This part of ISO 21018 specifies methods and techniques that are applicable to the monitoring of particulate contamination levels in hydraulic systems. It also describes the relative merits of various techniques so that the correct monitor for a given application can be selected. 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. This part of ISO 21018 can also be applicable for other liquids (e.g. lubricants, fuels and process liquids). NOTE Instruments used to monitor particulate contamination are not considered as or claimed to be particle counters, even if they use the same physical principles as particle counters.