ISO 4406: fluid cleanliness coding (SAE AS4059; NAS 1638 legacy)

The result is a shorthand for cleanliness: a lower code means cleaner fluid. Component manufacturers specify the target code their equipment needs.

Anyone responsible for the reliability of fluid-power and lubricated machinery uses these codes: hydraulic-system designers and operators in manufacturing, mobile and construction equipment, wind turbines, aerospace, and marine, along with lubricant suppliers and condition-monitoring engineers. Fuel handlers use them to grade the cleanliness of diesel and aviation fuel. Because most hydraulic and bearing failures trace back to particulate contamination, maintenance teams set a target cleanliness code for each system and manage filtration and oil changes to keep the fluid within it.

The cleanliness code is derived from a particle count, so the measurement behind both ISO 4406 and SAE AS4059 is counting and sizing the solid particles in a fluid sample, historically by laboratory optical counting and increasingly by inline optical or light-blockage sensors that report a live code. An inline counter mounted in the circuit converts its count into an ISO 4406 code (and often SAE AS4059 or legacy NAS 1638 classes) continuously, so the fluid's condition can be trended against that target code and filtration triggered before contamination causes wear. Continuous coding turns cleanliness from an occasional laboratory result into a live maintenance signal, catching ingress or filter breakthrough as it happens.