In this paper a methodology is described to be implemented in a simple and low cost automatic measuring apparatus for on line measurement of Cr(VI) concentration in water, which is able to be controlled by a PC and to be inserted in an integrated environmental control multi-sensor network. Measuring section is a miniaturized fiber optic probe, whose working principle is light absorption of the complex Chromium (VI) – Diphenylcarbazide. Device design and set up guidelines of an automatic measuring device are described in order to get a reliable, effective and user friendly configuration and to reduce measurement uncertainty depending on interfering quantities of different type, fluid dynamic, optical, environmental, taking into account “in situ” measuring conditions