7/5/2023 0 Comments Kinematic viscosity![]() ![]() Sometimes, viscosity is erroneously referred to as thickness (or weight). Generally speaking, viscosity is a fluid’s resistance to flow (shear stress) at a given temperature. Given the importance of viscosity analysis coupled with the increasing popularity of onsite oil analysis instruments used to screen and supplement offsite laboratory oil analysis, it is essential that oil analysts understand the difference between dynamic and kinematic viscosity measurements. By contrast, most onsite viscometers measure dynamic viscosity, but are programmed to estimate and report kinematic viscosity, so that the viscosity measurements reported reflect kinematic numbers reported by most labs and lube oil suppliers. Most used oil analysis laboratories measure and report kinematic viscosity. The two are easily confused, but are significantly different. Viscosity can be measured and reported as dynamic (absolute) viscosity or as kinematic viscosity. However, there is more to viscosity than meets the eye. Likewise, there is no property more critical to effective component lubrication than base oil viscosity. Of all the tests employed for used oil analysis, none provides better test repeatability or consistency than viscosity. Viscosity reporting is only valid when the temperature at which the test was conducted also is reported - for example 23 cSt at 40 degrees C. This value is converted to standard units such as centistokes (cSt) or square millimeters per second. It is determined by measuring the time in seconds, required for a fixed volume of fluid to flow a known distance by gravity through a capillary within a calibrated viscometer at a closely controlled temperature. In the kinematic measuring method, gravity is the only force that acts on the sample.Kinematic viscosity is a measure of a fluid's internal resistance to flow under gravitational forces. The mass (or weight) of a fluid is determined by gravity. Therefore, we say that steel has a greater density than ice cube. ![]() They may be the same size, but the steel cube weighs more than the ice cube. Think about an ice cube and a cube of steel. Dynamic (cP) / Density = Kinematic (cSt)įor a given sample, with a density greater than one, dynamic viscosity will always be the higher number.ĭensity is the ratio of the mass (or weight) of the sample divided by the volume of the sample.Kinematic (cSt) x Density = Dynamic (cP).Density actually provides a way to convert between a kinematic and a dynamic viscosity measurement. The unit of measure of kinematic viscosity is Centistokes (cSt).Ī basic difference between the dynamic and kinematic viscosity measurements is density. The time is converted directly to kinematic viscosity using a calibration constant provided for the specific tube. There are several ways to find the kinematic viscosity of a fluid, but the most common method is determining the time it takes a fluid to flow through a capillary tube. Put another way, kinematic viscosity is the measure of a fluid’s inherent resistance to flow when no external force, except gravity, is acting on it. The other way is to measure the resistive flow of a fluid under the weight of gravity. One way is to measure a fluid’s resistance to flow when an external force is applied. ![]() Water at 20 ☌ has a kinematic viscosity of about 1 cSt. Other units are: 1 St ( Stoke) = 1 cm 2/s = 10 −4 m 2/s. The SI unit of the kinematic viscosity is m 2/s. The kinematic viscosity is the ratio between the dynamic viscosity and the density of a fluid. ![]()
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