Quality Control of Lubricants

Significance of the Topic

Ensuring the quality of lubricants is essential for the reliable operation of mechanical systems, extending equipment lifespan, and minimizing maintenance costs. Conventional laboratory tests for key parameters such as acid number, viscosity, moisture content, and color number involve multiple techniques, significant sample preparation, and chemical reagents. These traditional workflows can be time-consuming and resource-intensive.

Objectives and Study Overview

This application note evaluates the performance of a visible-near infrared (Vis-NIR) spectroscopic approach—using the XDS RapidLiquid Analyzer—to rapidly and simultaneously determine four critical quality parameters of lubricants without chemical reagents or lengthy sample preparation.

Methodology and Instrumentation

Measurements were carried out in transmission mode over 400–2500 nm at a controlled temperature of 40 °C. Disposable borosilicate vials with an 8 mm path length eliminated the need for cleaning between samples. Model development and routine predictions were managed using Vision Air Complete software.

• XDS RapidLiquid Analyzer (Metrohm 2.921.1410)
• Disposable 8 mm glass vials (Metrohm 6.7402.000)
• Vision Air Complete spectroscopy software (Metrohm 6.6072.208)

Main Results and Discussion

Calibration and cross-validation metrics demonstrate strong agreement between Vis-NIR predictions and reference methods:

• Acid Number (ASTM D664): R² = 0.898; SEC = 0.422 mg KOH/g; SECV = 0.439 mg KOH/g
• Viscosity (ASTM D445): R² = 0.987; SEC = 1.77 cSt; SECV = 1.84 cSt
• Moisture Content (ASTM D6304): R² = 0.907; SEC = 0.0059 %; SECV = 0.0062 %
• Color Number (ASTM D1500): R² = 0.700; SEC = 0.841; SECV = 0.916

All four parameters were determined in a single measurement cycle lasting under one minute, compared to a combined 15 minutes or more using conventional methods.

Benefits and Practical Applications

The Vis-NIR approach offers rapid, multi-parameter analysis without chemical reagents, reduces operational costs, and streamlines sample throughput. Laboratories can integrate this method for routine quality monitoring in research, production, and QA/QC environments.

Future Trends and Applications

Further enhancements may include inline process monitoring, miniaturized spectrometers for field use, advanced chemometric algorithms leveraging machine learning, and expanded spectral libraries for novel lubricant formulations.

Conclusion

This study confirms that Vis-NIR spectroscopy with the XDS RapidLiquid Analyzer provides a fast, reliable, and environmentally friendly alternative to traditional wet-chemical methods for lubricant quality control, achieving high precision and significant time savings.

References

• Metrohm AG. AN-NIR-041: Quality Control of Lubricants with NIRS RapidLiquid Analyzer.