Retrotec DM32 - Manual (Page 79)

Page 79 of 94
©Retrotec Inc. 2014
Appendix C: Manually estimate 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
“Set 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.