This International Standard specifies an automatic particle counting procedure for determining the number and sizes of particles present in hydraulic-fluid bottle samples of clear, homogeneous, single-phase liquids using an automatic particle counter (APC) that works on the light-extinction principle. This International Standard is applicable to the monitoring of the cleanliness level of fluids circulating in hydraulic systems, the progress of a flushing operation, the cleanliness level of support equipment and test rigs, the cleanliness level of packaged stock. NOTE1 Measurements can be made with particles suspended in the original liquid or in a sample of the liquid diluted with a compatible liquid to reduce coincidence error. NOTE2 The presence of a fluid interface obstructs the light beam and gives false signals.