ISO 4406

Hydraulic Oil Analysis - what do the values tell you?

Q: How often should you take an oil sample?

For meaningful trends, standard industrial hydraulics are usually sampled every 500 to 1000 operating hours or twice a year. Critical or heavily loaded systems are sampled more often.

Q: What does the ISO 4406 code 18/16/13 mean?

The three numbers give the particle count larger than 4, 6 and 14 µm per millilitre, expressed as classes. Each class higher means roughly double the particles.

Q: When is water in hydraulic oil dangerous?

Free water is always critical. Mineral oils should stay below 200 ppm and sensitive servo systems often below 100 ppm, since water drives corrosion and oxidation.

Q: Can analysis postpone the oil change?

Yes, that is the core of condition-based maintenance. As long as cleanliness, viscosity and additives stay in the target range, the oil remains usable - often far beyond the time interval.

An oil sample reveals a machine's health long before failure. This guide explains the four key values - particles, water, viscosity and additives - and how to turn them into oil change intervals and wear trends for maintenance.

5 minStand: 2026-07Geprüft: Technical editors

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ISO 4406

particle cleanliness code

< 200 ppm

water target level

± 10 %

viscosity tolerance

3 numbers

code at 4/6/14 µm

Inhalt

The four values
Reading the values
Deriving maintenance
Frequently asked questions

What values does an oil sample deliver?

A lab analysis returns four core values: the particle count as a cleanliness code per ISO 4406, the water content in ppm, the viscosity at 40 °C per ISO 3448 and the state of the additives. Together they form a picture of the fluid and, indirectly, of the machine.

The ISO 4406 cleanliness code is three numbers for particles larger than 4 µm, 6 µm and 14 µm per millilitre, for example 18/16/13. Each code step higher means roughly double the particle count, so a shift from 20/18/15 to 22/20/17 is already a clear warning.

For meaningful trends, always take the sample at the same point on a warm, running system - otherwise settling and temperature distort the values.

How do you read water, viscosity and additives?

Water is the silent enemy: even 200 to 300 ppm of free water drives corrosion and speeds up oil oxidation. Mineral oils should stay well below 200 ppm, sensitive servo hydraulics often below 100 ppm.

Viscosity off by more than ± 10 % from target (e.g. ISO VG 46 = 46 mm²/s at 40 °C): ageing, overheating or mixing with the wrong oil.
Falling zinc, phosphorus and calcium: anti-wear and detergent additives are spent and the oil is ageing.
Rising iron (Fe) and chromium (Cr): wear on pumps, cylinders and bearings.
High silicon (Si) with a high particle count: dust ingress from outside - check the breather filter.
Rising copper (Cu): wear on plain bearings or cooler tubes.

Never judge a single value in isolation. Only the combination - such as high water plus rising iron - gives a reliable diagnosis.

Choose a hydraulic filter

Find the right filter rating for your target cleanliness class.

Read the guide

Change interval or wear trend?

The big win of oil analysis is the move from a rigid time interval to condition-based maintenance. Instead of dumping the oil every 2000 operating hours, the real condition decides - saving oil while preventing failures.

What matters is the trend across several samples. A single outlier may just be a bad draw; three consecutive samples with rising iron, however, point to a genuine wear trend you can investigate before failure.

Rule of thumb: an oil change is only really due once additives are spent, the cleanliness class is permanently exceeded or viscosity drifts beyond 10 % - much can be saved earlier by filtering and dewatering.

Frequently asked questions

How often should you take an oil sample?

For meaningful trends, standard industrial hydraulics are usually sampled every 500 to 1000 operating hours or twice a year. Critical or heavily loaded systems are sampled more often.

What does the ISO 4406 code 18/16/13 mean?

The three numbers give the particle count larger than 4, 6 and 14 µm per millilitre, expressed as classes. Each class higher means roughly double the particles.

When is water in hydraulic oil dangerous?

Free water is always critical. Mineral oils should stay below 200 ppm and sensitive servo systems often below 100 ppm, since water drives corrosion and oxidation.

Can analysis postpone the oil change?

Yes, that is the core of condition-based maintenance. As long as cleanliness, viscosity and additives stay in the target range, the oil remains usable - often far beyond the time interval.

Keep your hydraulics clean?

We supply filters, dewatering and sampling accessories so your oil analysis delivers reliable trends and change intervals become plannable.

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Standards based

Assessment per ISO 4406 and ISO 3448.

Trend not snapshot

We help read several samples correctly.

Matched filtration

Filter rating tuned to your target class.

Expert advice

Maintenance specialists support your evaluation.