Usually, solid particles in oil are one of the main factors causing increased wear of hydraulic components and a significant decline in reliability and performance. For servo valves, contaminants will increase the hysteresis, while for pump components, contaminants will cause increased wear, heat generation, and reduced efficiency, thereby greatly shortening their service life.

The standard for measuring the degree of contamination is the oil contamination level obtained through contamination detection technology. For the detection of oil contamination, the earliest methods were rough methods such as the "visual method" or "gravimetric method" to detect "mechanical impurities". These methods are still used by a few industries (such as coal) today. However, they can only reflect whether there are visible particles in the oil or the total weight of particles, and cannot reflect the size and distribution of particles at all.



Later, the "microscope counting method" began to be adopted, which can be considered a major advancement in the determination of solid particle contamination in oil. However, due to its cumbersome operation, inability to compare results, and high variability in detection results, its effectiveness is still unsatisfactory. With the development of science and technology, laser automatic particle counting technology based on the light-blocking principle began to be applied to oil contamination detection. Because laser-type automatic particle counters have advantages such as fast counting speed, high accuracy, and easy operation, they quickly became the primary detection method.



Currently, the vast majority of instruments used for automatic oil contamination detection are laser-type automatic particle counters. The accuracy of automatic particle counters in measuring particle size distribution and concentration mainly depends on the calibration of the particle counter. Over the past 30 years, the main basis for this calibration has been ISO 4402 {Calibration of automatic particle counters — Method using air cleaner fine test dust (ACFTD)}, which has consistently used air cleaner fine test dust (ACFTD) as the standard material. The accuracy of the given particle size values of ACFTD directly affects the accuracy of calibration. Standards related to oil contamination detection are currently in a period where old and new standards coexist and gradually transition to new standards.



Although there are still many difficulties in replacing old standards with new ones, since ACFTD standard dust is no longer produced, the transition of calibration standards for contamination detection methods from ACFTD (ISO 4402) to ISO MTD (ISO 11171) is inevitable and irreversible. In summary, it is necessary to accelerate the formulation and publication of relevant new domestic standards.