6/22/2023 0 Comments Viscosity index calculator![]() Of course, there is no difference at 40 degrees C.Īs mentioned previously, the machine dictates the viscosity requirements based on its design and operating conditions. Of lubrication professionals consider an oil’s viscosity index when selecting a lubricant, according to a recent survey at Ĭomparing the 95 VI oil to the 150 VI oil, there is a 236-percent difference in viscosity at minus 20 degrees C and a minus 25-percent difference at 100 degrees C. Now let’s examine the viscosity of these oils from minus 20 degrees C (minus 4 degrees F) to 100 degrees C (212 degrees F). However, one of these oils (Oil A) has a VI of 95 (mineral oil), while the other (Oil B) has a VI of 150 (synthetic). Let’s take a look at two different oils that share one common property - they’re both ISO VG 150. However, the vast majority of lubricants on the market will have VIs in the range of 90 to 160. Other lubricants can have VIs ranging over 400. Lubricants with VIs as low as minus 60 are available today. Better yet, you can enter a single known viscosity (and temperature) and VI (commonly found on a lubricant’s product data sheet) to calculate the viscosity of the same lubricant at any other temperature (say, a machine’s operating temperature). For instance, if you know the viscosity of a lubricant at two different temperatures, you can use the calculator to give you an estimation of the VI. You can use these calculators in a variety of ways. 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.
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