Simultaneous monitoring of multiple quality parameters of lubricating oils by Vis-NIR spectroscopy

The possibilities of visible (Vis) and near-infrared (NIR) spectroscopy in the petrochemical industry were demonstrated on the example of the determination of various physical and chemical quality parameters in lubricating oils according to ASTM E1655. Specific NIR methods were developed using a large sample set with different oils from several manufacturers. The unique combination of near-infrared spectroscopy with visible spectroscopy enabled the determination of additional quality parameters like the color number. The determined standard errors of cross-validation were found to be 0.21 mg KOH g-1 for acid number, 0.14 mg KOH g-1 for base number, 0.9 for color number, 0.0031 g cm-3 for density, 0.11 % for moisture, 0.54 cSt for viscosity at 100 °C, 11.7 for viscosity at 40 °C in cSt and 1.5 for viscosity index. The determined errors were comparable with errors of conventional test methods for lubricating
oil. The developed methods were successful validated in an additional step by using an independent set of samples. In summary, Vis-NIR spectroscopy has been proven to be a rapid, time- and cost-saving method for quality control of lubricating oil.