![]() ![]() Because no viscosity index is defined, the viscosity at other temperatures then at 40 degrees C can differ quite considerably for the same ISO VG grade. The ISO viscosity grades just define a certain viscosity range at 40 degrees C.(nominal value plus or minus 10%). Oils with a lot of VI-improver usually exhibit this type of behaviour at low temperatures (ATF, multigrade engine oils). For non-Newtonian fluids this type of calculation is less accurate, especially at lower temperatures. ![]() If you know the viscosity of a Newtonian fluid at two different temperatures the equation can be solved and kinematic viscosities at other temperatures can be calculated. The socalled Walther equation describes the temperature-viscosity relationship of Newtonian fluids.
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