Long time cave diver here. I mix tri-mix in my garage. The T cylinders of O2 you get at your local gas company is the same 02 if it welder's O2 or medical O2. The come from the same supply. The only difference is that they suck out the contents of the cylinder if it is medical grade O2 to ensure that there is no contamination, while they just open the valve and drain any leftover on returned tanks. It's really a non-issue for me and lots of other divers. Someone did the math and its like a possible 0.3% reduction of O2 quality.
And there are the divers who don't mind paying more for the stamp because it makes them feel good and they perhaps they justify skipping the 'gas quality' verification step. Invest in a good O2 monitor and verify every T cylinder.
And there are the divers who don't mind paying more for the stamp because it makes them feel good and they perhaps they justify skipping the 'gas quality' verification step. Invest in a good O2 monitor and verify every T cylinder.