Lubezine Magazine Vol. 9 April - June 2014 | Page 23

SAE is a viscosity grading system for oils used in the
automotive industry while ISO VG is a grading system
that is generally used to describe industrial oils.
measuring kinematic viscosity, namely 400C
and 1000C. The type of oil under consideration and its properties determine which temperature is employed. Industrial lubricants
are classified at 40 degrees and automotive
lubricants are classified at 100 degrees. In
terms of used oil analysis engines and compressors have the viscosity measured at both
temperatures whilst all other components
usually have their viscosity only measured
at 40 degrees, based on a typical operating
temperature of the component. If the viscosity index (which will be explained later) is
to be calculated then the viscosity at both
temperatures needs to be measured.
Kinematic viscosity is not the only viscosity measurement that can be made; there is
also dynamic viscosity (sometimes called
absolute viscosity) which is a measurement
of a fluid’s resistance to shear at a specified
temperature. Dynamic viscosity is measured in centipoise and one centipoise equals
one millipascal second.

Dynamic viscosity = 1 centipoise (cP) = 1
mPa.S
The two viscosities are related by the
fluid’s density. Although centistoke units
will be used throughout this article, different units may be used in other parts of the
world e.g. Engler Degrees (Europe), Redwood
Seconds (UK), and Saybdt Universal Seconds
(USA). The different systems are convertible
but only for measurements made at the same
temperature.

The ISO VG system
The International Standards Organisation,
Viscosity Grade (ISO VG) is a grading system
that is generally used to describe industrial
oils i.e. oils used in statio