Appendix C
NAS1638 Cleanliness Code System
45
NAS1638 Cleanliness Code System
The NAS system was originally developed in 1964 to define contamination
classes for the contamination contained within aircraft components. The ap-
plication of this standard was extended to industrial hydraulic systems simply
because nothing else existed at the time. The coding system defines the max-
imum numbers permitted of 100ml volume at various size intervals (differ-
ential counts) rather than using cumulative counts as in ISO 4406:1999. Al-
though 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 and this convention is used on the LPA² software.
Figure I
CONTAMINATION LEVEL CLASSES
according to NAS1638 (January 1964).
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 differ-
ential basis, in a given size bracket.
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