Definitions of Viscosity

Definitions

• Absolute Viscosity is the ratio of the shear stress in a fluid to the rate of shearing strain.

• Unit of absolute viscosity in the metric system: poise and centipoise;

1 poise

1 gm/(cm)(sec.)

• Unit of absolute viscosity in the English system:

slug/(ft.) (sec.)

;

1 slug/(ft.)(sec.) = 478.8 poise

• Kinematic Viscosity is the absolute viscosity ÷ 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 centistokes
1 centistoke	=	1/076 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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