Abstract: An oxygen concentrator comprises a plurality of adsorbent columns, each having an inlet, an outlet and abed of adsorbent material. The oxygen concentrator also includes a vacuum pump for removing separated nitrogen rich gas from the column inlets, a control valve for controlling flow of fluids in and out of the columns, and a breakthrough sensor for signaling the position of a mass transfer zone (MTZ), the breakthrough sensor controlling operation of the control valve as a function of the breakthrough sensor signal. An electric control module (ECM) receives the signal from the breakthrough sensor, and adjusts the control valve based on the signal.