Schedule 80 PVC (the thick grey stuff) is rated for 8 bar at least (if I recall correctly and our pipeline specifications at work are correct). Don't know about the white plumbing stuff, but unless I had something from the manufacturer of the actual pipe/fittings you were going to use I wouldn't be doing it. Compressed gas is a dangerous thing, thats why they hydrotest cylinders with water and not pressure test them with air.