Abstract : The respirable gases generated by an on board oxygen generation system (OBOGS) are comprised of a mixture of oxygen, argon, and nitrogen. This gas mixture can be measured ultrasonically to determine the concentration of oxygen within the mixture. This final report describes an ultrasonic measurement system that has been successfully tested to a simulated altitude of 20,000 ft. The concentration of oxygen calculated from ultrasonic data is accurate to approximately 1% over the range of 21 to 95%. In addition to measuring the concentration of oxygen, the ultrasonic oxygen sensor can also measure the flow rate of the respirable gases produced by an OBOGS accurate to approximately 1.6 L/min. Keywords: Ultrasonic, Respirable gases, Altitude chamber.