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Electrochemical amperometric transducers monitor the electric current through the electrochemical cell and measure, with high sensitivity, the concentration of biological or chemical species. The present study examined the physical phenomena of mass transfer in the vicinity of an electrochemical sensor operating in flow conditions using a theoretical model. A three-dimensional (3-D) geometry of the electrochemical cell with computational fluid dynamic simulations of time-dependent convection and diffusion are used. The study focused mainly on the contribution of the convection, while migration due to ion movement by the electric field and kinetics limitations due to enzyme activity have been neglected. The concentration of the electroactive species from a thin layer close to the electrode is calculated by numerical simulations. This calculated concentration has been compared with the concentration obtained from the electric current measured by an amperometric experimental system. Comparison between the simulations and experimental patterns shows good agreement. Only a short delay at the onset of the measured experimental concentration was observed, compared to the simulation data. The source for the small disagreements could be connected to the manual procedures of the incomplete bubble removal in the experimental setup and to the migration effect that was neglected in the model assumptions.