An ISO 4406 code such as 18/16/13 describes how many solid particles are in one millilitre of hydraulic oil: the first number covers particles of 4 µm(c) and larger, the second 6 µm(c) and larger, the third 14 µm(c) and larger. Each step up in a number roughly doubles the particle count, so 19/17/14 is about twice as dirty as 18/16/13. Most industrial and marine hydraulic systems should run at 18/16/13 or cleaner, and systems with proportional or servo valves cleaner still.
Particle contamination causes most hydraulic component wear. Knowing your code, and setting a target, is one of the cheapest ways to extend pump, valve and seal life.
How the code works
The current standard is ISO 4406:2021. A laboratory or online particle counter counts particles at three sizes and converts each count per millilitre to a scale number.
| Scale number | 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 |
Example: a sample with 1,800 particles ≥4 µm, 450 particles ≥6 µm and 60 particles ≥14 µm per mL is 18/16/13.
Some older reports give only two numbers (for example -/16/13), from microscope counting that does not measure 4 µm particles. Our oil cleanliness tool converts counts to codes and back.
Target codes by component
Set a target for each system based on its most sensitive component, pressure and how critical it is. These are typical targets for systems with mineral oil at normal working pressures; for pressures above about 210 bar, or critical duties, go one step cleaner on each number.
| Most sensitive component | Typical target |
|---|---|
| Servo valves | 16/14/11 or cleaner |
| Proportional valves | 17/15/12 |
| Variable piston pumps | 17/15/12 to 18/16/13 |
| Fixed piston pumps, vane pumps | 18/16/13 |
| Gear pumps, directional valves, cylinders | 19/17/14 |
| Low-pressure, non-critical systems | 20/18/15 |
Equipment makers often state a required cleanliness in the pump or valve manual. Where they do, use the maker’s figure; it is also often a condition of warranty.
NAS 1638 rough equivalents
Many ship manuals and older specifications still quote NAS 1638 classes. NAS 1638 was withdrawn and does not convert exactly to ISO 4406, because it counts different size bands. The usual approximate equivalents are:
| NAS 1638 class | Approximate ISO 4406 |
|---|---|
| 4 | 15/13/10 |
| 5 | 16/14/11 |
| 6 | 17/15/12 |
| 7 | 18/16/13 |
| 8 | 19/17/14 |
| 9 | 20/18/15 |
| 10 | 21/19/16 |
| 11 | 22/20/17 |
| 12 | 23/21/18 |
Use these for rough comparison only. When a specification matters, ask the lab to report against the standard the specification names. SAE AS4059 is the modern replacement for NAS 1638 in many sectors.
New oil is not clean oil
Oil from a new drum is often 21/19/16 or worse, after filling, handling and storage. Pouring it straight into a system that needs 17/15/12 adds dirt. Good practice:
- Fill through a filter cart or the system’s return filter, never through an open hatch.
- Keep drums sealed, stored lying down or under cover, out of sun and rain.
- Use dedicated, clean transfer equipment.
How to take a meaningful sample
A bad sample makes the code meaningless.
- Sample from a running system at operating temperature, so particles are in suspension.
- Use a dedicated sampling valve on a turbulent line, ideally after the pump and before the filter, or a return line before the filter.
- Flush the valve first: discard several hundred millilitres.
- Use a certified clean bottle. Fill to about 80%, cap immediately.
- Label with equipment, point, date, oil temperature and running hours.
- Do not sample from the bottom of the tank or a drain plug, unless you are looking for sludge and water.
Sample regularly, for example every three or six months on critical systems, so you can see trends.
Reading the trend
- Codes rising across all three numbers: filter bypassing, filter element failed, or a new ingress path (breather, seals, new oil).
- Third number (14 µm) rising more: wear particles from a component, or a filter that is too coarse.
- Sudden jump after maintenance: dirt introduced during work.
Ask the lab for water content, viscosity, acid number and wear metals too. A particle count alone does not show water or oxidation.
UAE-specific contamination sources
- Fine dust and sand through tank breathers, worn wipers and open fill points. Use desiccant or fine particulate breathers on tanks in dusty sites.
- Humidity and condensation, especially in equipment that heats up by day and cools at night, and on deck. Water speeds wear and oil breakdown.
- Salt spray on deck machinery tanks and open reservoirs.
- Heat that ages oil and creates varnish and sludge.
Getting from where you are to the target
- Fix ingress first: breathers, seals, wipers, fill procedure.
- Upgrade filtration where needed: a finer element, or an offline kidney-loop filter.
- Change elements on condition, using differential pressure indicators.
- Check the codes again after a few weeks.
Each code step down roughly halves the particle count, and many component makers show service life rising sharply as cleanliness improves.
When to call us
If a component keeps failing, ask for an oil sample and particle count before replacing it again. We look at valves and pumps alongside the oil results and tell you what is causing the contamination. See our power pack service and valve service. Convert counts and codes with the oil cleanliness tool.
