Retrotec DM32 User Manual download pdf (Page 83)

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©Retrotec Inc. 2014

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:

 











If the test pressure (20 in this case) is close to the desired reference pressure (25 Pa in this case), then the

correction is small and the value of n does not play as large a role. However, if the test pressure is much

higher or lower than the reference pressure, the error can be greater.

The @ Pressure feature is very useful for ensuring that results taken when the pressure was not adjusted

perfectly are still accurate.

To continue the above example: The flow at 20 Pa is 100 CFM. Actual n is 0.7, but this is unknown.

Instead, 0.6 will be used.

The gauge would calculate:

 





 





 





 





  

However, if it was known that n was 0.7, the flow at 25 should have been:

 





 





  

This value is less than 3% off from what it should be. If the test pressure was within 1 or 2 Pa of the

reference pressure of 25 Pa, the @ Pressure reading would be exact.

“n value” for @Pressure extrapolation

The “n value” is a number set between 0.5 and 1.0 that is used to extrapolate [Channel B] results to a

pressure that is not the pressure read on [Channel A]. It can be changed by tapping [Settings][…]

[Ducts=0.6, House=0.65]. Set it to 0.65 for houses, and 0.60 for ductwork. Set to 0.5 for tests using the

Retrotec House or duct 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.

 



  

      

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Retrotec DM32 User Manual Page 83

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