Oil Cleanliness: ISO vs. NAS

Oil cleanliness is defined as the measure of the level of particle contaminants in the oil. Particle contaminants can include both insoluble (solid or liquid materials that won’t mix in with the oil) and hard (e.g. metal shavings) particles. OEMs often provide recommendations for acceptable oil cleanliness levels for their equipment. Contamination control through proactive maintenance can ensure cleanliness levels are met, including both insoluble and hard particles.

Rating Systems

Cleanliness is an important indicator of the quality of your lubricating oil. This is often referred to as the oil’s particle count. Oil cleanliness is generally defined by two different particle count rating systems: the National Aerospace Standard (NAS) 1638 and the International Standards Organization (ISO) 4406. These both provide a classification of the number of particles to rate cleanliness

NAS 1638

NAS 1638 includes five “counting ranges” of particle size:

• 5 to 15 microns
• 15 to 25 microns
• 25 to 50 microns
• 50 to 100 microns
• >100 microns

These counting ranges are then classified into contamination levels by a number ranging from 00 (cleanest) to 12 (dirtiest) based on number of particles per 100 mL of fluid for each of the particle size counting ranges.

ISO 4406

ISO 4406 uses three overlapping size classes to rate particles instead of size ranges:

• >4 microns
• >6 microns
• >14 microns

Contamination levels are classified based on the number of particles greater than the three size ranges (4, 6 and 14 microns). This is determined in the equivalent of one millilitre of a fluid sample.

Converting NAS 1638 to ISO 4406

While both standards are applicable for industrial systems, ISO 4406 cleanliness codes are the more common standard in use. Since NAS and ISO use different counting methods, it’s not possible to have an exact conversion between these two standards. Therefore, only approximate comparisons between NAS and ISO are possible.

Additional Resources

Testing: ISO Particle Count
PDF: Understanding ISO Particle Counts