e. Aside from avoiding instrument error, the most significant requirement in
making valid velocity or air flow measurements is the location selected for the
measurements and the traverse of that location. The reason for these requirements is
that airflow is not uniform in the cross section of a duct. This is especially true near
such interferences as elbows, entries, and so forth. Therefore, for greater accuracy,
measurements should be taken at least 7.5 diameters of straight run downstream or 1.5
can be conducted. Figure 4-10 shows a cutaway drawing of both a round and a
rectangular duct, illustrating the principle of measuring the velocity pressure of equal
areas. Note that for round ducts it is advisable to traverse in two planes perpendicular
to each other. The suggested optimum number of measurements per plane for ducts of
stated dimensions is given below.
Number of Measurements
Duct Diameters (Inches)
48 and greater
Like round ducts, rectangular ducts are traversed in terms of equal area segments.
Rectangular ducts should be divided into a minimum of 16 to a maximum of 64 equal
area rectangles with readings taken in the center of each rectangle, as shown in
Figure 4-9. Traverse of a round and a rectangular duct.