The units of measurement are reported in Table 1. Thus it is imperative to report the temperature at which the viscosity is measured. The viscosity of a fluid, either kinematic or absolute, varies according to the temperature at which it is measured. So we have just performed a kinematic viscosity test, and have shown too that the kinematic viscosity of honey is greater than that of water, because it has a greater resistance to flow out of the jar. The fluids are both going to flow out of the jars, and in this case, the forces causing the flow are introduced by gravity. Now take those jars, unfix them from the table, and tip them on their sides. And obviously the fluid with the greater resistance to the stirring is the honey, so we can conclude that the absolute viscosity of honey is greater than that of water. Note that these forces are not created by gravity, so what you are doing is an absolute viscosity test. If you put a spoon into each jar and begin stirring, you are introducing shear forces into the fluid. The jars are fixed to the table, so they can't move. Suppose you have a jar of honey and a jar of water sitting on the table.

Let's illustrate this difference with an example. The shear forces introduced by gravity are actually very small compared to the shear forces introduced by the mechanical interaction of machine components. I like to think of kinematic viscosity as a specialized case of absolute viscosity.

Kinematic viscosity, strictly speaking, is defined as the ratio of absolute viscosity to density.ĭensity is a mass-derived property, and because mass and weight are, for practical purposes, proportional anywhere on the surface of Earth, kinematic viscosity is often interpreted as the fluid's resistance to flow under forces of gravity. Put more simply, the thicker the fluid, the more energy that is needed to get it to flow. In this column, I'll explain the differences between them, and give a few tips on how to apply them to lubricating fluids.Ībsolute viscosity is defined as a fluid's resistance to shear, or the fluid's resistance to deform when subjected to a force. The two ways of measuring and reporting viscosity, kinematic and absolute (also known as dynamic), often cause confusion in the minds of those who work with them on a less-than-regular basis.

Test oil viscosity