Wrench Tips #26:
Back Purge Your TIG Welds The Cheap Way
by Dave Coleman
When you're welding tubing, the backside of your weld (inside the tube) is still exposed to oxygen and oxidation reactions can weaken your weld. Even when strength isn't a big concern, with some materials (like stainless) the oxidation will build up a large, hard lump on the inside of the pipe, which can actually be a real issue for flow if you're welding something relatively small and flow-sensitive like a header primary.
The standard solution is some combination of Solar Flux (a paste that creates an inert shielding gas when heated by the welder) and/or back purging.
Back purging is simply using a second bottle of inert gas to flood the inside of the tube and flush out the oxygen. Kojima showed some hot, back purging action a few weeks ago, but Tim Taylor showed us some back purging tricks that let you use less of your expensive shielding gas while he was welding up some of the Miatabusa's exhaust.
The trick is simple: use a cheap rubber glove to seal the ends of the pipe. This lets you easily visualize the gas pressure inside the pipe, so you can adjust the flow rate on your regulator to keep the glove barely inflated. This is just enough flow that you can be confident no oxygen is leaking into the pipe. Keep the purge bottle nearby so you can easily adjust the flow rate as your welding seals up leaks in the pipe.
Also, use blue painters tape to seal unwelded joints and to seal around the gloves. Other tapes are more likely to leave adhesive residue when exposed to heat. Get it hot enough, and painters tape will leave residue too, so don't weld too close to it.
Feeding the gas into the pipe is as simple as poking a hole in the glove, slipping the gas line in, and taping it up.
Since the glove inflation indicates pressure inside the pipe, you can often run very low flow rates, saving you from wasted purge gas.
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1 comment
I’m putting together a small stainless exhaust at the moment, and this is probably the first back-purging method I’ve seen that doesn’t require buying another pile of equipment. Using the glove as a visible pressure indicator is such a simple idea. I’ve always just listened for the gas and hoped the flow was right, which obviously isn’t very scientific.
Your point about keeping the glove only slightly inflated made me read more about gas flow and turbulence. I found this explanation of root-shielding gases and why simply increasing the flow can sometimes pull oxygen back into the purge area: https://www.formiersystem.com/en/root-protection-gases/ That leaves me wondering how sensitive the glove method is. If it stays gently inflated, is that normally enough to confirm a useful flow, or could the gas inside still be turbulent enough to contaminate the root?