A blogger, this one at least, is a person who can't learn anything without developing an urgent impulse to communicate it. Have you ever wondered how an oxygen concetrator works? (If not, you can safely skip the following) An oxygen concentrator, if you are wondering, is one of those devices you see people with clear plastic tubes in their noses using - recognizable by it's own characteristic raspy breathing - sort of a pssst, pssst, pssst with a period of a few seconds. As its name indicates, it concentrates oxygen from the air and delivers it to the wearer, usually an ill person, but also to some pilots flying at high altitudes.
Knowing that air is 78% nitrogen and about 21% oxygen, how does it do that? What difference between the molecules is being exploited to do the separation. This stumped me, so I had to look it up, but if you don't know the answer, try to guess.
It turns out to be molecular size. Oxygen atoms, with 8 protons, are about 5% smaller than the 7 proton nitrogen atoms - both have partially filled outer electron shells. The separation is effected by running the air through a zeolite - a microporous mineral structure with appropriately sized pores, making it a good molecular sieve.
So you run the air under pressure though some zeolite and oxygen gets through, but nitrogen can't. The nitrogen quickly clogs the pores, so they need to be depressurized and backflushed with a bit of the oxygen to clean them out. Usually a two bed system is used so that one can be used while the other is being flushed. I assume that it's that cycle that produces the characteristic pssst, pssst, pssst.