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Has anyone ever used a helium tank to make a smoker. If so, do they hold heat well like a propane tank. Or can a helium tank even be used to make a smoker?
Helium is an inert gas so you don't have to worry about launching yourself into orbit when you cut into it, if there are any residual elements left after a good burn out I can't answer that question, what I can say is it's used in balloons and when they explode or deflate and release their helium it's evidently not dangerous.
As to how well it would retain heat would depend on the wall thickness, can you show us a pic?
Helium is a harmless gas so there are no issues in that regard. It can be breathed with no harm, and is actually mixed with oxygen to make a mix for scuba diving.
As for holding heat, as JPT said above that depends on the wall thickness, not the gas that was inside.