ISO 4406:2021

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Hydraulic fluid power — Fluids — Method for coding the level of contamination by solid particles

ISO 4406:2021 is an international standard that defines a coding system to measure the level of solid particle contamination in hydraulic fluids, essentially providing a way to categorize the cleanliness of a fluid based on the number of particles of specific sizes present in a given volume, expressed as a three-digit code that represents the particle count at different micron levels (typically 4 microns, 6 microns, and 14 microns.