This document establishes methods for:

—   validating equipment used to prepare secondary calibration suspensions for automatic particle counters;

—    performing online secondary calibration of automatic particle counters;

—    matching two or more online particle counters, i.e. to count the same number of particles at a given size by two APCs associated online;

—    validating online particle counting systems with and without online dilution as used, for example, to measure the filtration efficiency of a hydraulic filter as described in the multi-pass filter test in ISO 16889.