ISO 4406 Cleanliness Code
An international standard that expresses solid particle contamination in hydraulic fluid as a three-digit code. The digits correspond, in order, to the count ranges per milliliter of particles larger than 4, 6, and 14 microns; each increment of one digit means roughly a doubling of the particle count.
The code is written, for example, as 18/16/13. Lower numbers indicate cleaner fluid, and even a single-digit improvement makes a serious difference because the scale is logarithmic.
- Systems with proportional and servo valves require noticeably cleaner fluid than ordinary on/off valve systems; the manufacturer usually gives the target code in its catalog and it must be followed.
- The most commonly overlooked entry path for contamination is the tank breather filter and carelessness while adding fluid.
- Fluid taken from a new drum is not automatically clean; in many cases it must be filtered before being put into the system.
The cost of contaminated fluid in proportional control is concrete: the spool sticks, the deadband grows, hysteresis increases, and motion becomes "jumpy." Before fiddling with controller settings, having a fluid analysis done or at least checking filter condition usually delivers results faster.
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