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The viscosity index (VI) is an arbitrary, unit-less measure of a fluid's change in viscosity relative to temperature change. It is mostly used to characterize the viscosity-temperature behavior of lubricating oils. The lower the VI, the more the viscosity is affected by changes in temperature.
In the automotive industry the viscosity index is used to describe the change of viscosity with temperature. The reciprocal of viscosity is fluidity , usually symbolized by ϕ = 1 / μ {\displaystyle \phi =1/\mu } or F = 1 / μ {\displaystyle F=1/\mu } , depending on the convention used, measured in reciprocal poise (P −1 , or cm · s · g ...
Consequently, if a liquid has dynamic viscosity of n centiPoise, and its density is not too different from that of water, then its kinematic viscosity is around n centiStokes. For gas, the dynamic viscosity is usually in the range of 10 to 20 microPascal-seconds, or 0.01 to 0.02 centiPoise. The density is usually on the order of 0.5 to 5 kg/m^3.
Understanding the temperature dependence of viscosity is important for many applications, for instance engineering lubricants that perform well under varying temperature conditions (such as in a car engine), since the performance of a lubricant depends in part on its viscosity.
Viscosity index (ASTM D2270 [11]) is a measure of the extent of viscosity change with temperature; the higher the VI, the less the change. VI is calculated from viscosity measurements at 40°C and 100°C. The viscosities of paraffinic and naphthenic base oils have very different behavior with temperature change.
The grades include single grades, such as SAE 30, and also multi-grades such as SAE 15W-30. A multi-grade consists of a winter grade specifying the viscosity at cold temperatures and a non-winter grade specifying the viscosity at operating temperatures. An engine oil using a polymeric viscosity index improver (VII) must be classified as multi ...
Dimensionless numbers (or characteristic numbers) have an important role in analyzing the behavior of fluids and their flow as well as in other transport phenomena. [1] They include the Reynolds and the Mach numbers, which describe as ratios the relative magnitude of fluid and physical system characteristics, such as density, viscosity, speed of sound, and flow speed.
Group IV base oils have a viscosity index range of 125 - 200. Polyalphaolefin oils have a higher oxidative stability in extreme temperatures, and also have exceptionally low pour points , which makes them much more suitable for use in very cold weather (as found in northern Europe ), as well as in very hot weather (as in Middle East ).