Overview
ISO/TR 4808:2021 - Hydraulic fluid power - Interpolation method for particle count and filter test data - provides a recommended, practical method for estimating particle concentrations and filter Beta Ratio/removal-efficiency values at particle sizes that are not directly measured. The Technical Report addresses unit and size mismatches between test results and specifications (for example, µm(b) vs µm(c) or slightly different integral sizes) and prescribes a constrained cubic spline interpolation workflow to obtain comparable data for contamination control and filter performance assessment.
Key topics and requirements
- Scope: Interpolation of particle concentration and filter Beta Ratio data to intermediate particle sizes within the range of available data; extrapolation is not permitted.
- Unit conversion: Convert µm(b) sizes to µm(c) by multiplying by 0.898 (as defined in ISO 11171:2016). Conversion is valid up to 38 µm(c); above this threshold µm(b) and µm(c) values are treated as equivalent.
- Interpolation method: Use constrained cubic spline interpolation (the same approach adopted in ISO 11171:2020) to avoid overshoot and improve accuracy versus traditional cubic splines.
- Workflow steps: (1) convert sizes to µm(c); (2) apply constrained cubic spline interpolation to concentrations or Beta Ratios; (3) visually validate the interpolation curve for smoothness and monotonicity; (4) extract interpolated values at required µm(c) sizes.
- Data quality: Minimum of 3 data points is required; ideally 6 or more, evenly spaced and bracketing the particle sizes of interest. Accuracy depends on the quality and distribution of the original data.
- Tools: ISO provides an accompanying Excel spreadsheet (Interpolation – Concentration and Interpolation – Beta Ratio) to perform the conversion and interpolation.
Applications
- Comparing laboratory particle counts and filter performance results against cleanliness specifications (e.g., ISO 4406 codes) when measured sizes differ from specified sizes.
- Converting historical data reported in µm(b) to µm(c) for trend analysis and contamination control programs.
- Evaluating filter Beta Ratios and removal efficiencies at specified particle sizes for procurement, quality control, and filter selection.
- Supporting maintenance decisions (oil change, filter replacement) and reducing risk of incorrect actions caused by mismatched size units.
Who uses this standard
- Contamination control engineers and hydraulic system maintainers
- Test laboratory technicians and calibration specialists
- Filter manufacturers and quality assurance teams
- System designers and reliability engineers concerned with hydraulic fluid cleanliness
Related standards
- ISO 11171 - Calibration of automatic particle counters (unit definitions and conversion guidance)
- ISO 4406 - Coding the level of contamination by solid particles
- ISO 16889 - Multi‑pass method for evaluating filtration performance
ISO/TR 4808:2021 bridges measurement-unit gaps and provides a reproducible interpolation approach to make particle-count and Beta Ratio data comparable and actionable for contamination control and filter performance decisions.