Hydraulic oil cleanliness conversion relationship

The servo mechanism in the hydraulic system is very precise (the general clearance is 3μm), which is easily blocked by the particles in the oil, leading to a failure of control. Particles of lubricating oil will accelerate the wear of seals in the system, thereby reducing the reliability and service life of equipment. Therefore, it is necessary to measure the cleanliness of oil products in an accurate way, which is called “cleanliness”.

At present, ISO 4406 and NAS 1638 are widely used to express the cleanliness of oil. There is a certain correspondence between the two standards. China equivalent adopted ISO 4406-1999 standard, formed “hydraulic transmission oil solid particle pollution level code” GB/T 14039-2002 national standard.

The ISO 4406-1999 standard is to determine the number of particles >4μm, >6μm, >14μm in 100ml of oil, The ISO 4406 cleanliness standard was tabled by the International Organization for Standardization in 1991 for reference to pollution grade codes (cleanliness).

ISO 4406 is selected for 5μm, 15μm these two sizes, because studies have shown that 5μm particles will cause serious wear, and >15μm particles will cause hydraulic components stuck, so these two sizes can basically reflect the wear of the oil, stuck conditions.

How to convert the hydraulic oil cleanliness under different standards? The following table shows the conversion method between ISO4406 cleanliness table and different standards:


The tables in this guide allow users of using automatic portable particle counters to see the relationship between raw particle counts at various sizes and the reporting code numbers of various contamination standards.

Note that some of the table entries are defined as cumulative counts (e.g. “> 6µm”) and others are defined as differential counts (e.g. 6–14µm”).

Instances of particle sizes given as “µm” refer to ACFTD (i.e. Air Cleaner Fine Test Dust) distributions. Instances of particle sizes given as “µm(c)” refer to MTD (i.e. ISO Medium Test Dust) distributions.

All standards are in counts per volume, and provide easy methods for converting particle counts into limits that are simple to interpret. By noting the requirements of the standard,

particle counts can be accurately converted to contamination levels.





The NAS 1638 cleanliness standard was developed for aerospace components in the US and is still widely used for industrial and aerospace fluid power applications.

The figures are differential counts, and the NAS class is usually reported as a single figure representing the maximum allowed particle counts (i.e. worst case) for designated particle size ranges.


Post time: Aug-18-2022
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