Tags: viscosity

Viscosity is the utmost characteristic of a lubricant. The definition of lubricant viscosity is the fluid’s resistance to flow and shear. This resistance is measured by two different methods. Sometimes this can be confusing. This article explains the differences.

Around 1840, a French mathematician named Jean Leonard Marie Poiseuille conducted tests involving the flow of blood through small glass tubes. Poiseuille found that different blood flowed at different speeds through the glass tubes with the same amount of force.

This led him to conclude that different fluids have an internal friction which must be overcome by an external force in order to flow. This internal friction is measured by the force needed to make it flow and was given the measurement name of poise. To make readings easier, centipoise (cP) is preferred for lubricant viscosities. The term dynamic or absolute is used for this viscosity measurement.

The formula for dynamic or absolute viscosity is 1 centipoise (cP) equals 1 millipascal-second (mPa-s). Pascal is a unit of force just like horsepower. Therefore, this type of viscosity measurement requires an external force in order to be measured.

About the same time Poise was performing his tests, an Irishman named Sir George Stokes was dropping particles into fluids and measuring how fast they fell to the bottom. He discovered that the same particle sank at different rates in different fluids.

Stokes surmised there was some type of internal friction in the fluid causing the different rates of falling. He tested this theory by putting fluid in a glass tube and measuring how long it took for the fluid to flow a certain distance. These tests led to Stokes’ law and a different form of viscosity measurement. Again, centistokes (cSt) is used for easier readings. This viscosity measurement was given the term kinematic.

The formula for kinematic viscosity is 1 centistoke (cSt) equals 1 millimeter squared per second (mm^{2}/s). This is a rate of flow. It is the time it takes to have a known amount of fluid flow a given distance. There is no external force pushing the fluid. Only gravity is used. This means the fluid’s weight or density helps it to flow. Kinematic viscosity incorporates fluid density as part of its measurement.

Thus, dynamic viscosity is a measure of force, while kinematic viscosity is a measure of velocity. That’s the difference. If you divide kinematic viscosity by the fluid density, you get absolute viscosity. It appears Stokes and Poise got the same answer just in two different ways.