Viscosity Definitions

Absolute Viscosity: the force required to move a unit plane surface over another plane surface at unit velocity when surfaces are separated by a layer of fluid of unit thickness.

Unit of Absolute Viscosity in the metric system: poise and centipose
1 poise = 1 gram/(cm)(sec) and
1 centipoise = 1/100 poise

Unit of Absolute Viscosity in the English system: slugs/(ft)(sec)
1 slug/(ft)(sec) = 1/0.002089 poise

Kinematic Viscosity: the absolute viscosity divided by density.

Unit of Kinematic Viscosity in the metric system and commonly used in the countries using the
English system.

  • stoke and centistoke
  • 1 stoke = 1 poise/density (gm/ml)
  • 1 centistoke = 1/100 stoke

Other units of Kinematic Viscosity in the English system, the most practical unit for making
calculations is ft.2/sec.; 1 ft.2/sec. = 92903 centistoke and 1 centistoke = 1.0760 x 10-5 ft.2/sec.

Saybolt Universal Seconds, SSU, is the kinematic viscosity as determined by the time in seconds required for 60 cc of fluid to flow through a standard orifice.

Saybolt-Furol, SSF, utilizes a larger orifice and is used for very viscous fluids. Time of efflux is approx. 1/10 that of Universal.

Engler degrees are obtained by dividing the outflow time of a specified amount of fluid through a specified orifice by the outflow time of water at 68°F. The method is used predominantly in European countries.

Viscosity Index is an empirical number indicating the effect of temperature change on viscosity. Fluids with the same viscosity at a given temperature do not necessarily have the same viscosity index.

SAE Viscosity Numbers are a means of classifying crankcase lubricating oils in terms of viscosity. Other factors are not considered.

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