Introduction
ISO 4402 is a legacy international calibration standard used for automatic particle counters (APCs) measuring particulate contamination in hydraulic fluids. The method uses AC Fine Test Dust (ACFTD) as the calibration contaminant to establish particle size thresholds and counting accuracy.
Although ISO 4402 played a key role in early contamination control programs, it has largely been replaced by ISO 11171, which introduces NIST-traceable calibration materials and the modern µm(c) particle sizing framework used in ISO 4406 cleanliness coding.
Method Design
ISO 4402 is a calibration method rather than a routine cleanliness measurement standard. It specifies procedures for preparing calibration suspensions, determining particle size distributions, and adjusting APC channel thresholds to match expected ACFTD particle concentrations.
How the Method Works
- Prepare a calibration suspension by dispersing AC Fine Test Dust in a clean suspension fluid.
- Measure cumulative particle counts using the APC across repeated runs.
- Calculate average particle counts for each size threshold.
- Compare measured results with expected values derived from the ACFTD distribution table.
- Adjust instrument channel thresholds to match expected particle counts.
- Verify calibration using repeated measurements and acceptance tolerances.
Key Advantages
- Provided early international consistency in particle counter calibration.
- Enabled standardization of hydraulic cleanliness measurements.
- Maintained compatibility with legacy contamination control datasets.
Comparison to Other Particle Counting Standards
- ISO 11171 – Modern APC calibration standard using NIST-traceable reference materials.
- ISO 4406 – Cleanliness coding system based on particle counts.
- ISO 4407 – Manual microscopic particle counting method.
- SAE AS4059 – Aerospace particle contamination classification.
Applications
- Legacy hydraulic contamination monitoring programs
- Historical data comparisons between old and modern particle counting systems
- Calibration checks for older APC instruments
Operational Considerations
Calibration accuracy depends strongly on proper suspension preparation, dust dispersion, dilution accuracy, and avoidance of contamination or particle settling.
Because ISO 4402 is a legacy standard, its primary modern use is interpreting historical cleanliness data and legacy specifications.
Conclusion
ISO 4402 played an important role in establishing early particle counter calibration procedures but has been superseded by ISO 11171. Modern contamination monitoring programs rely on ISO 11171 calibration and ISO 4406 cleanliness coding for consistent global reporting.
