Comparison of Hydraulic Fluid Cleanliness Standards
This page on Comparison of Hydraulic Fluid Cleanliness Standards 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, 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 oil and gas production.
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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