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Appendix B. Traversing a Duct to Determine Average
Air Velocity or Volume
The following techniques can be used to measure air flow inside ducts, using a velocity probe or pitot-
static tube. When using a pitot-static tube, the individual velocities must be calculated for each pressure
reading and then averaged together. Averaging pressure with a pitot tube and then converting into
velocity will give an incorrect result, especially if many readings are more than ±25% from the average
pressure. Remember that for a pitot tube, velocity is proportional to the square root of pressure.
Where to Take the Measurement
In order to make air velocity measurements in a duct, it is best to measure at least 7.5 duct diameters
downstream and at least 3 duct diameters upstream from any turns or flow obstructions. It is possible to
do a traverse as little as 2 duct diameters downstream and 1 duct diameter upstream from obstructions, but
measurement accuracy will be impaired. When measuring rectangular ducts, use this formula to find the
equivalent diameter of the duct when calculating how much distance is 7.5 diameters downstream and
3 diameters upstream:
Equivalent Diameter = sq root of 4HV / Pi
Where:
H = horizontal duct dimension
V = vertical duct dimension
Pi = 3.14
It is also possible to take a single reading to measure air velocity or air volume flow in a duct, measuring
in the center of the duct and multiplying the reading by 0.9 to correct for the higher velocity at the center
of the duct. If conditions are very good an accuracy of ±5 or ±10 percent may be obtained this way. This
method is not reliable, however, and should only be used where small duct size or other conditions do not
permit a full traverse.
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.)
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