Breathing mixture controller

In an aircraft breathing system including an oxygen generator of the type used in pilot life support systems to draw gases from engine bleed air and to provide oxygen to the pilot and which generates oxygen/nitrogen mixture when overloaded, a device to cause an excess demand on the oxygen generator at lower altitudes so that a variable oxygen/nitrogen breathing mixture is drawn from the generator, and at higher altitudes a higher oxygen concentration breathing mixture is drawn from the generator, the device comprising:

• inlet means for receiving a portion of breathing mixture from the life support system in parallel to the pilot's demand on the system;
• means defining a chamber having substantially cabin pressure therein for receiving said portion;
• an oxygen partial pressure sensor contained in the chamber means to sense the partial pressure of oxygen in the breathing mixture;
• a valve positioned between the inlet and the chamber means and operable to change the rate of flow of breathing mixture to said chamber;
• control means coupled to the sensor and to the valve, the control means controlling the valve dependent on the sensed partial pressure of the oxygen so that if the partial pressure deviates from a predetermined pressure the valve is moved to adjust the rate of flow of said portion until the predetermined partial pressure is sensed by said partial pressure sensor so that at lower altitudes as the flow rate increases, an excess demand is created on the generator and a lower oxygen concentration breathing mixture is produced;
• and
• means sensitive to cabin pressure to prevent flow of said portion at higher altitudes when the cabin pressure is above a predetermined limit so that the device is then inoperative and the pilot receives a suitable higher oxygen concentration breathing mixture from the generator.