This part of ISO 21018 specifies a method for the semi-quantitative determination of the particulate contamination level using the filter blockage technique (also known as the mesh-obscuration method or the pore-blockage technique) either on-line or off-line in containers. It also defines procedures for calibrating the instruments and verifying their correct operation both in the laboratory and in service. In general, the techniques described in this part of ISO 21018 are suitable for monitoring a) the general cleanliness level in hydraulic systems, b) the progress in flushing operations, c) support equipment and test rigs. The use of this method is applicable to all single- or multi-phase liquid systems.