This International Standard specifies measuring methods for determining the level of solid particle contamination of hydraulic fluids used in hydraulic systems and equipment for aerospace purposes. Three methods are specified in this International Standard : a) microscopic method; b) automatic particle counting method; c) gravimetric method. In addition, this International Standard lays down the procedures for cleaning the apparatus and sample bottles, and specifies the solvents to be applied and how these solvents shall be prepared. Requirements as to the selection of the sampling point and to different sampling methods are also specified.