There seems to be some confusion. Tube is an engineering material. Pipe is a for conducting fluids. Both are sized differently and this probably why some people are having problems with pipe benders and the dies not fitting correctly. Tube is always sized on the OD, pipe is sized in many different ways, but rarely on its OD.
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Don't use scaffold.
Use blue or red band gas pipe, this is allowed by ARC/MSA regs, scaffold is not.