In the field of viscosity testing, dynamic viscosity and kinematic viscosity are two important viscosity values. Many novices will confuse these two viscosity values when choosing a viscometer. This article mainly introduces the difference between dynamic viscosity and kinematic viscosity, as well as the origin of its history.
Viscosity is the greatest characteristic of lubricants. The definition of lubricant viscosity is a fluid's resistance to flow and shear. This resistance is measured by two different methods. Sometimes this can be confusing. This article describes the differences.
Around 1840, French mathematician Jean Leonard Marie Poiseuille performed tests involving the flow of blood through small glass tubes. Poiseuille discovered that different blood flows through glass tubes at different speeds with the same force.
This led him to conclude that different fluids have internal friction that needs to be overcome by external forces in order to flow. Internal friction is measured by the force required to make it flow, and has been given the measurement name poise. For easier reading, it is better to use centipoise (cP) as lubricant viscosity. The terms dynamic or absolute are used for this viscosity measurement.
The formula for kinetic or absolute viscosity is that 1 centipoise (cP) equals 1 milliPascal second (mPa-s). Pascal, like horsepower, is a unit of force. Therefore, this viscosity measurement requires external force to perform the measurement.
Around the same time Poise was conducting his tests, an Irishman named Sir George Stokes was dropping particles into a liquid and measuring how fast they fell to the bottom. He found that the same particle sank at different rates in different fluids.
Stokes speculates that there is some type of internal friction in the fluid that causes the different rates of descent. He tested this theory by placing a fluid in a glass tube and measuring the time it took for the fluid to travel a certain distance. These tests led to Stokes' law and different forms of viscosity measurement. Likewise, centimeters (cSt) are used for easier reading. This viscosity measurement is called kinematics.
The formula for kinematic viscosity is that 1 centike (cSt) is equal to 1 square millimeter per second (mm2/s). This is the flow rate. This is the time it takes to bring a fluid of known flow for a given distance. There is no external force pushing the fluid. Only use gravity. This means that the weight or density of the fluid helps it flow. Kinematic viscosity incorporates fluid density into the measurement range.
Thus, dynamic viscosity is a measure of force whereas kinematic viscosity is a measure of velocity. That's it. If you divide the kinematic viscosity by the fluid density, you will get the absolute viscosity. Stokes and Bovas seem to have arrived at the same answer in two different ways.
