NAS 1638
NAS 1638 Cleanliness Requirements of Parts Used in Hydraulic Systems- Rev : The cleanliness limit of the representative fluid sample from parts ，assemblies， lines and fittings shall not exceed the permissible maximum contamination limits of the specified class given in Table I and Table II.2022
ISO 4406
Hydraulic fluid power- Fluids- Method for coding level of contamination by solid particles : In hydraulic fluid power systems, power is transmitted and con- trolled through a liquid under pressure within an enclosed circuit. Solid- particle contaminant is always present in the hydraulic fluid, and the amount needs to be determined because the contaminant may cause serious problems.2022
SAE 4059F
Aerospace Fluid Power - Cleanliness Classification for Hydraulic Fluids : This SAE Aerospace Standard (AS) defines cleanliness classes for particulate contamina tion of hydraulic fluids and includes methods of reporting related data (Appen- dix A). 2022