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 a) the cleanliness level of fluids circulating in hydraulic systems, b) the progress of a flushing operation, c) the cleanliness level of support equipment and test rigs, d) the cleanliness level of packaged stock. NOTE 1 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. NOTE 2 The presence of a fluid interface obstructs the light beam and gives false signals.