ISO 4406 defines a code system for reporting particle contamination in hydraulic fluids using three numbers X/Y/Z for particles ≥4μm(c)/≥6μm(c)/≥14μm(c) per mL. Each code number represents a range of particle counts doubling with each increment.

What target cleanliness for servo valves?

Servo valves require 16/14/11 or cleaner. Proportional valves: 18/16/13. Standard directional valves: 20/18/15. Gear pumps: 20/18/15. Vane pumps: 18/16/13. Piston pumps: 17/15/12.

What is a β-ratio (beta ratio)?

The β-ratio (filtration ratio) is the ratio of upstream to downstream particle counts at a given size. β₁₀ = 200 means the filter removes 99.5% of particles ≥10μm. Higher β = better filtration. Per ISO 16889.

How to improve oil cleanliness?

1) Install finer filters with higher β-ratio. 2) Add kidney-loop (offline) filtration. 3) Use sealed reservoirs with breather filters. 4) Filter new oil before adding. 5) Use clean fill practices. 6) Reduce ingression sources.

How often should oil samples be taken?

Critical systems (servo): monthly. Standard hydraulics: quarterly. Low-duty systems: semi-annually. Always sample after maintenance, fluid changes, or when problems occur. Use consistent sampling points and methods.

Free Engineering Tool

Hydraulic Oil Cleanliness Analyzer

ISO 4406 code calculator with target cleanliness lookup, filter β-ratio recommendation, and sampling interval guidance by component type.

ISO 4406Filter β-ratioTarget Cleanliness

Analysis Results

Current ISO 4406 Code

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Target for Component

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Status

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Filter β-ratio Required

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Recommended Filter Rating

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Sampling Interval

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Fluid Exchanges/Hour

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Est. Cleanup Time

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ISO 4406 Code Scale

Each code number R represents a particle count range: 2^(R-1) × 0.01 to 2^R × 0.01 particles per mL (simplified). The code doubles with each increment.

Target Cleanliness by Component

Component	Target ISO Code	Filter μm	β-ratio
Servo valves	16/14/11	3 μm	β₃ ≥ 200
Proportional valves	18/16/13	6 μm	β₆ ≥ 200
Piston pumps/motors	17/15/12	6 μm	β₆ ≥ 200
Vane pumps	18/16/13	10 μm	β₁₀ ≥ 200
Gear pumps, std. valves	20/18/15	10 μm	β₁₀ ≥ 75
Cylinders only	21/19/16	25 μm	β₂₅ ≥ 75

Cleanup Time Estimation

Approximate cleanup passes needed to reduce contamination by one ISO code level:

Practical Example

Example — System with servo valves, current 20/18/15

Target: 16/14/11 → need to reduce by 4 code levels

Each code level ≈ 2 system turnovers through proper filtration

200 L system, 60 L/min flow: turnover = 200/60 = 3.3 min

Est. cleanup: 4 levels × 2 turnovers × 3.3 min ≈ 26 minutes of kidney-loop filtration

Hydraulic Oil Cleanliness Analyzer | ISO 4406 | Free Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Analysis Results

ISO 4406 Code Scale

Target Cleanliness by Component

Cleanup Time Estimation

Practical Example