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