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Absolute viscosity is the measure of how a fluid resists flow in response to an external force. The density of the fluid is not a factor, and the measure relates to applications such as oil being forced through a tube, or an object being pushed through fluid by the force of a motor-powered spindle. The internal resistance of the fluid is calculated, and the shear resistance is directly correlated with the input force. To accurately measure either absolute or kinematic viscosity, or to measure of a fluid under the force of gravity, it is multiplied by its density. Kinematic viscosity is calculated by dividing the absolute number by the density of the fluid.
Measurements of absolute viscosity are conducted for the purposes of research for various fluids, including oils, for quality control, and to analyze the characteristics of grease for lubricating machinery. Laboratory measurements are common, and absolute or dynamic viscosity is also used to analyze oil via onsite viscometers. It is calculated by multiplying kinematic viscosity of a fluid by its specific gravity for Newtonian fluids because these fluids have the same viscosity no matter what the shear rate is.
Non-Newtonian fluids do not follow the Law of Fluid Mechanics defined by Sir Isaac Newton, and their viscosity decreases the higher the shear rate is. These are called thixotropic fluids, and their viscosity rises with a decrease in shear rate as well. They are comprised of a substance that is suspended in a fluid. It is not possible to directly measure the absolute viscosity of a non-Newtonian fluid. The apparent viscosity has to be determined by considering the shear rate and temperature of the fluid during the initial measurement.
The absolute viscosity of oil gives a number of the oil’s exact film thickness. Such a value is important for testing oil in the field or in manufacturing facilities. A standard based on the absolute viscosity of water at 68°F (about 20°C) was developed in the 1930s, which has made it much easier to determine the viscosity of other fluids. This value is used to calibrate viscometers and samples of oils, and is a crucial measurement in predicting the behavior of used oils and to give an assessment of their quality. Special laboratory and on-site instruments are used to provide an accurate assessment of fluid viscosity where it is most critical.
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