# Reference values for viscosity and density in incompressible NSE

I come from a pure mathematics background, so I have very limited physics knowledge. I'm currently working out the non-dimensional form for the Navier-Stokes equations and have some questions. Where do the reference values come from? Is this something people measure it for each type of fluid and tabulate it somewhere? (So are they in a text book or do people measure it experimentally in "real life" problems?)

Someone once told me that the reference value for density is often taken from water, but this may be wrong, or I remembered incorrectly.

I need a short sentence to say what these are and where do they come from.