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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.

 





















 



 









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 ×



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).

 



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.

 



  

      

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 the gauge.

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