Viscosity of Common Fluids Units for Kinematic Viscosity However, since the viscosity of most fluids is below 1 Pa-s (See Table 1), the millipascal-second (mPa-s) is often used instead. You can always check our application library to find examples of different fluids and their viscosity. The SI unit for dynamic viscosity η is the Pascal-second (Pa-s), which corresponds to the force (N) per unit area (m 2) divided by the rate of shear (s -1). It is not a coincidence that the viscosity of distilled water at 20☌ was used to define 1 cP! In order to give you an idea of the viscosity of some conventional fluids we have collected their viscosities in Table 1. This unit is used in honor of French physicist, Jean Léonard Marie Poiseuille (1797-1869), who worked with Gotthilf Hagen on the widely known Hagen-Poiseuille law which applies to laminar flow through pipes. The most commonly used unit for dynamic viscosity is the CGS unit centipoise (cP), which is equivalent to 0.01 Poise (P). In this page we briefly discuss the most common units for the two main types of viscosity: dynamic and kinematic. To further complicate things, different applications might use different unit systems such as SI, CGS. Sometimes it can be confusing since there are several types of viscosity, each with their own units. It appears Stokes and Poise got the same answer just in two different ways.We get asked about the units of viscosity all the time. If you divide kinematic viscosity by the fluid density, you get absolute viscosity. Thus, dynamic viscosity is a measure of force, while kinematic viscosity is a measure of velocity. Kinematic viscosity incorporates fluid density as part of its measurement. This means the fluid’s weight or density helps it to flow. There is no external force pushing the fluid. It is the time it takes to have a known amount of fluid flow a given distance. The formula for kinematic viscosity is 1 centistoke (cSt) equals 1 millimeter squared per second (mm 2/s). This viscosity measurement was given the term kinematic. Again, centistokes (cSt) is used for easier readings. These tests led to Stokes’ law and a different form of viscosity measurement. 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. Stokes surmised there was some type of internal friction in the fluid causing the different rates of falling. He discovered that the same particle sank at different rates in different fluids. Therefore, this type of viscosity measurement requires an external force in order to be measured.Ībout 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. Pascal is a unit of force just like horsepower. The formula for dynamic or absolute viscosity is 1 centipoise (cP) equals 1 millipascal-second (mPa-s). The term dynamic or absolute is used for this viscosity measurement. To make readings easier, centipoise (cP) is preferred for lubricant viscosities. This internal friction is measured by the force needed to make it flow and was given the measurement name of poise. This led him to conclude that different fluids have an internal friction which must be overcome by an external force in order to flow. Poiseuille found that different blood flowed at different speeds through the glass tubes with the same amount of force. This article explains the differences.Īround 1840, a French mathematician named Jean Leonard Marie Poiseuille conducted tests involving the flow of blood through small glass tubes. This resistance is measured by two different methods. The definition of lubricant viscosity is the fluid’s resistance to flow and shear. Viscosity is the utmost characteristic of a lubricant.
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