An apparatus 10,110 is provided that measures the speed of sound or acoustic disturbances propagating in a fluid or mixture having entrained gasair to determine the gas volume fraction of the flow 12 propagating through a pipe 14. The position is dynamically controlled from the outside, for example, through a hydraulic actuator connected between a pivoting arm supporting the magnetron and an arm fixed to the shaft, by two coaxial shafts independent controllable from the outside and supporting the magnetron through a frog-leg mechanism, or a cable connected between the pivoting arms and moved by an external slider. A first temperature sensor is provided in the exhaust outlet leading from the exhaust manifold of the engine and a second temperature sensor is provided in the catalytic converter.