I would caution against using any oxygen in your aircraft except "aviators breathing oxygen." I can't speak to the purity of welder's oxygen, but obviously medical grade oxygen is very pure. The problem with the use of either of these in an aircraft is their moisture content. Aviatior's oxygen is highly dried and has no appreciable moisture content. Medical grade oxygen has a significant moisture content that can cause freezing in the lines at altitude. This will block the line and ruin your day at altitude.
I just attended the high altitude physiology course at the FAA in Oklahoma City. I highly recommend this to all who fly at any altitude (It's also free). I have a new respect for hypoxia (even at low altitudes) and the insidious dangers of high altitude flight. The time spent in the altitude chamber at 25000 feet was a real eye opener. They teach you to recognize your own hypoxia symptoms (very different in different people). I watched a 20 year old pilot claim to the instructor that he could still fly though he was unable to put the plastic blocks in the cube (the toddler's toy). This was after only 3 minutes off oxygen.
Steve Johnson
RV9-A (Almost done)