Traversing A Square Duct - Alnor APM 151 Manual Del Usario

Figure 16—Location of measuring points when
traversing a round duct using the log-Tchebycheff
method.
*The APM 150 Multi-Purpose Meter is able to calculate
averages automatically, thus eliminating the need for
additional calculations. Refer to "AVE" on page 11 of this
manual.
Number of measuring
points per diameter
6
8
10
The preferred method is to drill three holes in the
duct at 60° angles from one another as shown in
Figure 16. 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 90° 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.)
16
Position relative to
inner wall
0.32, 0.135, 0.321, 0.679,
0.865, 0.968
0.021, 0.117, 0.184,
0.345, 0.655, 0.816,
0.883, 0.981
0.019, 0.077, 0.153,
0.217, 0.361, 0.639,
0.783, 0.847, 0.923, 0.981
16

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.
Figure 17—Location of measuring points for
traversing a rectangular duct using the log-
Tchebycheff method.
Number of points or
traverse lines per side
5
6
7
Position relative to
inner wall
0.074, 0.288, 0.500,
0.712, 0.926
0.061, 0.235, 0.437,
0.563, 0.765, 0.939
0.053, 0.203, 0.366,
0.500, 0.634, 0.797,
0.947