Measuring Hydraulic Fluid Cleanliness

This page on Measuring Hydraulic Fluid Cleanliness is an excerpt from the original Hydraulic Oil Cleanliness webpage, published on the Control And Instrumentation.com website, and is published here with their permission.

Hydraulic cleanliness is an important aspect to be considered when specifying hydraulic systems and components. It is usual to request that the system is flushed to achieve a specific degree of cleanliness, e.g. NAS 6. Measuring hydraulic cleanliness is not straight forward, consequently various standards exist on this subject. See our page on comparing hydraulic cleanliness classes.

ISO 4406

The ISO Cleanliness Code, ISO 4406, 1987 is the perhaps the most widely used International standard for representing the contamination level of industrial fluid power systems. Under ISO 4406 cleanliness is classified by a two number code, e.g. 16/13, based on the number of particles greater than 5 µm and 15 µm respectively in a known volume of fluid. However some manufacturers have expanded the code to three numbers by the addition of a code number representing the number of particles greater than 2 µm, e.g. 18/16/13. Using the table below, we can see a cleanliness rating of 18/16/13 would mean that there were
1300 - 2500 particles greater than 2 micron in size
320 - 640 particles greater than 5 micron in size, and
40 - 80 particles greater than 15 microns in size.

The full table of ranges for ISO 4406 is shown below

Range Number	No of Particles per ml
Range Number	More Than	Up to and including
24	80 000	160 000
23	40 000	80 000
22	20 000	40 000
21	10 000	20 000
20	5 000	10 000
19	2 500	5 000
18	1 300	2 500
17	640	1 300
16	320	640
15	160	320
14	80	160
13	40	80
12	20	40
11	10	20
10	5	10
9	2.5	5
8	1.3	2.5
7	0.64	1.3
6	0.32	0.64

NAS 1638

The NAS 1638 cleanliness standard was originally developed for aerospace components in the US but is still widely used for industrial and aerospace fluid power applications. It is used widely in the UK North Sea industries. NAS 1638 is comprised of fluid cleanliness classes, each class defined in terms of maximum allowed particle counts for designated particle size ranges. See NAS value table below.

**NAS Value Chart**
Class	Maximum Number of Particles / 100 ml
Class	5 - 15	15 - 25	25 - 50	50 - 100	>100
00	125	22	4	1	0
0	250	44	8	2	0
1	500	89	16	3	1
2	1000	178	32	6	1
3	2000	356	63	11	2
4	4000	712	126	22	4
5	8000	1425	253	45	8
6	16000	2850	506	90	16
7	32000	5700	1012	180	32
8	64000	11400	2025	360	64
9	128000	22800	4050	720	128
10	256000	45600	8100	1440	256
11	512000	91200	16200	2880	512
12	102400	182400	32400	5760	1024

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