Hey Autodesk! Revit's definition of Relative Roughness is upside down!

There have been a few inquiries lately regarding Revit's definition of Relative Roughness as documented here. 

Some eagle eyed users have noticed that this is the reciprocal of the defnition one may find commonly documented, such as here.

As the relative roughness factors into the computation of the friction factor, and the friction factor is used in the computation of pressure loss, the concern of these users has been that the computation of pressure loss may therefore be incorrect.

Fortunately, the developers had taken this into account in the computation of the friction factor.  For turbulent flow in piping, Revit's computation of the friction factor is based on the Colebrook equation as follows:

In both cases, computing f provides the same result.  So, even though Revit reports the friction factor as the reciprocal of how it may be defined elsewhere, you should find that the end result of the pressure loss on a given pipe segment is as expected.

We understand the concern of seeing large values of Relative Roughness  in the properties palette and pressure loss report, and we have logged this for consideration in future development.