I have feared the empty tank for years. Also, it made me a bit nuts when it was empty. However, there is a way to get some more O2 out of the welding tanks at least. Without the regulator, open the valve. I always hear a lot of O2 coming out with the regulator removed. In fear of running out one night, I took two garbage bags and set up a redneck bag. Just a garbage bag with a corner cut off to slip in the top and shoulder of a 2 liter bottle that I cut off. Then tape up the bottom so the bag is sealed except for the bottle opening. I held the opening of the two liter to the output of the tank and filled the garbage bag. It easily filled three bags with O2 left over. Those fluffy bags were waiting for me when I needed them.
The bags do leak, so you may need to top them up so they last over a day.
ATB!