64
Model 8715 DP-C
ALC
™ Micromanometer and Model 8380 A
CCU
B
ALANCE
®
Air Balancing Tool
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
6
0.032, 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, 0.979
10
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.)
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 effects of the duct wall on the 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 that side of the duct is. For duct sides less than 30 inches, 5 traversal points must be taken
along that side. For duct sides of 30 to 36 inches, 6 points must be taken. For duct sides greater than 36
inches, 7 points must be taken. Multiply the numbers in the table times the duct dimension to get insertion
depth for the probe.
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