NAS 1638 Code ...

The NAS system was originally developed in 1964 to define contamination classes for the contamination contained within aircraft components. The application of this standard was extended to industrial hydraulic systems simply because nothing else existed at the time

The 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 and this convention is used on the LPA2.

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.

MAXIMUM CONTAMINATION LIMITS (PER 100 mL)

                                                           size range in microns

approx ISO 4406 equiv.	NAS code	5-15	15-25	25-50	50-100	over 100
---	00	125	22	4	1	0
---	0	250	44	8	2	0
10/12/7	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	16000	2850	506	90	16
18/16/13	7	32000	5700	1012	190	32
19/17/14	8	64000	11400	2025	360	64
20/18/15	9	128000	22800	4050	720	128
21/19/16	10	256000	45600	8100	1440	256
22/20/17	11	512000	91200	16200	2880	512
23/21/18	12	1024000	182400	32400	5760	1020