Hey, this is a cool group. Just found it. I'm definitely a novice metalworker, though I have an ancient lathe and make stuff on it from time to time. I hope someday to own a better one, and a vertical mill to go with it (grin).
Around here, at least (mid-state NY), the price difference between welding O2 and USP (medical) O2 is negligible for the large (200cf-ish) cylinders. It's only a few dollars more for USP O2, though I don't recall exactly how much. Aviation O2 isn't available at my local gas supply company (I asked), so I never got a price on it.
As to whether one can breathe welding/industrial O2 safely, what follows is my opinion: It depends. For scuba diving, Technical Diving International gas mixing standards call for USP or Aviation O2 to be used for mixed-gas (Nitrox and Trimix) blending because the partial pressures of any contaminating gases rise as a function of depth, exacerbating their effects. Clearly, this also applies to using pure O2 under water.
At sea level or above, depending on whatever else was in there, you're still taking a chance. For example, consider a tank that contained 100% acetylene at atmospheric pressure before being filled with O2. The usual fill pressure of 2400 PSI would yield one part in 163 (2400/14.7), or ~6,100 ppm at sea level. The OSHA exposure limit for acetylene is
1,000 ppm. Oops. (Do your own math and research the exposure limits before you breathe anything!)For CO, 50 ppm is a problem and 70 ppm is a real problem. 6,100 ppm is obviously Right Out. Even at 10,000 feet, where if I recall correctly flight crew must use O2 after 20 minutes, that's still 70% or so of
6,100 ppm (~4,300 ppm), so aviation use would not be safe either in this case. There could be other contaminating gases I haven't thought about that might be worse than CO. And if the fill pressure is 2105 PSI, the percentages are even worse.Now, how an O2 cylinder would wind up *full* of a bad gas, or even with a high percentage of it, before being filled with O2 is not at all clear to me. Someone would almost have to work at it! Nonetheless, safe definitely beats sorry hands down in this case, and "probably OK" means that sometimes it won't be OK. Since "not OK" can translate to "dead," I'm going to continue to buy USP O2 for breathing.
My understanding is that USP and aviation grade O2 are put in cylinders that have been purged with O2 before final filling, so the maximum levels of contaminants are quite low no matter what's in the tanks before they are purged. This is a very "practical difference" to me when the O2 will be a breathing gas, even though the source of the O2 used to fill any tank (industrial, USP, or aviation grade) is very likely to be exactly the same.