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Dynamic Viscosity
The dynamic viscosity (sometimes called absolute viscosity) of a fluid can be defined as the resistance to flow and shear under the forces of internal friction. This internal friction is caused by the resistance of the fluid's molecules moving relative to each other. The larger the molecules, the higher the internal resistance and consequently the higher the dynamic viscosity. Dynamic viscosity can be measured using rotary viscometer. The metric unit for dynamic viscosity is dyne-second per square centimetre (or sometimes the numerically equivalent gram per centimetre-second is used) called a Poise. In practice the centipoise is used since the poise is a large unit. Imperial units are either slugs per foot-second, or the equivalent pound-seconds per square foot.
Kinematic Viscosity
Kinematic viscosity on the other hand is a measure of an oils resistance to flow and shear under the forces of gravity. Again, the larger the molecules, the greater resistance, the higher the kinematic viscosity. Most pipe friction charts and pump correction charts used by engineers refer to kinematic viscosity. Kinematic viscosity is usually measured using a Capillary Tube Viscometer, often referred to as a viscosity bath. This is the most common method used to determine a fluid's, note this is a lab based process - not on line analysis. The metric units of kinematic viscosity are square centimetre per second, called a stoke. In practice the centistoke is used since the stoke is a large unit. Imperial units are square foot per second.
Converting Viscosities
The dynamic viscosity of an oil can be calculated from the kinematic viscosity by dividing it by the fluid's density. e.g. centipoises (cp) = centistokes (cSt) / Density Temperature Effect Viscosity is temperature dependent, generally the warmer a liquid the less viscous it is whereas the warmer a gas the higher the viscosity. Therefore for a dynamic or kinematic viscosity to be meaningful a reference temperature must be quoted.
Other Viscosity Units
Occasionally other units of viscosity are quoted. These have often been named after the viscosity meter used to measure viscosity. For example:
Degree Engler
is still occasionally used in the UK. It is a measure of kinematic viscosity based on comparing a flow of the substance under test to the flow of water. Viscosity in Engler degrees is the ratio of the time of flow of 200 cubic centimeters of the fluid under test to the time of flow of 200 cubic centimeters of water at the same temperature in a standardized Engler viscosity meter. The base temperature is usually 20°C, though sometimes 50°C or 100°C is used.
Saybolt Universal Seconds (SUS) is a dated unit of measure of viscosity. The time in Saybolt Universal Seconds (SUS) required for 60 milliliters of a petroleum product to flow through the calibrated orifice of a Saybolt Universal viscometer. Sometimes referred to as the SSU number (Seconds Saybolt Universal) or SSF number (Saybolt Seconds Furol).
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