The Gas Introduction Module (GIM) is the basic system used in JET to provide the vacuum vessel with the amount of gas necessary for a successful plasma discharge. The Plasma Density Feedback (PDF) system controls the GIM Piezo Electric Valve allowing the desired amount of gas flowing in the vacuum vessel. The unavailability of a flow measurement is the main reason for the development of an accurate flow model, able to predict and calculate the number of particles flowing within an acceptable error margin. This paper describes a GIM model that uses the control signal of the piezo-electric valve to compute the pressure evolution inside the GIM reservoir and, using the pressure estimation, obtains the particles flow considering also the dynamic heat exchange effect between the gas and the structure of the GIM.