Traditional oil analysis methods - e.g. acidity and viscosity measurements - have been used to monitor lubricant conditions. These methods can detect when the useful life of a lubricant is over but fall short when trying to gain insight on how long a lubricant in current use could last. This makes it difficult to make proactive decisions and estimate oil drain periods. Lubricants do not start to degrade until the antioxidants, which prevent from oxidation, have depleted to a certain level where they no longer can protect the base oil from degradation. During the degradation process insoluble contaminants form that can lead to sludge and varnish.

Four engine oils were oxidized using oxygen pressurized vessels and four hydraulic oils were oxidized with turbine oil stability test (TOST). At different stages of oxidation, sample aliquots were withdrawn and analysed. A blend of engine oil and biodiesel was also tested as well as a mixture of hydraulic oil and water. Samples of engine oils were also tested from a rig test running at SCANIA’s facilities in Södertälje, Sweden. The samples were evaluated with Remaining Useful Life Evaluation Routine (RULER) and Membrane Patch Colorimetry (MPC). RULER is a voltammetric method that measures the antioxidant level in a lubricant sample and MPC measure the insoluble contaminants by spectrophotometric analysis. Results from these analyses were compared to conventional methods such as acid number, viscosity, and Fourier Transform Infrared spectroscopy (FTIR).

Results from the MPC-analyses showed that this method is dependent on the type of the lubricant tested. RULER performed well for all tested lubricants. It was shown that this analyse method can predict when the lubricant is going to start to degrade due to oxidation. Tests showed that the oxidation of the lubricant starts when there are 20-25% of the antioxidants remaining.