Page 61 of 72
©Retrotec Inc. 2012
Pressure
achieved
Cannot reach
50 factor
Pressure
achieved
Cannot reach
50 factor
50
1.00
25
1.57
49
1.01
24
1.61
48
1.03
23
1.66
47
1.04
22
1.71
46
1.06
21
1.76
45
1.07
20
1.81
44
1.09
19
1.88
43
1.10
18
1.94
42
1.12
17
2.02
41
1.14
16
2.10
40
1.16
15
2.19
39
1.18
14
2.29
38
1.20
13
2.40
37
1.22
12
2.53
36
1.24
11
2.68
35
1.26
10
2.85
34
1.28
9
3.05
33
1.31
8
3.29
32
1.34
7
3.59
31
1.36
6
3.97
30
1.39
5
4.47
29
1.42
28
1.46
27
1.49
26
1.53
Cannot Reach 25 Pa Factors
Rather than using the equation in the situation where the 25 Pa test pressure cannot be achieved, you
can use tables with the factors already calculated and the following procedure. Take a flow (CFM)
reading at the pressure achieved and multiply by the "Cannot reach 25 Pa factor” from Table 14 to get
an estimate of the flow that would result at 25 Pa.
For example: if you read 600 CFM at a building pressure of 19 Pa, the "Cannot reach 25 Pa factor"
from the table is 1.18. Make the following calculation:
600 𝐶𝐹𝑀 × 1.18 = 708 𝐶𝐹𝑀
Thus the estimated result for CFM at 25 Pa will be 708 CFM.
The factors depend on the
n
value chosen, and Table 13 is for
n
of 0.6 which is typical for duct systems.
