Retrotec DucTester 200 Series Residential Applications - Manual (Page 77)

Page 77 of 91
©Retrotec Inc. 2015
For most ducts, the n value is usually 0.60. For the flow at 50 Pa, replace “25” with “50.”
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 factor" from
Table 14 is 1.18. To get the estimated flow at 25 Pa, 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. Table 14 is for n of 0.60 which is typical for ducts.
Table 14: Factors for when a duct pressure of 25 Pa cannot be reached (assumed n value of 0.6).
Pressure
achieved
Cannot reach
25 factor
25
1.00
24
1.02
23
1.05
22
1.08
21
1.11
20
1.14
19
1.18
18
1.22
17
1.26
16
1.31
15
1.36
14
1.42
13
1.48
12
1.55
11
1.64
10
1.73
9
1.85
8
1.98
7
2.15
6
2.35
5
2.63
Cannot Reach 50 Pa Factors
Rather than using the equation in the situation where the 50 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 50 Pa factor” from Table 15 to get
an estimate of the flow that would result at 50 Pa.