Retrotec DM-2 - Manual (Page 60)

Page 60 of 72
©Retrotec Inc. 2012
Appendix B: Manually calculate Flow if required test
pressure cannot be reached
If you have a situation where you cannot reach the required 25 or 50 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
procedure is the same one the gauge uses to extrapolate flows with the
[@ Pressure] key. The
following equation describes the relationship between the desired flow at 25 Pa and the measured
values.
Flow @ 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 most houses the n value is usually 0.65. For large
buildings, the n value is usually 0.55. In the above equation, substitute 50 for 25 if 50 Pa is the
required test pressure.
The actual flow exponent n for an enclosure can be calculated by measuring building leakage at
multiple pressure differences, from 10 to 50 Pa, and determining the slope of the line:
Flow = K ×𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑛
The graph of pressure versus flow will be linear if graphed on a log-log scale, and the slope will be n.
The “Flow @” equation is used in the gauge [@ Pressure] function, based on the values set for n , and
the “@ Pressure” target value in the gauge. By using the [@ Pressure] function, the gauge performs
the extrapolation for you if you are in the situation where you need a flow at a test pressure that
cannot quite be reached. The [@ Pressure] values should only be used if the pressure achieved is
within 10 Pa of the required test pressure, unless you have measured the actual n value for the
building using a multiple point test and have entered that “n” in the gauge.
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 factor" from Table 13to get an
estimate of the flow at 50 Pa.
For example: if you read 1000 CFM at a building pressure of 30 Pa, the "Cannot reach 50 factor" from
the table is 1.36. Make the following calculation:
1000 𝐶𝐹𝑀 × 1.39 = 1390 𝐶𝐹𝑀
Thus the estimated result for CFM at 50 Pa will be 1390 CFM.
The factors depend on the “n” value chosen, and Table 13 is for “n” of 0.65 which is typical for houses.
Table 13: Factors for when a building pressure of 50 Pa cannot be reached (“n” value of 0.65)
Pressure
achieved
Cannot reach
50 factor
Pressure
achieved
Cannot reach
50 factor