A submersible system and method for measuring the gas volume fraction in an aerated fluid inside a reactor (1) wherein the aerated fluid comprises a gas dispersed in the form of bubbles in a fluid in the form of a solution, suspension, mixture of liquids or a combination thereof. The submersible system comprises: an open and pass-through gas exclusion device (20) of a variable cross section wherein the inlet opening whereby the fluid enters without gas bubbles towards the opened and through gas bubbles exclusion device (20) is greater than the outlet opening whereby the fluid exits without gas bubbles of the opened and through gas bubble exclusion device (20). The outlet opening abuts with an inlet pipe (23). A chamber (24) which can provide a sealed camera, can contain at least one flow meter to measure the gas-free fluid velocity when circulating between an inlet (27) and an outlet (28) of the chamber (24) or sealed camera The inlet (27) of the chamber (24) or sealed camera can be coupled to the inlet pipe (23). The outlet (28) of the chamber (24) or sealed camera can be coupled to an outlet pipe (26) of the liquid dispersion towards the reactor (1). A flow transmitter (29) connected to the flow meter, located inside or outside said chamber (24) or sealed camera, generates an outlet signal proportional to the bubbles-free fluid velocity through a gas bubble exclusion device and a calculation unit (30) which generates an output signal (31) proportional to the gas volume fraction in the aerated fluid.