ISO Cleanliness Codes Explained
Understanding ISO 4406 fluid cleanliness ratings and what they mean for your hydraulic system.
What is ISO 4406?
ISO 4406 is the international standard for reporting fluid cleanliness levels. It uses a three-number code (like 18/16/13) to describe the concentration of particles in a fluid sample. Each number represents the quantity of particles above a specific size threshold per milliliter of fluid.
Reading the Code: What the Numbers Mean
A typical ISO code looks like this: 18/16/13
Lower numbers = cleaner fluid. Each time the code number increases by 1, the particle count roughly doubles. So an 18/16/13 fluid has about twice as many particles as a 17/15/12 fluid.
ISO Code Range Chart
Each range code corresponds to a particle count range:
| Range Code | Particles per mL (min) | Particles per mL (max) |
|---|---|---|
| 6 | 0.32 | 0.64 |
| 7 | 0.64 | 1.3 |
| 8 | 1.3 | 2.5 |
| 9 | 2.5 | 5 |
| 10 | 5 | 10 |
| 11 | 10 | 20 |
| 12 | 20 | 40 |
| 13 | 40 | 80 |
| 14 | 80 | 160 |
| 15 | 160 | 320 |
| 16 | 320 | 640 |
| 17 | 640 | 1,300 |
| 18 | 1,300 | 2,500 |
| 19 | 2,500 | 5,000 |
| 20 | 5,000 | 10,000 |
| 21 | 10,000 | 20,000 |
| 22 | 20,000 | 40,000 |
Target Cleanliness by Application
Different hydraulic components have different cleanliness requirements. More precise components with tighter clearances require cleaner fluid.
| Application / Component | Target ISO Code |
|---|---|
| Servo valves, high-pressure piston pumps | 14/12/9 |
| Proportional valves, vane pumps | 15/13/10 |
| Directional valves, gear pumps | 16/14/11 |
| Flow control valves, cylinders | 17/15/12 |
| Low-pressure systems, general industrial | 18/16/13 |
| New oil (typical as-received) | 20/18/15 |
Important Note
New oil from the drum is NOT clean enough for most hydraulic systems. It typically arrives at 20/18/15 or worse and should always be filtered before use.
How to Achieve Target Cleanliness
Select the right filter micron rating
To achieve 16/14/11, you typically need a 10-micron absolute (β10 ≥ 200) filter. For 14/12/9, use a 3-5 micron filter.
Filter new oil before adding to system
Use a filter cart or kidney loop to pre-filter new oil. Never add unfiltered oil directly to a clean system.
Use breathers with desiccant
Standard breather caps let in dirt and moisture. Desiccant breathers filter incoming air and remove water vapor.
Perform regular oil analysis
Test oil samples quarterly to track particle counts and catch contamination issues before they cause damage.
Why Cleanliness Matters
Contamination is responsible for 70-80% of hydraulic system failures. Even particles you can't see (under 40 microns) cause wear in pumps, valves, and cylinders. The clearances in modern servo valves can be as tight as 1-4 microns—smaller than a human hair.
The Cost of Contamination
Need Help With Filtration?
Our team can help you select the right filters to achieve your target cleanliness level. We work with all major filter manufacturers and can cross-reference existing filters to find the best solution for your application.
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