Invacare "Mobilaire V" oxygen concentrator, $20.

The oxygen concentrator is another medical machine that's always worth picking up, and the Invacare Mobilaire V is one of the more common models you're going to see. Due to any number of medical reasons, some people require a continuous source of supplemental pure oxygen. We've all seen people wheeling or carrying around those little green tanks of compressed oxygen. But in curcumstances where a source of electricity is always close at hand, it's more practical to use a machine that simply extracts the oxygen from the air around us, and that's where the oxygen concentrator comes in so handy. But... how the heck does it work?

The air we breathe is composed of roughly 80% nitrogen and 20% oxygen. An oxygen concentrator uses ambient air as a source of oxygen by separating these two components. The concentrator does this by utilizing two canisters containing specialized granules, which literally absorb the nitrogen from the compressed air, emitting 95% pure oxygen.

Atmospheric air is drawn into the oxygen concentrator, where it's filtered and raised to a pressure of 20 pounds per square inch (P.S.I.) by a compressor. The compressed air is then introduced into one of the two canisters containing zeolite granules, where the nitrogen is absorbed, allowing nearly pure oxygen to be made available for patient use. After about 20 seconds of operation, the compressed air is automatically diverted to the other canister, where the process is repeated for another 20 seconds. This alternation allows for an uninterrupted output of oxygen. While the pressure in the second canister is at 20 P.S.I. the pressure in the first canister is being reduced to a near vacuum, which actually sucks the stored nitrogen out of the zeolyte granules and returns it to the atmosphere. The zeolite is now ready for the next absorption cycle. By alternating the pressure in the two canisters so that first one and then the other is at 20 P.S.I., a constant supply of oxygen is produced while the zeolite is continually being regenerated.

Don't let the word zeolite scare you, either. The term sounds pretty scientific, but zeolites have been around for many years and are nothing more than very fine micro-porous filter granules with very specifically shaped cavities or channels in their surfaces, which literally 'trap' atoms or molecules of a specific shape, while allowing other atoms or molecules to pass by. This action can be compared to a jigsaw puzzle, where only one shape will fit in a specific spot.

As you can imagine, a zeolite can be formulated to filter out just about anything you want, by simply changing the shape of the cavities in the granules. So, technically, you could make a nitrogen concentrator, an argon concentrator or a helium concentrator, by simply loading the canisters with zeolite granules of the correct cavity shapes to trap the required gases!

The better oxygen concentrators available nowadays have an oxygen output of five liters per minute, and the 'V' in this unit's name indicates that it can produce 5 liters of oxygen per minute. These units used to sell for around $300 on eBay, but since guys who refurbish them have begun listing them en masse, they go for around $100 to $150.

When you find yourself considering buying one of these machines, make sure that the machine runs quietly as it puts out the oxygen.
Let it run for ten minutes or so, to ensure that it has no problems that would cause the machine to pop any breakers and shut itself off.
Like most oxygen concentrators, the Mobilaire has an hour counter that records the total number of hours the machine has been used. Although just about any machine will sell as long as it works, you should put this number into your auction because your prospective buyers will want to know.
Make sure the machine has no cigarette odors to it. Keep in mind that most of the people who use these machines didn’t get that way from clean living. They smoked heavily, and often continued smoking even after they were put on oxygen. This means that many of the machines you find have become thoroughly permeated from cycling many hours of cigarette smoke through them.

Finally... are you thinking of selling one of these machines on eBay? If so, you might want to put in your auction that the machine is intended for use in the arts and crafts industry. EBay has gotten freaky about selling medical equipment, even though the particular piece you lise on eBay will very likely not be used for any medical use by the person who buys it from you! Artists and craftsmen involved in glass blowing, bead making, lamp making, or any other pursuit where glass must be worked with in a molten state, will involve heating and melting torches. And these little torches require oxygen as one of their components, which is easily obtained from a handy little oxygen concentrator like the Mobilaire. So, you're not really selling a piece of medical equipment, but a piece intended for the arts and crafts industry, which will be used to produce oxygen for melting torches. 

This Invacare Mobilaire V oxygen concentrator sold on eBay for $215. The buyer would be using it for her beadmaking business.

Photo of Invacare Mobilaire V oxygen concentrator