Retrotec DM-2A Manual de usuario descargar pdf (Pagina 30)

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The “n” value is typically 0.65 for houses and 0.60 for ductwork. 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). 

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 



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, so will the

flow.

 



  

      

Houses and ducts are composed 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.

These n values can be measured simply by doing a multi-point Door Fan or duct test. The result will be

an n and a C (coefficient) so flow at any pressure can be calculated by using the equation:

 

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 

The gauge uses the “n” and “C “values to extrapolate for flows at other pressures.

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 DM-2 will complete the following calculation for 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 could 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 DM-2 would calculate:

 





 





 





 

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  

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