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Variability in Oil Analysis Results

Evan Zabawski
Evan Zabawski | Senior Technical Advisor, TestOil

Variability in Oil Analysis Results

Two consecutive samples, at normal sampling intervals from the same machine, are never expected to have exactly the same results, but occasionally the second set of results appears to have either changed by more than a normal amount, or possibly have shifted in a counter-intuitive direction (i.e. gotten better without any top-ups or other actions). In these cases, it is quite common to suspect the lab has erred, but the following explanation shows why this trend occurs regularly.

Each test method will have an acceptable tolerance of variation. Repeatability is defined as the difference between two test results, obtained by the same operator with the same apparatus under constant operating conditions on identical test material. Reproducibility is defined as the difference between two single and independent results, obtained by different operators working in different laboratories on identical test materials. Most ASTM methods identify the acceptable tolerance level, or repeatability and reproducibility, of a test; typically, these are reported with a 95% confidence level, meaning the values can only exceed 1 case in 20.

Using elemental spectroscopy as an example, some significant differences in repeatability and reproducibility can be observed. Copper at 10 ppm has a repeatability of 1.0 ppm, and a reproducibility of 2.4 ppm; but lead at 10 ppm has a repeatability of 3.3 ppm, and a reproducibility of 6.9 ppm. Given that repeatability is akin to running the same sample twice in a row, whereas reproducibility is more like rerunning a sample on a different day, the expected difference between two samples pulled on different days should not be expected to be any tighter, and it can be either higher or lower than the previous result.

Some tests, like viscosity, have a variability of approximately 1%, meaning that differences are only noticeable on the first, or potentially second, decimal place. Other tests, like particle count, have a variability exceeding 100%, meaning that the difference between two results can be as high as two-fold (or 1 ISO code). Some tests even have differing variability depending on the magnitude of the result.

The lab is familiar with these limits and takes them into account when reviewing reports; therefore, it is important to have a general idea of just how accurate a test is expected to be before reacting to any differences in results not identified by the lab.

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About the Author

Evan Zabawski
Evan Zabawski Senior Technical Advisor, TestOil

Evan is a Certified Lubrication Specialist. Evan has extensive experience training tradesmen and professionals in a variety of fields including: lubrication fundamentals, contamination control, condition monitoring, RCM/FMEA and used oil analysis. Evan has been a member of STLE for over 20 years, serving as Chair of the Alberta Section for 8 years, and also as an instructor of the Condition Monitoring course at STLE Annual Meetings. Currently, Evan has Editor of TLT Magazine, and have served as the Editor for The STLE Alberta Section’s Basic Handbook of Lubrication – Third Edition, and contributed as one of the editors for STLE/CRC’s Handbook of Lubrication and Tribology, Volume II: Theory and Design, Second Edition. Evan has published several technical papers and am also a member in good standing of API and ASTM.