A Comparison of NAS 1638 and ISO 4406 Cleanliness Codes

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.

Contamination level classes according to NAS 1638 (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 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
—	00	125	22	4	1	0
—	0	250	44	8	2	0
12/10/7	1	500	89	16	3	1
13/11/8	2	1000	178	32	6	1
14/12/9	3	2000	356	63	11	2
15/13/10	4	4000	712	126	22	4
16/14/11	5	8000	1425	253	45	8
17/15/12	6	16,000	2850	506	90	16
18/16/13	7	32,000	5700	1012	190	32
19/17/14	8	64,000	11,400	2025	360	64
20/18/15	9	128,000	22,800	4050	720	128
21/19/16	10	256,000	45,600	8100	1440	256
22/20/17	11	512,000	91,200	16,200	2880	512
23/21/18	12	1,024,000	182,400	32,400	5760	1020