I was reading an article in Northern Rodder about airlines and a company called GARAGE-PAK
was highly recommended by them. This was recommended over PVC, copper, and black iron. The tubing is made from aluminum and powder coated. It slips into connectors so no welding, threading etc. Check out the site. Then look good at the prices. 1 pipe for $31.00 and 1 "T" connector for $29.00!!!!! I don't know about most people but to run lines around a shop with a few drops would run into the hundreds. For that kind of money I'll drag a hose around or run some iron.