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Understanding ISO Cleanliness ben lee
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Within the hydraulic industry there are certain buzz words that are spoken regarding Hydraulic fluid cleanliness. Before we can talk about Hydraulic oil cleanliness, let us try to picture how small a “micron” really is.


Now we can see how small a micron is relative to a human hair, ISO (International Organisation for Standardisation) have conducted a controlled experiment to measure how clean our hydraulic oil really is. The ISO Code is used to display contamination levels per millilitre of fluid for 3 sizes, these are 4 microns , 6 microns and 14 microns. The ISO code is represented with 3 numbers (example 20/17/14). Each number represents a contaminant level code for the 3 particle sizes.

The ISO Cleanliness Code references the number of particles greater than 4, 6, & 14 microns in one millilitre of fluid. The corresponding ISO Range Code, shown below, gives the cleanliness code number for each of the three particle sizes.

As you can see from the table every ISO Code has a specific particle range which identifies the cleanliness of the fluid.  From this table you can see that ISO Code 23 is twice as ‘dirty’ as ISO Code 22.

If you are wondering how we can measure these very small particles in Hydraulic fluid? There are a number of ways in which we do this we can use Particle counters  these use a shadow graph type technology , as the oil passes through the light the particles are picked up by shadows , the machine can calculate the amount of oil and how many ‘shadows’ are in that amount of oil.

Now there are drawbacks from using such a particle counter, this type of method will only tell you there are a specific number particles in the system and will give you a ISO Code number, this will not tell you what the particles consist of.

How do you find out what the very small particles consist of? We can provide sample bottles and have these sent off to a lab to be tested in a more controlled environment. This will break down the components within the hydraulic oil and highlight anything irregular.

From the list of contaminants we can have an idea what state our hydraulic power pack is in, for instance a high number of Aluminium particles could indicate the pump is failing. Water can be a huge problem for hydraulic systems, not only for rust issues but as water contains oxygen it can be compressed, Hydraulics relies on transmitting power using an incompressible fluid also water under pressure can give up a vapour which when sucked up by the pump and pressurised can implode causing explosion damage we call this Vaporous Cavitation. There are a number of ‘good’ particles within Hydraulic oil which help with things like anti-wear, anti-foaming and oxidation agents , we need to be careful not to over clean the oil with very fine filtration and take the good agents from the oil.

If you have any more questions on the ISO Code or Filtration, please speak to our technical team on 01452 523352.

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