Part 3: ISO cleanliness code.


To a novice, oil cleanliness codes can look very confusing. However, armed with a basic understanding of how the code attempts to represent the cleanliness of fluids and the table to interpret them and any operator can start making sense of the specs and therefore start making sound decisions.

test tubes isolated on white

The basics of ISO 4406:99

ISO 4406:99 is the reporting standard for fluid cleanliness. According to this standard, a code number is assigned to particle count values derived at three different micron levels: greater than 4 microns, greater than 6 microns and greater than 14 microns.

The ISO code is assigned based upon the table below.


As an example, the cleanliness specification for a Rexroth A10V  hydraulic pump is 20/18/15. The first number, 20, is the number of particles per millilitre above 4 microns in size. From the table above, code 20 represents between 5000 and 10000 particles per millilitre. Similarly, the second number, 18 represents between 1,300 and 2,500 particles per millilitre greater than 6 microns. The third number, 15 indicates that the spec requires between 160 to 320 particles per millilitre above 14 microns.

How the numbers are determined.

Typically an oil sample is taken and analysed. The test methods used most frequently for counting particles are:

Automatic particle count (according to ISO 11500) The contamination level of a liquid sample is determined by automatic particle counting, using the light extinction principle. Automatic particle counters: ≥4, ≥6 and ≥14 µm (some types also larger micron sizes)

Manual particle count (according to ISO 4407) Particles are counted manually with the use of membranes (pore size max 1,5 micron) and an optical microscope.

Good practice.

Previous parts of this series have shown the harm that contamination can do to any hydraulic system. Operators of these systems should strive to reduce the level of contamination to at least the spec required by the manufacturers of the equipment. To determine the contamination level in a hydraulic system, regular oil samples should be taken and analysed. Based upon the results, the existing filtration can be improved if required.

In the case of a spike in contamination, say for example from a catastrophic component failure or from an isolated contamination incident, off-line (kidney) filtration should be considered before running the equipment. Replacing the oil is another option.

Reference on our Website

A reminder that all parts of this series will be available on our website after they have been published for  quick and easy reference.


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