Power numbers for anchor and helical ribbon impellers are typically defined in terms of a viscous power number, which is constant for Reynolds numbers less than ten (Re<10).  See accompanying equations for Anchor - Power Number Viscous, Helix - Power Number Viscous, and Reynolds Number.  The power numbers and Reynolds numbers are dimensionless groups, so the choice of units must have conversion factors if the units are not coherent.
 
The viscous power number is different from the typical power number used for low viscosity impellers.  The viscous power number has viscosity in the denominator, rather than density, and different exponents on the rotational speed and impeller diameter.  Correction factors must be applied to the viscous power number when the operating conditions are in the transition range, Reynolds numbers greater than ten (Re>10).  The correction factors in the transition range are greater than one.

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