Retrotec DucTester 341 - Manual (Page 72)

Page 72 of 83
©Retrotec Inc. 2014
Assumed n = 0.6
Achieved
Pressure
(Pa)
Actual Flow Exponent (n) of Enclosure
0.50 0.55 0.60 0.65 0.70 0.75
60 1.8% 0.9% 0.0% -0.9% -1.8% -2.7%
65 2.7% 1.3% 0.0% -1.3% -2.6% -3.9%
70 3.4% 1.7% 0.0% -1.7% -3.3% -4.9%
Achieved pressure is the actual maximum pressure achieved in the enclosure (ducts in this case) during
the test.
Error = (
Pressure of Interest
Achieved Pressure
)
(Actual n−Assumed n)
In the tables, the Pressure of Interest is 50Pa, and Assumed n is 0.65 or 0.60
The actual flow exponent n for an enclosure can be calculated by measuring enclosure leakage at
multiple pressure differences, from 10 to 50 Pa, and determining the slope of the line on the graph of
pressure versus flow which will be linear if graphed on a log-log scale, and the slope will be n.
Cannot Reach 25 Pa Factors
If you have a situation where you cannot reach the required 25 Pa during the test, there is a way to
estimate the flow at that pressure based on the measured pressure and flow you did achieve. This is the
procedure the gauge uses for extrapolating the flow at the required pressure when you use the
[@] key.
The following equation describes the relationship between the flow at the required pressure of 25 Pa
and the actual measured flow at the measured pressure:
Flow at 25 Pa = (Flow at measured pressure) ∗ (
25 Pa
measured pressure
)
n
CFM
25
= CFM
P
∗ (
25
P
)
n
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 11 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 11 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 11 is for n of 0.60 which is typical for ducts.