The quantity defined above is sometimes called dynamic viscosity, absolute viscosity, or simple viscosity to distinguish it from the other quantity, but is usually just called viscosity. There are actually two quantities that are called viscosity. Ten poise equal one pascal second making the centipoise and millipascal second identical. The most common unit of viscosity is the dyne second per square centimeter, which is given the name poise after the French physiologist Jean Poiseuille (1799–1869). The pascal second is more rare than it should be in scientific and technical writing today. Despite its self-proclaimed title as an international system, the International System of Units has had little international impact on viscosity. The SI unit of viscosity is the pascal second, which has no special name. Or if you prefer calculus symbols (and who doesn't)… F The similarity to Newton's second law of motion ( F = ma) should be apparent. The more usual form of this relationship, called Newton's equation, states that the resulting shear of a fluid is directly proportional to the force applied and inversely proportional to its viscosity. (dynamic) viscosityįormally, viscosity (represented by the symbol η "eta") is the ratio of the shearing stress ( F/ A) to the velocity gradient ( ∆ v x/∆ y or dv x/ dy) in a fluid. Fluids resist the relative motion of immersed objects through them as well as to the motion of layers with differing velocities within them. In Table 2, we present most common units for viscosity and the conversion factors between them.Informally, viscosity is the quantity that describes a fluid's resistance to flow. Note that 1 cm 2/s is equivalent to 100 cSt. However, due to the viscosity values of most common fluids, square centimeters per second (cm 2/s) is used more often. The SI unit for kinematic viscosity is square meters per second (m 2/s). One stoke is equivalent to one poise divided by the density of the fluid in g/cm 3. Kinematic viscosity is often measured in the CGS unit centistokes (cSt), which is equivalent to 0.01 stokes (St). You guessed it! This one is named after Irish mathematician Sir George Gabriel Stokes (1819-1903) who, among other contributions to fluid mechanics, helped develop the Navier-Stokes equation for the conservation of momentum. 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. We get asked about the units of viscosity all the time.
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