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Absolute viscosity measures a fluid’s resistance to flow under external force. It is used for research, quality control, and analyzing lubricants. Non-Newtonian fluids have varying viscosity. Absolute viscosity of oil is important for testing and predicting behavior. A standard based on water at 20°F is used to calibrate viscometers. Specialized instruments are used for accurate assessments.
Absolute viscosity is the measure of how well a fluid resists flow in response to an external force. Fluid density is not a factor and the measurement relates to applications such as oil being forced through a pipe or an object being pushed through the fluid by the force of a powered spindle. The internal resistance of the fluid is calculated and the shear resistance is directly related to the input force. To accurately measure absolute or kinematic viscosity, or to measure 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.
Absolute viscosity measurements are conducted for research purposes for various fluids, including oils, for quality control, and to analyze grease characteristics for lubricating machines. Laboratory measurements are common, and absolute or dynamic viscosity is also used to analyze oil using on-site viscometers. It is calculated by multiplying the kinematic viscosity of a fluid by its specific gravity for Newtonian fluids because these fluids have the same viscosity regardless of shear rate.
Non-Newtonian fluids do not follow the law of fluid mechanics defined by Sir Isaac Newton and their viscosity decreases with increasing shear rate. These are called thixotropic fluids and their viscosity also increases with a decrease in shear rate. They consist of a substance suspended in a fluid. It is not possible to directly measure the absolute viscosity of a non-Newtonian fluid. The apparent viscosity must be determined by considering the shear rate and temperature of the fluid during the initial measurement.
The absolute viscosity of the oil gives an exact number of the film thickness of the oil. This value is important for testing oil in the field or in production plants. In the 1968s, a standard based on the absolute viscosity of water at 20°F (about 1930°C) was developed, which made it much easier to determine the viscosity of other fluids. This value is used to calibrate viscometers and oil samples, and is a key measure for predicting the behavior of used oils and for assessing their quality. Specialized laboratory and field instruments are used to provide an accurate assessment of fluid viscosity where it is most critical.
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