in an oxygen concentrator which is intended for aircraft use , a booster compressor is used to increase the pressure of the product gas from the concentrator in order to increase the amount of the gas which can be stored in a plenum . the booster includes two moving pistons which are rigidly linked together and a series of check valves which control the flow of gases through the compressor . one of the pistons is driven by air from the rotary valve in the concentrator , and the other piston compresses the product gas for delivery to the plenum . a small sample of concentrator product gas is monitored by an oxygen sensor for oxygen concentration . once the oxygen concentration has reached an acceptable level the booster compressor fills the plenum with product gas . thereafter , if oxygen concentration of product gas delivered to the crew from the concentrator falls below the concentration which is required at a particular altitude , the product gas stored in the plenum is delivered to the crew . the oxygen sensor monitors the concentrator output product gas to the breathing regulator when the stored plenum gas is not being used .