A semi-closed circuit gas recycling scuba diving breathing apparatus having a breathing loop. The breathing loop has a breathing bag to be supplied by at least one gas tank and a recycling chamber. The gas tank is connected to an inlet of the breathing bag by a first duct and a second duct having respectively a demand regulator and a fixed nozzle configured to deliver respectively a first gas input and a second gas input to the breathing bag. Advantageously, the apparatus has a third duct having a gas flow regulator configured to deliver at a variable mass flow, a third gas input to the breathing bag, and a diving condition sensor, configured to measure a physiological parameter of the diver or the depth. The apparatus is advantageously configured to make the variable mass flow vary according to data of the sensor to optimise the consumption of gas.