is a constant of proportionality, and it is known as the dynamic viscosity of the fluid. Where is the area of the layer and is the distance between the layers. The force required to move a layer of fluid in this way is related to the velocity at which the fluid layer will move by the equation: A force needs to be applied to a layer of fluid in order to make it flow at a constant speed relative to any other layer. Whenever a fluid flows against a surface, the different layers of fluid exert frictional forces between each other, causing them to flow at different speeds. The main difference between dynamic and kinematic viscosity is that dynamic viscosity is a measurement of how difficult it is for a fluid to flow whereas kinematic viscosity is the dynamic viscosity of a fluid divided by its density. Usually, two types of viscosity are quoted: dynamic and kinematic viscosity. Viscosity is very important to any process that depends on a flow of fluid. The viscosity index of hydraulic system oil should not be less than 90.Main Difference – Dynamic vs. Kinematic Viscosity The way viscosity changes with temperature is reflected by the Viscosity Index: the smaller the viscosity change is the higher the viscosity index.
Kinematic viscosity equation full#
You can download full chart in PDF: Viscosity-Temperature-Chart.pdf or use online tool: TEMPERATURE-VISCOSITY CHART. Viscosity-Temperature chart for most popular hydraulic oil: For example, a lubricant with a VG value of 22 will have an average viscosity of 22 cSt (centistokes) at 40 degrees C: The actual VG value signifies a lubricant’s average viscosity at 40 degrees C.
Kinematic viscosity equation iso#
The International Standards Organization created the ISO VG (Viscosity Grade) in response to the need for a globally recognized viscosity designation. This is a generic chart for viscosity grade selection in depend of ambient temperature: For example, see recommendations from EATON. Generally, optimum operating viscosity of the hydraulic oil should be between 16 cSt (80 SUS) and 40 cSt (180 SUS).Īs a rule, the manufacturers of hydraulic components give hydraulic fluid viscosity recommendations in accordance with type of their pump you use in the system. In general, an oil which matches the viscosity requirements of the pump, will also be satisfactory for valves. Conversely, a fluid which is too thin may allow an accelerated rate of wear and additional slip losses. A fluid which is too viscous may prompt cavitation. Hydraulic components will operate efficiently only within a specific viscosity range, optimum operating range for each of them. As the viscosity decreases (when temperature increasing), the mechanical efficiency will increase due to low forces: As the temperature increases, liquid viscosity decreases and leakage becomes more significant, reducing the volumetric efficiency. Kinematic viscosity for some common liquids you can see at The Engineering ToolBox. NOTE: Equations above are for fluid with specific gravity 0.876 (like petroleum oil) and at fluids temperature 37.8☌ (100☏). For a quick and an approximate conversion you can use next formulas depends on viscosity range: SUS to cSt The conversion from Centistokes to Saybolt Universal Seconds in terms of calculations is specified by the ASTM D2161 and is not simple. In North America are more popular kinematic viscosity units Saybolt Universal Seconds (SUS) or Seconds Saybolt Universal (SSU). The SI unit for kinematic viscosity is m 2/s but the more common unit is the centistoke (cSt): The symbol for kinematic viscosity is the Greek letter nu (ν). The kinematic viscosity is the ratio of the dynamic viscosity μ to the density of the fluid ρ: It is easier to measure and more common to report the kinematic viscosity of a fluid. s), but the more common unit is the centipoise (cP):įor example, the dynamic viscosity of water at 20☌ is 1.00 cPīritish unit of dynamic viscosity, is the reyn, named in honour of Osbourne Reynolds:.
The SI unit for dynamic viscosity is the pascal-second (Pa The symbol for dynamic viscosity is the Greek letter mu (µ). There are dynamic and kinematic viscosity are usually common for calculations. The viscosity is a measure of the fluid’s resistance to flow. Selecting the ISO VG (Viscosity Grade) for your system.Understanding the optimum operating viscosity range.Dynamic and kinematic viscosity in SI and British unit.