ALS Tribology eSource
January 2011

Oil CupsMonitoring Oil Oxidation
by David Doyle, CLS, OMA I & II
Vice President & Operations Manager

There are several schools of thought regarding determination of oxidation for in-service industrial lubricants. The most common routine tests for monitoring oxidation are viscosity, acid number, and oxidation number by infrared spectroscopy (FTIR). Of these tests, individuals will have varying opinions as to what tests give the best value. Some opinions contend that monitoring the increase in acid number gives you the most reliable monitoring tool. Other individuals hold to the view that oxidation number by FTIR is your only reliable monitoring tool. There is also the view that as long as viscosity is not increasing there is no problem.

The best answer to these varying opinions is: none of the above AND all of the above. When considering viscosity, acid number, and oxidation number, no single test parameter is definitive for establishing the level of oxidation in industrial lubricants. A combination of test parameters and evaluation of the overall effect shown by multiple tests provides the most reliable analysis of the rate of oxidation. To some extent, these tests can also be correlated with monitoring color change of the lubricant, where severe darkening of the oil would associated oxidation indicate by other tests. By comparing the changes in a matrix of tests the various factors that contribute to different analysis results due to oxidation give a better overall picture.

Factors that cause changes in acid number for used lubricants can be complex and, at times, an increase in the value may not occur until the very end of the fluid life . . . and then a sudden, dramatic increase may occur. Oxidation number monitoring is good for a consistent trend but is not an end-all and provides a limited picture. In addition, even if viscosity has not changed much there can still be detrimental oxidation products created that may not readily have an effect on viscosity. Therefore, by looking at a matrix of test parameters, the strength of each test can give an overall clearer picture.

There are additional tests that do a good job of monitoring lubricant oxidation life. These are somewhat more involved tests, tend to take more time, and are more expensive. One such test is oxidation by RPVOT. RPVOT does not necessarily look at the level of oxidation in a lubricant. The test actually monitors the depletion of oxidation inhibitor in the formulation in order to predict continued service life. Another such test is the RULER, which also monitors the active oxidation inhibitor remaining in a lubricant. Analysis for varnish potential is a useful test for determining the level of harmful degradation material created by the process of oxidation. These tests are useful as additional verification when the more routine tests for viscosity, acid number, and oxidation number show signs of service life depletion and increased lubricant oxidation

The best approach to monitoring oxidation in a lubricant is to perform a combination of tests and avoid relying on one single parameter to determine fluid integrity. This approach involves both analyzing the level of fluid oxidation and predicting the remaining service life that can be expected in a lubricant.

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