National Aerospace Standard (NAS) 1638 is one of the methods used in coding the level of particle contamination used in the fluid power industry.

In the 1970s and 1980s it was used in the offshore oil production industry, the iron and steel industry to further lead to the development of other coding systems (ISO 4406).

The coding method is the highest class value in the range: (5-15 m), (15-25 m), (25-50 m), (50-100 m), 100 m.