ISO 4406 Cleanliness Code

The ISO 4406 standard quantifies fluid cleanliness using a three-number code representing particle counts at three size thresholds:

• First number: Particles larger than 4 micrometers (um)
• Second number: Particles larger than 6 micrometers
• Third number: Particles larger than 14 micrometers

Each code number represents a range of particles per milliliter. A code of 18/16/13 means:

18 = 1,300-2,500 particles >4um per mL

16 = 320-640 particles >6um per mL

13 = 40-80 particles >14um per mL

ISO Code Scale

Beta Ratio (Filtration Ratio)

The beta ratio indicates filter efficiency at capturing particles of a specific size:

Beta(x) = Particles upstream of size x / Particles downstream of size x

For example, Beta 10 = 200 means for every 200 particles of 10um entering the filter, only 1 passes through. This equals 99.5% efficiency.

Beta Ratio to Efficiency Conversion

Component Sensitivity Levels

Different hydraulic components have varying tolerances to contamination based on their internal clearances:

Very High Sensitivity (clearance 1-5 um): Servo valves, high-pressure piston pumps

High Sensitivity (clearance 5-15 um): Proportional valves, vane pumps, piston motors

Moderate Sensitivity (clearance 15-25 um): Directional valves, gear pumps, cylinders

Low Sensitivity (clearance >25 um): Flow controls, simple actuators

Filter Selection Guidelines

Select filter micron rating based on the most sensitive component's clearance divided by a safety factor (typically 5-10):

Filter rating = Component clearance / Safety factor

For a servo valve with 5um clearance: Filter rating = 5 / 5 = 1um absolute