UEi Test Instruments DAFM4 - Manual (Page 9)

8
MEASURING AIR VOLUME CONT.
We recommend the following number of access holes depending on duct
size;
• 18” or less – Two access holes 6” apart
• 18”~30” – Five access holes
• 30”~36” – Six access holes
• Greater than 36” – Seven access holes
ACCESS HOLE SPACING
Use the Log-Tchebycheff Rule for rectangular ducts to determine spacing
for access holes.
Number of access
holes per side
Position relative to inner wall
(multiplier)
5 0.074, 0.288, 0.500, 0.712, 0.926
6 0.061, 0.235, 0.437, 0.563, 0.765, 0.939
7 0.019, 0.153, 0.217, 0.361, 0.639, 0.783,
0.847, 0.923, 0.981
Example – for a 20” inside dimension select 5 access holes.
• Access 1, 20 x 0.074 = 1.48” from wall (use 1 ½“)
• Access 2, 20 x 0.288 = 5.76” from wall (use 5 ¾”)
• Access 3, 20 x 0.500 = 10” from wall (use 10“)
• Access 4, 20 x 0.712 = 14.24” from wall (use 14 ¼“)
• Access 5, 20 x 0.926 = 18.52” from wall (use 18 ½“)
SAMPLE CALCULATED LOCATIONS
Access
Holes
5 Access
Holes
6 Access
Holes
7
Width 24 Width 30 Width 36
0.074 1.8 0.061 1.8 0.053 1.9
0.288 6.9 0.235 7.1 0.203 7.3
0.500 12.0 0.437 13.1 0.366 13.2
0.712 17.1 0.563 16.9 0.500 18.0
0.926 22.2 0.765 23.0 0.634 22.8
0.939 28.2 0.797 28.7
0.947 34.1