Oxygen concentrators operate on the principle of pressure swing adsorption of atmospheric nitrogen onto zeolite materials. At high pressure, nitrogen sticks to the surface of the zeolite. Because the zeolite is extremely porous, it has a very large surface area and can adsorb large volumes of gas. At low pressure the nitrogen is released.
An oxygen concentrator has two cylinders filled with zeolite pellets. Air is compressed to a few times normal atmospheric pressure (typically 20 psi/138 kPa gauge, or 2.36 atmospheres absolute) and passed through the zeolite bed. The bed adsorbs the nitrogen, leaving nearly-pure oxygen (plus small amounts of argon, CO₂, water vapor and other minor atmospheric components) in the output.
The zeolite becomes saturated within a few seconds, at which point the compressed air is fed to the second cylinder while the first is vented to ambient pressure, and the nitrogen escapes. When the second cylinder is saturated, the compressor switches back to the now-available first, and the cycle repeats.
Older units cycled with a period of about 20 seconds, and supplied up to 5 liters per minute of 90+% oxygen. Since about 1999, units capable of supplying up to 10 lpm have been available.