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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 cleanliness classes.
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 | |
| 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 |
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 | |
| 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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