Described and claimed is an interchangeable tip-open cell fluorometer comprising a housing and a fluorometric probe tip interchangeably connected to the housing, the probe tip including a probe tip housing defining an open cell and enclosing a probe optical arrangement, the probe optical arrangement including an excitation source and a fluorescence detector wherein the excitation source is aimed directly into the fluorescence detector such that a sample can be fluorometrically detected. Also claimed is a method of using this interchangeable tip-open cell fluorometer for detecting fluorescent signals emitted by one or more fluorophores from samples from a natural or industrial water system. The fluorometer, when coupled with a controller, is capable of monitoring and optionally controlling an industrial process or system.