An oxygen concentrator is a device providing oxygen therapy to a patient at minimally to substantially higher concentrations than available in ambient air. They are used as a safer, less expensive, and more convenient alternative to tanks of compressed oxygen. Common models retail at around $800. Leasing arrangements may be available through various medical-supply companies and/or insurance agencies.
Oxygen concentrators are also used to provide an economical source of oxygen in industrial processes.
How oxygen concentrators work 
Oxygen concentrators operate on the principle of pressure swing adsorption of atmospheric nitrogen onto zeolite minerals. 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 an air compressor, two cylinders filled with zeolite pellets, a pressure equalizing reservoir, and some valves and tubes. The concentration process is cyclical with each cycle having two halves of equal duration. The cycle can be explained with respect to the 1st cylinder as follows. In the first half of a cycle the 1st cylinder receives air from the compressor. The first half cycle lasts typically about 3 seconds. During that time the pressure in the 1st cylinder rises from atmospheric to a few times normal atmospheric pressure (typically 20 psi/138 kPa gauge, or 2.36 atmospheres absolute) and the zeolite becomes saturated with nitrogen so that pumping in additional air would begin to reduce the oxygen concentration. Part way through the first half of the cycle the gas in the 1st cylinder becomes close to pure oxygen (there are small amounts of argon, CO₂, water vapor and other minor atmospheric components) and a valve is opened to allow that gas to start flowing to the pressure equalizing reservoir to which the patient oxygen hose is connected. At the end of the first half of the cycle there is another valve position change so that the air from the compressor is directed to the 2nd cylinder. As what is in the 1st cylinder flows to the reservoir, the pressure in the 1st cylinder drops, and the nitrogen adsorbed on its zeolite begins to be released back into the gas, so that the oxygen concentration begins to drop. Part way through the second half of the cycle there is another valve change whereby the gas still in the 1st cylinder is vented back to the surroundings. That is to keep the concentration of oxygen in the pressure equalizing reservoir from falling below about 90%. The pressure in the hose delivering oxygen from the equalizing reservoir is kept steady by a pressure reducing valve.
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.
Portable oxygen concentrators 
Since the year 2000 a number of companies have produced portable oxygen concentrators. Typically, these devices produce one to five liters per minute of oxygen, and they use some version of pulse flow or "demand flow" to deliver oxygen only when the patient is inhaling. However, there are a few portable oxygen concentrators that produce up to three liters per minute of oxygen continuously such as the Invacare Solo2 or SeQual Eclipse. Also, they can provide pulses of oxygen either to provide higher intermittent flows or to reduce the power consumption.
The Federal Aviation Administration (FAA) of the United States has approved the use of portable oxygen concentrators on commercial airlines. However, users of these devices should check in advance as to whether a particular brand or model is permitted on a particular airline. Unlike in commercial airlines, users of aircraft without cabin pressurization need oxygen concentrators which are able to deliver enough flowrate even at high altitudes, such as OXYFLY.
Usually, "demand" or pulse-flow oxygen concentrators are not used by patients while they sleep. There have been problems with the oxygen concentrators not being able to detect when the sleeping patient is inhaling. Some larger portable oxygen concentrators are designed to operate in continuous-flow mode in addition to pulse-flow mode. Continuous-flow mode is considered safe for night use when coupled with a CPAP machine.
Military uses 
In both clinical and emergency-care situations, oxygen concentrators have the advantage of not being as dangerous as oxygen cylinders, which can, if ruptured or leaking, greatly increase the combustion rate of a fire. As such, oxygen concentrators are particularly advantageous in military or disaster situations, where oxygen tanks may be dangerous or infeasible.
Oxygen concentrators are considered sufficiently foolproof to be leased to individual patients as a prescription item for use in their homes. Typically they are used as an adjunct to CPAP treatment of severe sleep apnea. There also are other medical uses for oxygen concentrators, including emphysema and other respiratory diseases.
Used and refurbished units should be purchased through a reputable dealer. Temperamental units are worthless to the medical community since an individual's health frequently relies on the constant extended operation of the unit. However, such units are valuable to metal and glasswork hobbyists. Because oxygen is a "permanent gas" (cannot be liquefied at any pressure at room temperature), it is expensive to obtain in bottled form. Medical oxygen concentrators or specialized industrial oxygen concentrators can be made to operate small oxyacetylene cutting and welding torches.
Industrial oxygen concentrators 
Industrial processes may use much higher pressures and flows than medical units. To meet that need, another process, called vacuum swing adsorption (VSA), has been developed by the Air Products company. This process uses a single low pressure blower and a valve that reverses the flow through the blower so that the regeneration phase occurs under a vacuum. Generators using this process are being marketed to the aquaculture industry. Industrial oxygen concentrators are often available in a much wider range of capacities than medical concentrators.
Industrial oxygen concentrators are sometimes referred to as oxygen generators within the oxygen and ozone industries to distinguish them from medical oxygen concentrators. The distinction is used in an attempt to clarify that industrial oxygen concentrators are not approved medical devices by the Food and Drug Administration (FDA) and they are not suitable for use as bedside medical concentrators. However, applying the oxygen generator nomenclature can lead to confusion. The term, oxygen generator, is a misnomer in that the oxygen is not generated as it is with a chemical oxygen generator, but rather it is concentrated from the air.
The use of the oxygen generator terminology can also be a problem in shipping in the wake of the crash of ValuJet Flight 592. Non-medical oxygen concentrators can be used as a feed gas to a medical oxygen system, such as the oxygen system in a hospital, though governmental approval is required, such as by the FDA, and additional filtering is generally required.
See also 
- Portable oxygen concentrator
- The section on Storage and Sources of Oxygen in the Oxygen therapy article.
- Nitrogen separation membrane