Why Size Matters.

Your hydraulic oil might look clean. It probably isn’t.

When we analyse hydraulic systems, one of the most misunderstood metrics is the ISO 4406 cleanliness code. Usually seen as a three number code such as 18/15/13.

What the numbers actually mean

ISO 4406 reports particles in three size bands:

≥4µm / ≥6µm / ≥14µm

So a code like 19/17/16 means your system contains:

• 2,500–5,000 particles per ml, 4µm or larger

• 640–1,300 particles per ml, 6µm or larger

• 160–320 particles per ml, 14µm or larger

And if that code increases by just one number, those ranges double.

For example:

19/17/16 → 20/18/17

At first glance, that looks like a minor shift. It’s not.

Each increase of just ONE ISO code represents approximately DOUBLE the number of particles in that size range.

That means:

• 2× more ≥4µm particles

• 2× more ≥6µm particles

• 2× more ≥14µm particles

And in hydraulic systems where operating clearances can be 3-10 microns, that matters.

The reality we see in the field

From our historical database, a significant portion of hydraulic samples exceed recommended ISO cleanliness targets each month.

Most failures don’t start catastrophically. They start microscopically.