Oil in machines (like engines, hydraulics, or gearboxes) works the way, blood works in the human body. It keeps everything moving smoothly, reduces friction, and removes heat. But over time, tiny bits of metal, dust, or dirt can get into the oil.

These contaminants act like sandpaper inside the machine, wearing down parts faster. Even particles too small to see can cause big damage, especially in high-precision systems like hydraulics.

So, measuring oil cleanliness helps:

By regularly checking how clean the oil is (using special codes like ISO 4406), companies can know when it is time to change the oil or add filters — before damage happens.

Oil cleanliness class measurement refers to the classification of the level of contamination in hydraulic or lubricating oils, typically by measuring the number and size of particles in the oil.

As stated above, this is crucial for ensuring machinery longevity and reliability, especially in systems with tight tolerances.

Most Common Standards for Measuring Oil Cleanliness

ISO 4406 Cleanliness Code

• Most widely used standard.
• Measures the number of particles ≥4 µm, ≥6 µm, and ≥14 µm in a 1 mL sample.
• Represented as a 3-number code, e.g., 19/17/14.
• Each number corresponds to a range (or code) for the number of particles in each size category.
• The lower the numbers, the cleaner the oil.

NAS 1638 (National Aerospace Standard)

• Older standard, still in use in aerospace and military.
• Rates oil cleanliness from Class 00 (very clean) to Class 12 (very dirty) based on the number of particles in 100 ml.

SAE AS4059

• Aerospace-oriented standard similar to ISO 4406 but uses different particle size ranges and limits.
• Classifies contamination in categories based on specific size bands.