NAS 1638 Cleanliness Rating

Canonical Definition

A fluid cleanliness classification system originally developed by the National Aerospace Standard (NAS 1638), defining 14 classes (0–12) based on particle counts per 100 mL in five size ranges (5–15 µm, 15–25 µm, 25–50 µm, 50–100 µm, >100 µm). Higher NAS class numbers indicate higher contamination. NAS Class 6 is commonly specified for industrial hydraulic systems; Class 4–5 for servo and proportional valve systems. NAS 1638 has been largely superseded by ISO 4406 for new system specifications but remains common in legacy aerospace and military equipment documentation.

Engineering Context

NAS 1638 and ISO 4406 are not directly interchangeable — they use different size ranges and counting methods. Approximate correspondence: NAS 5 ≈ ISO 15/13/10; NAS 7 ≈ ISO 17/15/12; NAS 9 ≈ ISO 19/17/14. When converting between systems, use the ISO 11171 counting method (designated as "c") rather than older optical microscopy methods to maintain accuracy.

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