ISO 4406-2021 PDF

Full title and description

ISO 4406:2021 — Hydraulic fluid power — Fluids — Method for coding the level of contamination by solid particles. This International Standard specifies the coding system used to report and communicate the quantity of solid particulate contamination in fluids used in hydraulic fluid power systems.

Abstract

ISO 4406:2021 defines a three-part cleanliness code that expresses the concentration of particles in a fluid sample as cumulative counts for three particle-size thresholds. The code provides a concise, industry‑wide method to report particle contamination (per millilitre) so that equipment manufacturers, maintenance teams and testing laboratories can specify, compare and monitor hydraulic fluid cleanliness consistently.

General information

• Status: Published
• Publication date: 7 January 2021
• Publisher: International Organization for Standardization (ISO)
• ICS / categories: 75.120 (Hydraulic fluids); 23.100.60 (Filters, seals and contamination of fluids)
• Edition / version: 4 (2021)
• Number of pages: 6

Scope

This standard specifies the code to be used for defining the quantity of solid particles in fluids used in hydraulic fluid power systems. It standardizes how particulate contamination is reported (cumulative particle counts per millilitre) so that measurements from particle counters or membrane filter methods can be expressed in a common format for design, procurement, maintenance and quality control. It is a reporting/classification standard and does not prescribe sampling procedures or analytical instrumentation calibration methods (these are covered by related standards).

Key topics and requirements

• Three-part cleanliness code (example format: 18/16/13) representing cumulative particle counts for particles ≥4 µm(c), ≥6 µm(c) and ≥14 µm(c).
• Mapping of code numbers to particle-count ranges (each increment represents approximately a doubling of particle concentration within the coded band).
• Cumulative reporting convention — counts for larger sizes are included in the smaller-size counts (e.g., 6 µm and 14 µm counts are subsets of the 4 µm count).
• Intended output is a concise, repeatable cleanliness designation suitable for specifications, acceptance limits and trend monitoring.
• Standard clarifies reporting format and interpretation; it should be used alongside sampling, counting and calibration standards to ensure results are comparable (e.g., calibration of automatic particle counters and membrane filter counting methods).

Typical use and users

Used by hydraulic system designers, OEMs, filtration and hydraulic component manufacturers, laboratory analysts, reliability and maintenance engineers, oil analysis service providers and procurement/specification teams. Typical uses include defining target cleanliness for new systems, acceptance criteria for fluids and components, routine oil-analysis reporting, troubleshooting contamination problems and validating filtration performance.

Related standards

Commonly referenced standards to use with ISO 4406 include ISO 11171 (calibration and verification of automatic particle counters for liquids), ISO 4407 (membrane filter counting/microscope methods for particulate determination), and relevant ISO technical reports and test methods on sampling, particle‑count interpolation and filter performance. Equipment manufacturers and labs commonly pair ISO 4406 codes with calibrated particle-counter procedures and membrane filter counting protocols to ensure consistent results.

Keywords

hydraulic fluid, contamination, particle count, cleanliness code, ISO 4406, particle size, fluid cleanliness, filtration, oil analysis, particle counter

FAQ

Q: What is this standard?

A: ISO 4406:2021 is an international standard that defines a coding system to report the level of solid-particle contamination in hydraulic and related fluids.

Q: What does it cover?

A: It covers the format and interpretation of the fluid‑cleanliness code (typically three numbers) that correspond to cumulative particle counts at specified micrometre thresholds (commonly reported as ≥4 µm(c), ≥6 µm(c) and ≥14 µm(c)).

Q: Who typically uses it?

A: Hydraulic system designers, OEMs, filtration suppliers, oil analysis labs, maintenance and reliability engineers, and procurement/specification teams use it to specify and communicate fluid cleanliness requirements.

Q: Is it current or superseded?

A: ISO 4406:2021 (4th edition) was published in January 2021 and supersedes the previous 2017 edition. It is the current edition for reporting cleanliness; users should confirm status if a specific national or industry update has been released after 2021.

Q: Is it part of a series?

A: It is part of a family of hydraulic fluid contamination and testing standards. Complementary standards cover particle counting methods and instrument calibration (for example, standards for membrane filter counting and for calibration/verification of automatic particle counters).

Q: What are the key keywords?

A: hydraulic fluid, contamination, particle count, cleanliness code, ISO 4406, filtration, oil analysis.