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Measuring Hydraulic Fluid Cleanliness

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

comparingcleanliness classes.

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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 m and 15 m 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 m, 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

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

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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 below.

Class

Maximum Number of Particles / 100 ml

5 - 15 15 - 25 25 - 50 50 - 100 >100

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