This part of ISO 9555 deals with the use of fluorescent tracers in discharge measurements by the dilution method. Apparatus and methods of general application are set out in ISO 9555-1 and are not repeated here, with the exception of those relating specifically to fluorescent tracers. The use of fluorescent tracers is attractive because of the small amounts of tracer needed to make a discharge measurement. Certain fluorescent tracers can be measured at concentrations of less than 1 μg/l. Fluorescent tracers are easy to handle and their concentrations can be readily determined.