The purpose of this technique is to provide a means of determining the severity of particulate contamination in fluid power systems. AS598 was prepared by SAE A6C1 panel and supersedes ARP598C. AS598 retained the technical content of ARP598C with more prescriptive requirements for the material used to carry out the analysis. The widely used technique has since been replaced in many instances with automatic particle counters (APC) but remains a proven, accurate and reliable technique. This SAE Aerospace Standard (AS) defines the materials, apparatus and procedure for sizing and counting of particulate contamination, 5 um or greater, in hydraulic fluid samples by membrane filtration with microscopic counting. The microscopic counting method is capable of counting particulate matter in samples withdrawn from fluid power systems as identified by the 12 classes of AS4059 and projected beyond these for the five standard ranges specified and can thus serve as the primary document to determine acceptability. It is also capable of revealing but not measuring evidence of abnormal amount of water, other fluids, fine particulate and other materials, especially fibers and metals. It is applicable to all military, civil, space vehicles and test equipment.