Retrotec DucTester 340 - Cannot Reach 50 Pa Factors

Page 79 of 89
©Retrotec 2017
Table 11: 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 12 to get
an estimate of the flow that would result at 50 Pa.
For example, if you read 1000 CFM at a building pressure of 30 Pa, the "Cannot reach 50 factor" from
Table 12 is 1.36. To get the estimated flow at 50 Pa, make the following calculation:
    
Thus the estimated result for CFM at 50 Pa will be 1360 CFM.
The factors depend on the “n” value chosen, and Table 12 is for “n” of 0.60 which is typical for ducts.
Table 12: Factors for when a duct pressure of 50 Pa cannot be reached (assumed “n” value of 0.60).
Pressure
achieved
Cannot reach
50 factor
Pressure
achieved
Cannot reach
50 factor
50
1.00
25
1.52
49
1.01
24
1.55