The temperatures used to determine the VI are 40 degrees C to 100 degrees C.Īn Internet search on “viscosity index calculator” will direct you to several Web pages. The higher the VI, the more stable the viscosity across a range of temperatures (more desirable). Halfway in between was a VI of 50, and so forth. If it was similar to Texas Gulf crude, it was assigned a VI of 0. If a lubricant was similar to the Pennsylvania crude, it was assigned a VI of 100. Based on the methodology, Pennsylvania crude (paraffinic) was set as a benchmark at one extreme, representing low viscosity changeability relative to temperature.Īt the other extreme was Texas Gulf crudes (naphthenic). It is an empirically derived, unitless number. The viscosity index was developed for this purpose (ASTM D2270) by E. Most importantly, for any candidate oil, you need to know how viscosity changes relative to a change in temperature. Remember, the ISO Viscosity Grade system only reports viscosity at a single temperature: 40 degrees C. That said, because machines require a certain viscosity, and temperature is known to have a dramatic influence on viscosity, it is imperative that you take the average operating temperature and temperature range into account when selecting viscosity. You may recall that the well-known Stribeck curve doesn’t have a temperature variable. Instead, a machine’s viscosity requirement is based on such things as component design (e.g., bearing), loads and speeds. Minimum, maximum and optimum viscosity requirements demanded by machines don’t take temperature into account. Conversely, machines don’t care about temperature as it relates to viscosity (loosely speaking). However, a lubricant’s viscosity is meaningless unless the temperature is noted, i.e., the temperature at which viscosity is measured.

Viscosity index calculator