🔍 Particle cleanliness class in hydraulic oils
Classification of solid particle contamination level in hydraulic and lubricating oils using automatic laser particle counter. Result as ISO code (e.g. 18/16/13).
Overview
Hydraulic oil cleanliness class according to ISO 4406 is fundamental parameter in diagnostics and maintenance of hydraulic systems. Solid particle contamination (metals, sand, oil degradation products, fibers) causes ~75% of hydraulic system failures — causing abrasive wear of pumps, proportional valves, cylinders and seals.
ISO 4406 standard defines three-digit cleanliness code, where each digit corresponds to particle count range in 1 mL oil larger than respectively: ≥4 µm(c), ≥6 µm(c) and ≥14 µm(c). For example code 18/16/13 means: 1,300–2,500 particles ≥4 µm, 320–640 particles ≥6 µm and 40–80 particles ≥14 µm in 1 mL. Lower code means cleaner oil.
Required cleanliness classes depend on system type: gear pumps — ISO 20/18/15, piston pumps — ISO 18/16/13, servohydraulics — ISO 16/14/11, proportional valves — ISO 17/15/12. Component manufacturers (Bosch Rexroth, Parker, Eaton) specify cleanliness requirements in technical documentation.
Measurement is performed with automatic laser particle counter (APC — Automatic Particle Counter) by light obscuration method. Particle flowing through laser beam causes intensity drop — drop magnitude corresponds to particle size. Modern counters (HIAC PODS+, Beckman Coulter, Parker iCount) analyze sample in less than 60 seconds.
Method principle
Hydraulic oil sample is introduced into measurement cell of automatic particle counter (APC). In cell laser beam (laser diode) passes through flat channel filled with flowing oil. When solid particle flows through beam, it blocks part of light — photodiode registers intensity drop proportional to particle cross-section. Electronics counts particles in size ranges: ≥4 µm(c), ≥6 µm(c) and ≥14 µm(c) (suffixes "c" indicate calibration per ISO 11171 with NIST SRM 2806 test dust). Based on counted particle number in 1 mL, range codes are assigned (scale from 1 to 28+). Result is recorded as three-digit ISO code, e.g. 18/16/13.
Applications
- Diagnostics of hydraulic systems in industrial machinery
- Hydraulic oil monitoring in mines and steel mills
- Oil quality control in servohydraulic systems (CNC machine tools)
- Hydraulic filter acceptance (filtration efficiency verification)
- Oil analysis in gas and steam turbines (power industry)
- Lubricating oil monitoring in industrial gearboxes
- Oil cleanliness control when filling new systems
- Oil analysis programs (predictive maintenance)
Key parameters
| Parameter | Value |
|---|---|
| Threshold sizes | ≥4 µm(c), ≥6 µm(c), ≥14 µm(c) |
| Result format | Three-digit ISO code, e.g. 18/16/13 |
| Code range | 1–28+ (lower code means cleaner oil) |
| Sample volume | 50–100 mL (depends on instrument) |
| Measurement time | <60 s (portable), 2–5 min (laboratory) |
| Calibration | ISO 11171 with NIST SRM 2806 (MTD test dust) |
Standard
- Standard number
- PN-EN ISO 4406:2023
- Title (PL)
- Hydraulika siłowa — Ciecze — Metoda kodowania poziomu zanieczyszczenia cząstkami stałymi
- Title (EN)
- Hydraulic fluid power — Fluids — Method for coding the level of contamination by solid particles
Step-by-step procedure
1. Oil sample collection
Collect oil sample from hydraulic system under operating conditions (warm oil, active flow). Use clean bottle and tubing. Flush 3× before actual collection. Min. 100 mL.
2. Counter preparation
Turn on particle counter min. 15 min before measurement (laser stabilization). Flush cell with clean calibration oil. Perform background measurement — result ≤code 8/6/4.
3. Calibration verification
Measure NIST SRM 2806b calibration standard. Result must be within ±1 ISO code from certified value. If deviation — recalibrate.
4. Sample homogenization
Mix bottle with sample by vigorous shaking (30 s) or use ultrasonic mixer (2 min). Do not create air bubbles (false particles).
5. Dilution (if required)
For dark, turbid oils or viscosity >425 cSt: dilute sample 1:1 or 1:4 with clean solvent. Use dilution method (ISO 4402).
6. Measurement
Introduce sample into counter (suction or pressure). Measurement volume per instrument requirements (typically 5–25 mL). Measurement time ~30–60 s. Instrument automatically counts particles in 3 size channels.
7. ISO code reading
Instrument displays result as three-digit ISO 4406 code, e.g. 18/16/13. First digit = particles ≥4 µm(c), second = ≥6 µm(c), third = ≥14 µm(c).
8. Interpretation
Compare result with component manufacturer requirements. E.g. piston pump: max 18/16/13, servohydraulics: max 16/14/11. Result out of range → oil or filter replacement.
9. Post-measurement cleaning
Flush measurement cell with clean oil or solvent (3 cycles). Measure background — must return to ≤code 8/6/4. Turn off instrument.
10. Reporting
Report: ISO 4406 code (e.g. 18/16/13), collection date, collection point, oil temperature, oil type, trend vs previous measurements. Attach recommendation (oil OK / filter replacement / oil replacement).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Portable laser particle counter | Beckman Coulter HIAC PODS+, Parker iCount LCM20, MP Filtri LPA3 | 40,000–120,000 PLN |
| Laboratory particle counter | Beckman Coulter HIAC 8011+, Pamas S50, Eracount XS | 80,000–200,000 PLN |
| Sample collection bottles | Clean glass or plastic bottles 100–250 mL with cap, cleanliness class ISO 15/13/10 | 15–50 PLN / pc |
| Collection syringe | 50 mL syringe with PTFE tubing, disposable tips | 200–500 PLN / set |
| Calibration standard | NIST SRM 2806b (medium test dust in oil), Beckman Coulter AccuCount | 2,000–5,000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Calibration oil (clean) | — | Base oil filtered to class ≤12/10/7, for zeroing and instrument background verification |
| MTD standard (Medium Test Dust) | — | NIST SRM 2806b — test dust suspension in oil with certified particle count, for calibration per ISO 11171 |
| Dilution solvent | — | Isopropanol or purified naphtha, filtered 0.45 µm — for dilution method (dark/turbid oils) |
| Clean calibrated bottles | — | Sample bottles with guaranteed cleanliness ≤15/13/10, for sample collection without contamination |
Health and safety (OHS)
- Hydraulic oil — may be hot (up to 80°C), burn risk when collecting from operating system
- Hydraulic oils — may contain irritant additives (ZnDTP), use nitrile gloves
- System pressure — disconnect pressure before opening collection point, oil ejection risk
- Laser in particle counter — class 1 (safe), but do not disassemble optical covers