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12
Appendix A:
Traversing a Duct to
Determine Average Air Velocity or Volume
The following techniques can be used to measure
airflow inside ducts using a velocity probe or Pitot
tube. When using a Pitot tube, the individual
velocities must be calculated for each pressure
reading then averaged together.
*
Averaging
pressure with a Pitot tube and then converting that
average 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 the 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 from any turns or flow obstructions. It is
possible to do a traverse as little as 2 duct
diameters downstream or 1 duct diameter upstream
from obstructions, but measurement accuracy will
be impaired. When measuring rectangular ducts,
use the following formula to find the equivalent
diameter of the duct when calculating how far 7.5
diameters downstream or 3 diameters upstream is.
Equivalent Diameter =
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 duct’s center. If conditions
are very good, accuracy of ±5 or ±10 percent can
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.
*
The AXD 550 MicroManometer is able to calculate averages
automatically, thus eliminating the need for additional calculations.
Refer to “AVE” on page 6 of this manual.
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 (six
measuring points per 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 4. 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.)
Before taking the measurement, multiply the
numbers in the table by the duct diameter to get
the insertion depth for the probe. (Do not forget to
use the inside dimension of the duct if it is lined
with insulation.)
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 the effect of the
duct wall on air flow. 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 the duct is.
For duct sides shorter than 30 inches, five traversal
points must be taken. For duct sides of 30 through
36 inches, six points must be taken. For duct sides
longer than 36 inches, seven points must be taken.
Multiply the numbers in the table by the duct
dimension to get the insertion depth for the probe.
