Investigation of the measurement precision of oil analysis instruments, using fully formulated oils. Part 2: contamination-measuring instruments

摘要

Purpose - The aim of this study is to determine the variation of the different oil analysis instruments in terms of standard deviation and CV-values, when measuring samples of fully formulated hydraulic and gear oils taken from working systems. Design/methodology/approach - In this investigation two different spectrometric techniques, ICP-OES and RDE-OES, have been studied to determine the instruments' precision of measurement and ability to measure the absolute level of contamination. Findings - The ICP has better precision of measurement of the two instruments, but cannot predict the absolute values of contamination when oil samples are only treated by organic solvent dilution if the samples include large or dense particles. It is therefore not too good, with the sample pretreatment method used, at detecting wear processes that produce dense/large particles, such as pitting failure. For instance, microwave-assisted acid digestion could be used for sample pre-treating to obtain accurate results in that case. It should, however, be able to detect wear mechanisms that produce small particles such as abrasive wear in any case: the ICP has a repeatability value of r = 3 per cent and a reproducibility value of R = 12 per cent for contamination levels of between 50-400 PPM and r = 0.6 PPM and R = 2 PPM, respectively, at values below 50 PPM; the RDE cannot predict the absolute value of contamination if this includes large or dense particles if proper sample pre-treatment is not used. It is therefore not good at detecting wear mechanisms that produce dense/large particles (if the oil samples are not pre-treated properly) such as pitting but should be able to detect abrasive wear and similar processes that produce small particles in any case; the RDE's precision of measurement is not as good as the ICP, with a reproducibility variation of R = r = 25 per cent for contamination levels between 20-500 PPM and R = r = 6 PPM for contamination level below 20 PPM. Research limitations/implications - Measuring only on fully formulated oils from hydraulic and gear systems. Practical implications - The study will be of significant support regarding industrial interpretation of measurement results from the most common oil particle measurement methods. Originality/value - No other similar studies are known.