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How was the ASTM formula for Viscosity Index derived? Specifically with reference to the reference oils considered and the table for it.

The viscosity index can be calculated using the following formula:

$$V = \frac{100*(L-U)}{(L-H)}$$

Check this reference for details.

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back when it was invented they measured all the oils they could find. They got the worst (most viscosity change) and the best in each group and ranked these as 0 to 100. All others were plotted on this "linear" scale.

The unit was used for dome time until the best was bettered, then they invented VIE (E is a subscript, and stands for extension, strictly all VI's above 100 are VIE's).

It is a horrid unit because it is far from having any useful meaning, unfortunately it has some significance.

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