The NAS system was originally developed in 1964 to define contamination classes for contamination contained in aircraft components. The application of this standard was extended to industrial hydraulic systems as nothing else existed at the time. It is still referred to in some industries, although the ISO 4406 cleanliness codes are more commonly found.
The NAS 1638 coding system defines the maximum numbers permitted of 100mL volume at various size intervals (differential counts) rather than using cumulative counts as in ISO 4406.
Although there is no guidance given in the standard on how to quote the levels, most industrial users quote a single code which is the highest recorded in all sizes. This convention is employed with laser particle size analyzers.
The contamination classes are defined by a number (from 00 to 12) which indicates the maximum number of particles per 100 mL, counted on a differential basis, in a given size bracket. Note that the comparison to ISO 4406 is necessarily an approximation.
Maximum Contamination Limits (per 100 mL)
|Size range in microns|
|Approximate ISO 4406 Equivalent||NAS 1638 code||5-15 µm||15-25 µm||25-50 µm||50-100 µm||Over 100 µm|