But at higher temperatures, the polymers expand and that helps increase viscosity.
At low temperatures, the polymer chains in the modifiers contract or fold and don’t have much effect on fluid viscosity. These modifiers are temperature sensitive. Synthetic oils usually have a higher VI than do mineral oils.įluid manufacturers can improve the viscosity index of base oils by using polymer additives to form multigrade-viscosity oils. Highly refined mineral oils with few contaminants tend to have higher VIs, all things being equal. Standard ASTM D2270 calculates viscosity index in part by measuring a fluid’s kinematic viscosity at 40° and 100° C. That, in turn, means consistent, high performance from a machine. The best oils with the highest VIs remain stable and don’t vary much in viscosity over a wide temperature range. Typical mineral-oil fluids used in hydraulics have a viscosity index of around 100, although products range from below 100 to well beyond 200. High-VI fluids, in contrast, are less affected by temperature changes.
Thus, a fluid with a low viscosity index will experience a relatively large swing in viscosity as temperatures change. (It also applies to fluids like engine and automatic transmission oils, gear lubricants, and power-steering fluids.) The greater the viscosity index (VI), the smaller the change in fluid viscosity for a given change in temperature, and vice versa. Viscosity index is a dimensionless number that represents how the viscosity of a hydraulic fluid changes with temperature.
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