ducts of 10-inch
diameter and smaller. Four or five concentric circles (8 or 10 measuring points per diameter) are used for
ducts larger than 10-inch diameter.
The preferred method is to drill 3 holes in the duct at 60° angles from each other as shown in Figure 31
below. Three traverses are taken across the duct, averaging the velocities obtained at each measuring
point. Then the average velocity is multiplied by the duct area to get the flow rate. (A different method
uses 2 holes at 90° from each other, decreasing the number of traverses with the probe by one.)
0.032dia
0.135dia
0.321dia
0.679dia
0.865dia
0.968dia
dia
Figure 31: Location of Measuring Points when Traversing a
Round Duct using Log-Tchebycheff Method
#of Measuring
Points
Per Diameter
Position Relative to Inner Wall
0.032, 0.135, 0.321, 0.679, 0.865, 0.968
0.021, 0.117, 0.184, 0.345, 0.655, 0.816, 0.883, 0.979
0.019, 0.077, 0.153, 0.217, 0.361, 0.639, 0.783, 0.847, 0.923, 0.981
Before taking the measurement, multiply the numbers in the table times the duct diameter to get insertion
depth for the probe. (Do not forget to use the inside dimension of the duct if it is lined with insulation.)
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