Most hydraulic pump and valve failures start with particles you cannot see. A speck of 10 microns is about a tenth of the width of a human hair, yet it is big enough to score a pump, jam a servo valve or wear a cylinder seal. The ISO 4406 code is how the industry measures this dirt.
An ISO 4406 code such as 18/16/13 gives three particle counts per millilitre of oil: particles of 4, 6 and 14 microns and larger. Each number is a range, and every step up roughly doubles the count. Typical targets are around 16/14/11 for servo valves, 17/15/12 for proportional valves and 18/16/13 for most vane and piston pump systems.
A particle counter counts the particles in each millilitre of oil and reports three numbers, separated by slashes:
Each number is a range code, not the count itself:
| Range code | Particles per mL (more than – up to) |
|---|---|
| 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 |
So 18/16/13 means 1,300–2,500 particles of 4 µm or more, 320–640 of 6 µm or more and 40–80 of 14 µm or more in every millilitre. Moving from 18/16/13 to 21/19/16 means roughly eight times more dirt.
Targets depend on the most sensitive part in the system and on operating pressure. The table shows commonly used starting points; your component maker's figure overrides it.
| Most sensitive component | Typical target |
|---|---|
| Servo valves | 16/14/11 |
| Proportional valves | 17/15/12 |
| Variable piston pumps, high pressure | 17/15/12 |
| Vane and fixed piston pumps | 18/16/13 |
| Gear pumps, low-pressure systems | 19/17/14 |
| Industrial gearboxes | 18/16/13 to 20/18/15 |
Oil can pick up dirt in drums, transfer containers, funnels and hoses before it ever reaches the machine. New oil filled straight from a drum is often well above the target for servo or proportional systems. That is why many plants filter new oil into the machine through a filter cart.
Many Indian machine manuals and oil reports still quote NAS 1638 classes. NAS is an older standard based on different particle sizes, so conversions are only approximate. As a rough guide, NAS 6 is close to ISO 17/15/12, NAS 7 to 18/16/13, NAS 8 to 19/17/14 and NAS 9 to 20/18/15.
ISO 4406 counts particles, not water. Water causes rust, additive loss and faster oxidation. Ask for water content (ppm) in the same oil report, and aim to keep it as low as practical, typically below a few hundred ppm for hydraulic oil.
Cleaner oil means fewer pump and valve failures, longer oil life and less downtime. Moving a system from 21/19/16 to 17/15/12 is roughly a sixteen-fold drop in particles at each size, and component makers publish life-extension tables showing large gains from improvements of that kind. Cleanliness is usually the cheapest reliability improvement a plant can make.
What does ISO 18/16/13 mean?Per millilitre of oil there are 1,300–2,500 particles of 4 µm and larger, 320–640 of 6 µm and larger, and 40–80 of 14 µm and larger.
Is NAS 6 the same as ISO 17/15/12?They are close but not identical, because the two standards count different particle sizes. Treat any conversion as approximate.
How often should hydraulic oil be tested?For critical machines, every three months is a common starting point; for less critical ones, every six months. Test sooner after a failure or a top-up from a new source.
Can I clean oil instead of changing it?Often yes. If viscosity, acid number and additives are still in range, filtration can bring a dirty oil back to target at a fraction of the cost of a change.
Want to set cleanliness targets for your hydraulic machines? Share your oil reports or machine list and our team will help you read them.