This International Standard describes the performance testing of on-line sensors/analysing equipment for water. The standard is applicable to most sensors/analysing equipment, but it is recognized that, for some sensors/analysing equipment, certain performance tests cannot be carried out. This International Standard  defines an on-line sensor/analysing equipment for water quality measurements;  defines terminology describing the performance characteristics of on-line sensors/analysing equipment;  specifies the test procedures (for laboratory and field) to be used to evaluate the performance characteristics of on-line sensors/analysing equipment.