ISO 21018-3:2008

Overview

ISO 21018-3:2008 - "Hydraulic fluid power - Monitoring the level of particulate contamination of the fluid - Part 3: Use of the filter blockage technique" specifies a semi‑quantitative method to monitor particulate contamination in hydraulic liquids using the filter blockage technique (also called the mesh‑obscuration or pore‑blockage method). The standard covers both on‑line and off‑line (container/sample bottle) use, and defines procedures for calibrating and verifying instruments in the laboratory and in service. It applies to all single‑ or multi‑phase liquid systems and is intended for monitoring system cleanliness, flushing operations and support equipment or test rigs.

Key topics and technical requirements

• Principles: Two measurement principles are defined
  • Constant differential pressure across the filter (monitor change in flow)
  • Constant flow through the filter (monitor change in differential pressure)
• Calibration & verification: Procedures for preparing calibration dust (ISO 12103‑A3), creating verification samples, and validating instrument operation to improve consistency and correlation with optical particle counters (APC).
• Equipment & sample handling: Requirements include cleaned sample bottles (ISO 3722), reference instruments, oven for dust drying (100–150 °C), sample agitating devices (ultrasonic or shaker), filtered solvent dispensers (0.8 µm), vacuum de‑aeration, and validated test rigs (ISO 11943).
• Cleanliness thresholds: Flushing liquids and sample bottle RCLs are specified (examples: flushing liquid < 10 particles ≥6 µm(c)/ml; sample bottles <100 or <5 particles ≥6 µm(c)/ml depending on use).
• Operating procedures: Guidance for sampling from pressurized lines, system reservoirs, bulk containers and sample bottles, plus back‑flushing to remove retained particles between analyses.
• Health & safety: Electrical, mechanical and chemical safety measures, solvent handling, grounding and environmental disposal.
• Limitations: The technique does not count individual particles or provide direct traceability of particle sizes like APCs; non‑particulate contaminants (gels, additives) can influence results.

Applications and who uses it

• Practical uses:
  • Routine cleanliness monitoring in hydraulic systems
  • Assessing progress of flushing and maintenance operations
  • Monitoring support equipment, mobile machinery, test rigs and hydraulic test benches
• Typical users:
  • Hydraulic engineers and maintenance technicians
  • Condition monitoring and reliability teams
  • Laboratory technicians validating contamination instruments
  • OEMs, filtration suppliers and test facility operators

Related standards

• ISO 21018‑1 (General principles)
• ISO 11171 (Calibration of APCs)
• ISO 11943 (On‑line APC calibration/validation)
• ISO 12103‑1 (Arizona test dust)
• ISO 3722 (Sample container cleaning)
• ISO 4021, ISO 5598

ISO 21018‑3 is a practical reference for anyone implementing filter‑blockage monitoring to control particulate contamination in hydraulic fluid power systems.