Innovative in-pile instrumentation is crucial for advanced experimental programs in research reactors. In this field, we developed a specific acoustic sensor to improve the knowledge of fission gas release in Pressurized Water Reactor fuel rods when irradiated in Material Testing Reactors. In order to perform experimental programs related to the study of the fission gas release kinetics, the CEA acquired the ability to instrument a pre-irradiated PWR fuel rod. The detector, which has been developed and is patent pending by CEA, SCK.CEN (Belgian Nuclear Research Center) and IES (French research laboratory of Montpellier II University and French National Research Center), is the subject of this paper. This original acoustic sensor has been designed to measure the molar mass and pressure of the gas contained in the fuel rod plenum. For in-pile instrumentation, the fraction of fission gas, such as Krypton and Xenon, in Helium, can be deduced on-line from this measurement. Design and testing program of this innovative sensor will be discussed.