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Comparison of Fluid Cleanliness Standards
Hydraulic cleanliness is an important aspect to be considered when specifying hydraulic systems and components, e.g. valve actuators. 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 Hydraulic Fluid Cleanliness for a review of the 2 most common.
The ISO Cleanliness Code, ISO 4406, 1987 is the perhaps the most widely used International standard however the dated NAS 1638 standard is still widely quoted
by industries involved in UK North Sea.
Inevitably, there is a requirement to be able to compare these cleanliness classification codes. The table below gives a good approximation;
| ISO Code | NAS Class |
| 23/21/18 | 12 |
| 22/20/18 | -- |
| 22/20/17 | 11 |
| 22/20/16 | -- |
| 21/19/16 | 10 |
| 20/18/15 | 9 |
| 19/17/14 | 8 |
| 18/16/13 | 7 |
| 17/15/12 | 6 |
| 16/14/12 | -- |
| 16/14/11 | 5 |
| 15/13/10 | 4 |
| 14/12/9 | 3 |
| 13/11/8 | 2 |
| 12/10/8 | -- |
| 12/10/7 | 1 |
| 12/10/6 | -- |
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