This SAE Aerospace Standard (AS) defines cleanliness classes for particulate contamination of hydraulic fluids and includes methods of reporting related data (

Appendix A

). The contamination classes selected are based on the widely accepted NAS 1638 cleanliness classes. Conversion from NAS 1638 cleanliness class specifications to AS4059 class specifications is defined. Comparison of the NAS 1638 classes to AS4059 classes is defined and the differences explained (

Appendix B ). This document provides versatility in identifying a maximum class in multiple size ranges, total number of particles larger than a specific size or designating a class for each size. NAS 1638 classes based on weight of particles are not applicable to either of these classes and are not included.