175 psi, i need to hide the air tank of my bag set up under the rear seat, here its illegal to have adjustable suspension and risked to have the car inpounded once already. I need a small tank, and because im making a step notch i thought of incorporating a small diameter tank into the design as the smaller 2 or 3 gallon tanks available are too bulky and would be kinda of an hassle to fit. As for open root welding i did a search and no i never did, althought i usually bevel both joints to get better penetration, but usually if i leave a gap and make a root pass its not in the same way as in pipe weldin. I could even use my old stick welder if you think its better, i used to be quite good at it, plus i have a 100pk of electrodes that is just sittin there so i might as well put it to good practice use. I'll do some test panels, or tubing sections then picture them and post'em.