![]() ![]() The relationship between the shear stress and the shear rate of fluid under mechanical stress is established by Newton’s law of viscosity. The following table states the viscosity of some popular substances. Temperature (☌) Kinematic Viscosity (m 2/s X 10 -6) 10 1.3059 20 1.004 30 0.801 50 0.553 70 0.413 90 0.326 100 0.294 Table 2: Kinematic Viscosity of water with respect to temperature Viscosity of Some Common Substances Table 2 below provides the kinematic viscosity of water. The kinematic viscosity of water can easily be obtained by dividing the above dynamic viscosity values by the water density. The following table provides the dynamic viscosity of water with respect to various temperatures. As for liquids, the viscosity decreases with an increase in temperature, the same is true for water. The viscosity of water at 20 0 C is 1 centipoise or 1 cP. Suspended Particles: Suspended materials increases viscosity.Multiphase flow: The viscosity of multiphase flow is affected by the volume of each phase.Liquids being incompressible does not have much impact. Pressure: With an increase in pressure, the viscosity of gases usually increases.Flow Conditions: For laminar flow, the viscosity of liquid remains constant while for turbulent flow viscosity changes.On the contrary, the viscosity of gases increases with an increase in temperature. Fluid Temperature: Usually the viscosity of liquids decreases with an increase in temperature.Lower viscosity motor oils also result in better fuel economy. These lower viscosity motor oils are needed to move through the thin engine pathways to protect and clean metal surfaces. This has led over time to motor oil viscosity decreasing – currently 5W-30 and 5W-20 are the most popular grade and 0W-20 is the fastest growing grade. But as engine technology has advanced over the years, the size of engines and pathways in engine that oil flows through have gotten smaller and thinner. In the past, it was common for vehicles to require higher viscosity oils such as 20W-50’s, 10W-40’s and 10W-30’s. The 5W-20 will have a lower viscosity and provide less resistance than the 10W-30 at both start-up and normal engine operating temperatures. If we compare a 5W-30 and a 10W-30, they will behave very similarly as the engine heats up, but at start-up, the 5W-30 will provide less resistance and start easier than the 10W-30. Less resistance results in improved fuel economy but the oil is not as thick and forms a smaller layer of protection between metal surfaces. But as the engine heats up, the 5W-20 will move with less resistance than an 5W-30. Again, the lower the number, the lower the viscosity of the oil and the faster the oil will move around the engine.Īs an example, let’s compare 5W-20, 5W-30 and 10W-30 motor oils. A 5W-20 and 5W-30 will have very similar if not equal viscosity at lower start-up temperatures. In the 5W-30 example, the 30 defines the viscosity of the oil at normal engine temperatures. The high temperature viscosity is the number after the dash and is related to the viscosity of the oil as it is moving around your engine after the car has warmed up and is at normal engine temperature. The lower the number, the faster the oil will flow at vehicle start up. For example, if the oil is a 5W-30, the 5W part describes the viscosity of the oil at low temperatures. ![]() That value has the letter “W” after the number and has a dash after the W. The low temperature viscosity of the oil is a measurement that simulates starting a car on a cold winter day. But as motor oil technology advanced, additives such as viscosity index improvers allowed for the use of the same grade of oil year-round. Years ago, most vehicles used one viscosity grade oil in the summer and a different viscosity grade oil in the winter. The measurement of viscosity at high and low temperatures are properties of multi-grade oils. ![]() Motor oil viscosity grades are based on a scale developed by the API (American Petroleum Institute) lubricant organization. The values are defined in a specification known as API 1509 and are based on the resistance the oil gives to flowing at two different temperatures – cold and high temperature. Viscosity is the resistance to flow of a fluid. ![]()
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