Short answer
An ISO 4406 code is three numbers that summarize how many solid particles are in one millilitre of fluid: the first counts particles 4 µm(c) and larger, the second 6 µm(c) and larger, the third 14 µm(c) and larger. Each number is a scale step, and each step up roughly doubles the particle count. A lower code means cleaner oil.
What the three numbers count
The counts are cumulative. The first channel includes every particle at or above 4 µm(c), so it always contains the particles counted in the 6 and 14 µm(c) channels too. The “(c)” means the sizes are based on particle counters calibrated to ISO 11171; older reports from before that calibration method used 2, 5 and 15 µm channels, so very old results are not directly comparable.
Each code number corresponds to a range of particles per millilitre. In a code of 19/17/14, the 19 means roughly 2,500 to 5,000 particles per mL at 4 µm(c) and above, the 17 means roughly 640 to 1,300 at 6 µm(c) and above, and the 14 means roughly 80 to 160 at 14 µm(c) and above.
Why the scale doubles
The scale is logarithmic: each step represents about twice as many particles as the step below. Dropping from 19/17/14 to 16/14/11 is three steps in every channel, or roughly one-eighth as many particles. Small changes in the numbers therefore represent large changes in contamination.
When particles are counted by microscope instead of an automatic counter, the smallest channel may not be reported and a dash takes its place, as in -/17/14.
Target codes
Component makers and reliability programs set target cleanliness codes based on how sensitive the system is. Servo and proportional valves and high-pressure piston pumps generally need cleaner oil than gear pumps or low-pressure circuits. Equipment manuals, pump and valve manufacturers, and filter suppliers publish recommended targets; use those rather than a generic number.
New oil is not automatically clean. Handling, transfer and storage can add particles, which is why many plants filter oil as it goes into a reservoir and check codes on new deliveries.
Getting a reliable result
A cleanliness code is only as good as the sample. Take samples from a live, turbulent zone with the system at operating temperature, use clean bottles and a consistent sample point, and flush the valve before filling. Trends over time are more useful than any single reading.
Frequently asked questions
Is a lower ISO 4406 code better?
Yes. Lower numbers mean fewer particles per millilitre. Each step down is roughly half as many particles in that size range.
Does ISO 4406 measure water?
No. ISO 4406 covers solid particle counts only. Water content is measured separately, for example by Karl Fischer titration in an oil analysis report.
How do I get a cleanliness code for my hydraulic oil?
Send a properly drawn sample to an oil analysis lab that offers particle counting, and ask for results reported to ISO 4406.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].
