3.3.4.2 Discussion—Formerly, kinematic viscosity was defined
specifically for viscometers covered by this test method
as the resistance to flow under gravity. More generally, it is the
ratio between momentum transport and momentum storage.
3.3.4.3 Discussion—For gravity-driven flow under a given
hydrostatic head, the pressure head of a liquid is proportional
to its density, ρ, if the density of air is negligible compared to
that of the liquid. For any particular viscometer covered by this
test method, the time of flow of a fixed volume of liquid is
directly proportional to its kinematic viscosity, ν, where
ν = η ⁄ρ, and η is the dynamic viscosity.