Described and claimed is a modular fluorometer and a method of using same to control an industrial water system, wherein said modular fluorometer comprises: one or more units with each unit comprising: a channel comprising an optically appropriate cell through which a water sample flows; a light source positioned outside said optically appropriate cell; an optional excitation filter positioned between the light source and the optically appropriate cell; a reference detector positioned outside the optically appropriate cell; an emission detector positioned outside the optically appropriate cell; an optional emission filter positioned between said emission detector and said optically appropriate cell; wherein if more than one unit is present in said modular fluorometer, said optically appropriate cells through which said water sample flows are aligned such that only one channel through which a water sample flows exists in said fluorometer.