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Appendix D: Details about the @Pressure extrapolation function
There is a way to estimate the flow at a target pressure based on the measured pressure and flow you did
achieve. This procedure is the one the gauge uses to extrapolate flows with the
[@ Pressure]
key. The
following equation describes the relationship between the desired flow at the target pressure and the
measured values.
Flow @ target Pa = (Flow at measured pressure) ∗ (
target Pa
measured pressure
)
n
CFM
target
= CFM
P
∗ (
target
P
)
n
The actual flow exponent n for an enclosure can be calculated by measuring building leakage at multiple
pressure differences, for instance from 10 to 50 Pa, and determining the slope of the line:
Flow = C ×
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 gauge
uses the “
n”
value to extrapolate for flows at other pressures. Because the extrapolation
function is a ratio of two flows from the same fan, the
C
value cancels out.
The “
n
” value is
generally set to 0.65 for houses, and 0.60 for ductwork. Set to 0.5 for tests on the
simulator, air handler flow and any large hole that is not composed of long thin cracks.
A wide open hole has an
n
of 0.5, meaning that when the pressure is quadrupled, the flow doubles. That is
due to completely turbulent flow going through that hole (flow = square root of pressure, a constant for
that particular hole).
𝑓𝑙𝑜𝑤 = 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒
0.5
An
n
value of 1.0 represents tiny little holes, so small that the air would not be turbulent but rather would
go through the holes as laminar flow. This means that when pressure is quadrupled, the flow will also be
quadrupled.
𝑓𝑙𝑜𝑤 = 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒
1
𝑓𝑙𝑜𝑤 = 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒
4 ∗ 𝑓𝑙𝑜𝑤 = 4 ∗ 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒
Houses and ducts have many holes that will have both turbulent and laminar flow going through them.
Duct holes tend to be slightly larger, whereas houses have more prevalent long tiny cracks, and therefore
tend to have lower
n
values.
The “Flow @target Pa” equation is used in the gauge [@ Pressure] function, based on the values set for
n
,
(in Settings>>Enclosure>>Enclosure Type),
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 t
he gauge.
For example: If we guess at the n value of a duct as being 0.6 and measure 100 CFM at 20 Pa (by accident
or by design), then the gauge will complete the following calculation to estimate the flow at 25 Pa:
