Ozone and oxygen
Molecular sieve
A molecular sieve is a porous crystalline material, usually a zeolite, with pores of a uniform size that trap some gas molecules and let others pass. In an oxygen concentrator the sieve holds back nitrogen from compressed air, so the gas leaving the bed is mostly oxygen.
Also called zeolite bed, sieve bed
The everyday version
Think of a sieve that separates by molecule rather than by grain size. A zeolite has a rigid crystal lattice riddled with cavities of a precisely uniform diameter. Gases whose molecules fit and bind to those cavities are held; the rest go through.
In an oxygen concentrator
Air is roughly 78% nitrogen, 21% oxygen and about 1% argon. The zeolite used in medical concentrators adsorbs nitrogen preferentially under pressure. Air is compressed into one bed, oxygen-rich gas leaves it, and when that bed approaches saturation the machine drops the pressure to release the captured nitrogen to atmosphere while a second bed takes over. Alternating between beds — pressure swing adsorption — produces a continuous flow. The concentrators supplied with our saunas use compressed packed molecular beds.
Why the sieve sets the purity ceiling
Argon behaves too much like oxygen for the sieve to separate it, so it passes through with the oxygen. That is why concentrator output tops out in the low-to-mid nineties rather than reaching the 99%+ of cylinder oxygen, why the United States Pharmacopeia’s monograph for sieve-produced oxygen is written as a 90.0–96.0% band rather than a single figure, and why the concentrators supplied with a sauna are specified as 93% ± 3%.
Looking after it
Sieve material is degraded by moisture and by oil, which is why medical concentrators use oil-free compressors and inlet filtration. Dust starving the air intake makes the machine work harder for less output. Keeping the intake and filters clean is the main thing an owner can do to protect purity over the years.
Why it matters here
It is the reason a concentrator's purity is quoted as a band rather than a single number, and the reason keeping the air intake and filters clean is worth the couple of minutes it takes.