Hydraulic contamination control starts with a target, not with a filter micron label. An oil cleanliness code records the concentration of particles in a sample. The system designer then chooses filtration, filling and sampling practices that can maintain a suitable target for the most sensitive component under the real operating conditions.
What an ISO 4406 code means
The commonly used three-number ISO 4406 code reports particle-count ranges at sizes of at least 4, 6 and 14 micrometres, using calibrated particle-counting methods. A code such as 18/16/13 is a cleanliness classification, not a direct statement that a filter has “18 micron” media. Each step in a scale number approximately doubles or halves the permitted particle-count range. The code should always be tied to an actual sample location, date, fluid and test method.
Set the target from the components
Ask the pump, valve, servo-control and cylinder-seal suppliers for their required fluid cleanliness. The most sensitive component normally sets the system target; operating pressure, contamination ingress and maintenance conditions also matter. HYDAC’s fluid-care guidance recommends beginning with component requirements and then selecting filtration to reach and maintain the resulting ISO target. A universal “best” code for every hydraulic system would be misleading.
Filter rating is more than nominal micron size
Compare filter elements by the particle size at which their efficiency is measured, the stated beta ratio under the relevant test, dirt-holding capacity, pressure-drop curve, bypass arrangement and compatibility with the fluid and temperature range. Check both clean and loaded pressure drop at cold-start viscosity. A fine element with insufficient flow capacity can open a bypass or starve a pump; a large filter housing alone does not guarantee a clean system.
Sampling and maintenance make the target real
- Sample from a consistent, representative point with clean equipment and a documented procedure.
- Record operating state and oil temperature so later results can be compared.
- Filter new fill oil where the system specification requires it; new oil is not automatically clean enough.
- Trend particle counts with filter differential pressure and inspect abnormal rises for ingress or wear.
Particles are only one part of fluid condition. Water, viscosity change and oxidation need their own checks. Our oil particle detection system and oil health management system illustrate monitoring options; choose the sampling plan and acceptance limit for the actual hydraulic circuit.
Technical references: HYDAC fluid-care guide; HYDAC filtration overview and ISO 4406 table. The cover is a concept illustration.