If this reduction is significant, the system may not be sufficiently lubricated over the entire operating temperature range. Oil viscosity (see ASTM D445) usually decreases with increasing temperature. Below you will find a simple VI calculator. Normally, all things being equal, highly refined mineral oils with few contaminants have high VIs and Synthetic oils generally have a higher VI than mineral oils. Standard ASTM D2270 Calculates Viscosity Index by Measuring the Kinetic Viscosity of Liquids at 40° and 100☌ and ASTM D567 Method for Calculating Viscosity Index from Viscosity at 100✯ and 210✯. In turn, this means consistent, high performance in the machine. The best oils with the highest VI are stable and do not vary greatly in viscosity over a wide temperature range. Viscosity Index was measured by a scale of 0 to 100 however, modern science of lubrication has led to the development of oils with very high VI. High-VI liquids, 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 temperature changes. The higher the VI, the smaller the change in fluid viscosity for a given change in temperature and vice versa. We can say that the it is the dimensionless number that shows how the temperature change can affect viscosity of an oil (engine oil and automatic gear oils, and power-steering fluids). The viscosity index (VI) is an arbitrary, unitless measure of a fluid’s viscosity change relative to a temperature change.
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