[AndersK]

I'm about to weld the top of a SS tank, 16-17 gauge or 1,5 mm.

Would filling it with water to 1/2" below top level be stupid or a good idea?
Im thinking about possible steam might screw it up.

I want to do it to save gas and keep it as cool as possible to reduce distortion.

[Braehill]

Anders,
You can get a cylinder of Nitrogen for a lot cheaper than Argon and purge it with that. Is the opening large enough to get Aluminum HVAC tape in there as a backer? How large is the tank? Is it 100% necessary for it to be back purged? What's going to reside in the tank?

I think you'll be disappointed with the results if you try the water route, the water isn't a solid enough barrier to keep the gas from entering the liquid. You're introducing the 2 things that you most want to keep out if they get hot enough to disassociate, Hydrogen and Oxygen. That's just the way I see it working out.

Len

[AndersK]

Thanks Len
Its for fuel so its not absolutely necessary.
About 45 liters and just a 50 mm hole for the fuel sensor so tape is not good here.