ISO Cleanliness Codes
Understanding ISO 4406 fluid cleanliness codes, particle counting, target cleanliness levels for hydraulic components, and filter selection guide.
Understanding ISO 4406 Codes
The ISO 4406 cleanliness code is the international standard for reporting fluid contamination levels. It uses three numbers separated by slashes that represent particle counts at specific size thresholds.
(per mL)
(per mL)
(per mL)
Each code number represents a range of particle counts per milliliter of fluid. Lower numbers indicate cleaner fluid. The three thresholds (4, 6, and 14 micron) correspond to particle sizes that affect different component types.
ISO Code Range Numbers
Each ISO code number corresponds to a range of particle counts. The particle count doubles between adjacent code numbers.
| ISO Code | Particles per mL (Minimum) |
Particles per mL (Maximum) |
Typical Level |
|---|---|---|---|
| 6 | 0.32 | 0.64 | Ultra-clean |
| 7 | 0.64 | 1.3 | Ultra-clean |
| 8 | 1.3 | 2.5 | Ultra-clean |
| 9 | 2.5 | 5 | Very clean |
| 10 | 5 | 10 | Very clean |
| 11 | 10 | 20 | Clean |
| 12 | 20 | 40 | Clean |
| 13 | 40 | 80 | Moderate |
| 14 | 80 | 160 | Moderate |
| 15 | 160 | 320 | Acceptable |
| 16 | 320 | 640 | Acceptable |
| 17 | 640 | 1,300 | Marginal |
| 18 | 1,300 | 2,500 | Marginal |
| 19 | 2,500 | 5,000 | Contaminated |
| 20 | 5,000 | 10,000 | Contaminated |
| 21 | 10,000 | 20,000 | Very dirty |
| 22 | 20,000 | 40,000 | Very dirty |
| 23 | 40,000 | 80,000 | Severely dirty |
| 24 | 80,000 | 160,000 | Severely dirty |
Target Cleanliness by Component
Different hydraulic components have varying sensitivity to contamination. More precise components with tighter clearances require cleaner fluid.
| Component Type | Typical Clearance | Target ISO Code | Filter Rating |
|---|---|---|---|
| Servo Valves | 0.5-4 micron | 14/12/9 | 3 micron |
| Proportional Valves | 2-8 micron | 15/13/10 | 5 micron |
| Piston Pumps/Motors | 0.5-5 micron | 16/14/11 | 6 micron |
| Vane Pumps/Motors | 0.5-13 micron | 17/15/12 | 10 micron |
| Gear Pumps/Motors | 0.5-25 micron | 18/16/13 | 12 micron |
| Directional Valves | 2-40 micron | 18/16/13 | 12 micron |
| Pressure/Flow Controls | 2-40 micron | 18/16/13 | 12 micron |
| Cylinders | 2-40 micron | 19/17/14 | 15 micron |
| Test Stands/Aerospace | < 1 micron | 13/11/8 | 1-3 micron |
| Mobile Equipment | Various | 18/16/13 | 10-12 micron |
Pressure Impact on Cleanliness
Higher system pressures increase wear from contamination. Adjust target cleanliness based on operating pressure.
| System Pressure | Impact on Wear | Cleanliness Adjustment |
|---|---|---|
| < 1000 psi (70 bar) | Lower stress | Standard target acceptable |
| 1000-2000 psi (70-140 bar) | Moderate stress | Standard target |
| 2000-3000 psi (140-210 bar) | Higher stress | 1 code level cleaner |
| 3000-5000 psi (210-350 bar) | High stress | 2 code levels cleaner |
| > 5000 psi (350+ bar) | Severe stress | 3+ code levels cleaner |
Beta Ratio Explained
The Beta ratio (Beta_x) measures filter efficiency at capturing particles of a specific size. It is the ratio of upstream particle count to downstream particle count.
Means: For every 200 particles >= 10 microns entering the filter,
only 1 particle >= 10 microns passes through.
| Beta Ratio | Efficiency (%) | Particles Removed | Particles Passed |
|---|---|---|---|
| Beta 2 | 50.0% | 1 of 2 | 1 of 2 |
| Beta 10 | 90.0% | 9 of 10 | 1 of 10 |
| Beta 20 | 95.0% | 19 of 20 | 1 of 20 |
| Beta 75 | 98.7% | 74 of 75 | 1 of 75 |
| Beta 100 | 99.0% | 99 of 100 | 1 of 100 |
| Beta 200 | 99.5% | 199 of 200 | 1 of 200 |
| Beta 1000 | 99.9% | 999 of 1000 | 1 of 1000 |
| Beta 2000 | 99.95% | 1999 of 2000 | 1 of 2000 |
Efficiency Formula: Efficiency = (1 - 1/Beta) x 100%
Example: A filter rated Beta_10 = 200 captures 99.5% of particles 10 microns and larger.
Reading Filter Ratings
| Filter Rating | Meaning | Typical Application |
|---|---|---|
| 3 micron Beta_3 >= 200 | 99.5% efficient at 3 microns | Servo systems |
| 6 micron Beta_6 >= 200 | 99.5% efficient at 6 microns | Piston pumps |
| 10 micron Beta_10 >= 200 | 99.5% efficient at 10 microns | Vane pumps, general |
| 12 micron Beta_12 >= 200 | 99.5% efficient at 12 microns | Industrial standard |
| 25 micron Beta_25 >= 75 | 98.7% efficient at 25 microns | Suction strainers |
Filter Selection Guide
Pressure Line Filters
Located after pump, before components. Protects sensitive components from pump-generated particles. High collapse strength required. Most effective location for cleanliness control.
Return Line Filters
Located in tank return. Catches wear particles from all system components. Lower pressure rating acceptable. Most common industrial filter location. Cost-effective protection.
Suction Strainers
Located at pump inlet. Coarse filtration (100+ mesh) only. Protects pump from large debris. Must have low pressure drop to prevent cavitation. Not for fine filtration.
Offline/Kidney Loop
Separate circulation system. Continuously polishes fluid. Excellent for achieving very clean levels. Does not affect main circuit. Ideal for sensitive systems.
Filter Sizing Guidelines
| Filter Location | Flow Capacity | Max Pressure Drop (Clean) | Bypass Setting |
|---|---|---|---|
| Pressure Line | 100% of pump flow | 15-25 psi (1-1.7 bar) | 50-75 psi (3.5-5 bar) |
| Return Line | 150% of pump flow | 5-15 psi (0.3-1 bar) | 25-50 psi (1.7-3.5 bar) |
| Suction | 150-200% of pump flow | 2-3 psi (0.15-0.2 bar) | Not recommended |
| Offline Loop | 10-25% of reservoir volume/min | 25-50 psi (1.7-3.5 bar) | Optional |
Common Contamination Sources
| Source | Contaminant Type | Prevention Methods |
|---|---|---|
| Built-in | Manufacturing debris, welding slag, sand, paint chips | Flush new systems, clean components before assembly |
| Ingressed | Dust, dirt, water through breathers, seals, fill ports | Desiccant breathers, proper seals, filtered fill |
| Generated | Wear particles from pumps, motors, valves, cylinders | Proper filtration, maintain cleanliness targets |
| Maintenance | Dirty tools, open reservoirs, contaminated oil | Clean procedures, covered reservoirs, filter new oil |
| Degradation | Oxidation products, varnish, sludge | Temperature control, regular oil analysis |
Oil Sampling Best Practices
Sampling Location
Take samples from live, turbulent zones - not dead legs or tank bottoms. Best locations: return line (before filter), pressure line (after filter), dedicated sample valves.
Sampling Procedure
System must be at operating temperature. Flush sample port and lines first. Use clean bottles and equipment. Fill bottles completely to minimize air exposure.
Frequency
Critical systems: Weekly to monthly. Standard systems: Monthly to quarterly. Mobile equipment: Every 250-500 hours. Always sample after maintenance or filter changes.
Trending
Single samples provide limited value. Track trends over time. Sudden changes indicate problems. Consistent sampling location is essential for valid comparisons.