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Reply to "Hydraulic oil blackening"

Micro-dieseling is caused by the implosion of air bubbles as they move from a low pressure zone (reservoir) to a high pressure zone (pump). When the bubble implodes, a very high temperature is created essentially cracking the hydrocarbon molecule. This typically produces by-products with both light-ends and heavy-ends. The heavy-ends will agglomerate and form deposits, (the golden coating that you referred to) while the light-ends may flash off. This explains why micro-dieseling in hydraulic oil develops a distinct sulfur odor, not unlike rotten eggs. Does your oil have a peculiar smell? If so, micro-dieseling may be the cause.

From an analytical standpoint, FT-IR is a good method to detect micro-dieseling. I prefer to view the used oil graph overlaid on top of a new oil graph to interpret the results. Look for a peak in the 1600-1630cm-1 range.

Finally, switching to a heavier viscosity fluid will not help out your situation, as it will take even longer for air bubbles to escape. Exhaust all efforts to minimize the air in your system. Is it possible to look in your reservoir while your system is running to see if there are any air bubbles? You may want to investigate the “bubble eliminator”, an innovative product designed to remove air from a system.

Best of luck.
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