Overview - ISO/TR 4813:2021 and ISO 11171:2020 context
ISO/TR 4813:2021 is a technical report that explains the background, impact and practical use of ISO 11171:2020 in hydraulic fluid power particle counting and filter testing. It documents why ISO 11171 was revised, summarizes an international inter‑laboratory study (ILS) using NIST SRM 2806d and RM 8632a, and explains the implications of adopting µm(c) as the sole particle‑size reporting unit for automatic particle counters (APC). The report helps users interpret particle count data, understand calibration traceability and assess effects on filter performance metrics.
Key topics and technical requirements
- Single size unit (µm(c)): ISO 11171:2020 mandates µm(c) as the only acceptable particle‑size unit to eliminate confusion caused by the earlier µm(b)/µm(c) option (µm(b) sizes were ~10% larger).
- Consensus SRM certification: Beginning with SRM 2806d, NIST certifies SRM 2806x as a consensus standard to minimize batch‑to‑batch particle‑size shifts.
- ILS evidence: ISO/TR 4813 summarizes ILS design and results used to certify SRM 2806d and to evaluate reproducibility of APC measurements across laboratories.
- Standardized calibration methods:
- Use of at least 12 threshold settings spaced logarithmically across the size range.
- Lowest threshold = 1.5 × instrument noise level.
- Calibration curve generation via constrained cubic spline to reduce inter‑lab uncertainty.
- Dilution and small‑size calibration: ISO 11171:2020 permits defined dilution methods (derived from ISO 11500) to enable reliable calibration below 4 µm(c), with cleanliness verification for dilution fluids and glassware.
- Large‑particle calibration (>30 µm(c)): Primary calibration at large sizes uses monodispersed latex particles with guidance for selecting sizes to improve traceability.
Practical applications
- Ensures consistent, traceable particle count data for hydraulic fluids used in mobile and industrial systems.
- Provides basis for filter performance testing and multi‑pass evaluations where accurate Beta ratios and particle size distributions are critical.
- Supports quality assurance, supplier verification, and specification compliance when reporting fluid cleanliness (e.g., per ISO 4406).
- Guides laboratories and manufacturers in calibrating APCs to reduce measurement variability and prevent misinterpretation of contamination levels.
Who should use this standard
- Test laboratories and calibration houses performing APC calibration
- Filter manufacturers and test labs conducting multi‑pass filter evaluations
- Hydraulic system designers, maintenance engineers and reliability teams
- Standards bodies and procurement/specification authors requiring consistent cleanliness metrics
Related standards
- ISO 11171 - Calibration of automatic particle counters for liquids
- ISO 4406 - Fluid cleanliness coding by solid particles
- ISO 16889 - Multi‑pass filtration test method
- ISO 5725, ISO 3534 series - Statistical principles referenced in ILS design
Keywords: ISO/TR 4813:2021, ISO 11171:2020, hydraulic fluid power, particle count, APC, µm(c), SRM 2806d, RM 8632a, calibration, filter testing, contamination control.
Open preview (PDF)Hydraulic fluid power — Background, impact and use of ISO 11171:2020 on particle count and filter test data
