Oscar wrote:25 feet of 5/16" OD tube with 0.020" wall has ~17 in³ of internal volume. It would take 1 CFH about 30 seconds to fully purge it. At 2 CFH would be about 15 seconds to fully purge.

I should have done a little math. You're right Oscar, this shouldn't be a big deal.

I think you would, let the copper tube be long enough to go at least an inch past the weld joint and then plug the end of the 5/16" tube (that the copper tube goes into) so it doesn't draw air in. You may be able to use aluminum foil as a plug or...if you have access to a lathe turn a small pl...

I need to weld stainless fittings to the end of 5/16" stainless tubing with 0.020" wall thickness. The tubing is formed into a heat exchanger and the length of tubing is around 25 feet. I need a fitting on each end. I know I need to shield the back side of the weld, but don't want to fill ...