In one aspect, a product gas concentrator is provided. In one embodiment, the apparatus may include: a first process separating adsorbable components from a source gaseous mixture, a second process providing concentrated product gas in a continuous output mode, a third process providing concentrated product gas in a pulsed output mode, and a fourth process selectively switching between the continuous and pulsed output modes. In another embodiment, the apparatus may include: a first process pressurizing a source gaseous mixture, a second process separating adsorbable components from the pressurized gaseous mixture, a product tank accumulating concentrated product gas for dispensing, an output path, a third process selecting a volume to be dispensed during a predetermined time, a pressure sensor monitoring pressure of the concentrated product gas, and a fourth process controlling flow of the concentrated product gas in response to the selected volume and the monitored pressure.