🔍 Particle cleanliness class in hydraulic oils

Petrochemistry PN-EN ISO 4406

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

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.

   ⏱ Time: 5 min

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.

   ⏱ Time: 15 min

3. Calibration verification

   Measure NIST SRM 2806b calibration standard. Result must be within ±1 ISO code from certified value. If deviation — recalibrate.

   ⏱ Time: 5 min

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).

   ⏱ Time: 2 min

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).

   ⏱ Time: 5 min

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.

   ⏱ Time: 1 min

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).

   ⏱ Time: 1 min

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.

   ⏱ Time: 5 min

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.

   ⏱ Time: 5 min

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).

    ⏱ Time: 10 min

Required equipment and apparatus

Reagents, media and consumables

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

Frequently asked questions