I started building a fermenter from a keg using Harpo's design. Thanks Harpo 
So to get a sanitary weld without purging the whole keg I put this little back gas cup together. Its made from a 1 1/4" copper pipe cap, a piece of 1/4" tubing, and a piece of perforated plate I cut out of an old colander. The inside is stuffed with a copper scrubby. I was able to fish it through the uncut keg opening with a piece of string and with a nut and washer tighten it down over the coupler. So it takes care of purging the back and clamps the coupler in place for welding. It worked so well I would use it for open kegs as well.
My welds don't look anywhere near as good as Harpo's
So to get a sanitary weld without purging the whole keg I put this little back gas cup together. Its made from a 1 1/4" copper pipe cap, a piece of 1/4" tubing, and a piece of perforated plate I cut out of an old colander. The inside is stuffed with a copper scrubby. I was able to fish it through the uncut keg opening with a piece of string and with a nut and washer tighten it down over the coupler. So it takes care of purging the back and clamps the coupler in place for welding. It worked so well I would use it for open kegs as well.
My welds don't look anywhere near as good as Harpo's
