Keg diptube-thermowell conversion: seeking tips on soldering stainless steel

[IanMC]

Hi all,

I'm about to do my first stainless steel solder, and am seeking some advice. I'm very familiar with soldering electrical connections, but this will be my first plumbing application.

I'm trying to seal up a corny keg liquid dip tube in order to use it as a thermowell for keg fermenting. I've got a straight 21" dip tube (https://www.homebrewing.org/21-Straight-5-gallon-Soda-Keg-Liquid-Dip-Tube-No-Ears-Used-_p_6477.html) that I plan to seal up with a 1/4" stainless steel barbed plug (https://growmasters.com/products/ss-plug-1-4-barb), and then solder in place to seal up any gaps.

My plan is to do the following:
1) Scuff up the barb and dip tube interior with some light sand paper
2) Clean both surfaces with Barkeepers Friend and isopropyl alcohol
3) Dip the barb in flux; coat the dip tube interior in flux. I will be using Harris Stay Clean.
4) Push the barb into the bottom of the tube. May need a hammer to get it in fully.
5) Heat it with a propane torch and apply solder once at temp. I will be using some kind of silver solder. Hoping to get Harris Stay Brite 8 if I can find it reasonably cheap.

I've considered cutting small pieces (like 1mm in length) of solder and dropping them into the dip tube to rest atop the barb, so they would melt and seal from the inside also. I know this is probably overkill, but don't know if this may cause unexpected issues. Any thoughts or suggestions? Do I need to leave some kind of a lip at the bottom of the tube so that the solder can wick inside, or should I just pound the barb fully in such that the bottom is flush with the tube?

Thanks!

 

[Dr_Jeff]

I'd pound the barb in along with some flux, drop the pieces of solder inside and heat it with a torch.

When I made a couple of thermowells, I just pounded the end flat with a bit of flux inside, dropped in several pieces of solder, then heated it until it was glowing red. I saw a bit of solder drip through one. I let them cool for a couple of minutes then put them into a bucket of water to quench them to finish cooling. I eventually took a file and filed off the end where I had pounded it closed and continued to work it to the shape I wanted, finishing it up with a fine file, and then a fine sharpening stone, until it was pretty much polished. I likely went a bit overkill and there is, at best guess 1/2 inch of solder in the tip of the tube, possibly more.

What you propose sounds like a solid plan.

 

[IanMC]

By crazy coincidence, I was actually working on this at the exact time you posted your advice, and so I didn't see it until afterwards. Still, I ended up doing basically what you said.

I started out by filing down the barb significantly so that the head would fit into the keg post, then cleaning both it and the bottom/inside of the diptube with PBW, BKF, and then 91% isopropanol. I covered the barb in flux, sprayed some flux into the tube, and then pounded the barb into the tube with a hammer, which required significant effort. I probably should have filed down the diameter of the barb also, as it ended up bulging the tube outward slightly. This made it too big to fit into the post, so then I had to sand the tube OD down a bit also. Unfortunately, all this hammering and sanding dirtied up the whole thing a lot, so I had to clean it again, which was not perfect. Then, I heated it with a propane torch until the solder would melt on contact, but it did not wick into the crevice or stick to the metal at all. I tried to leave a small gap between the end of the tube and the head of the barb, thinking that this would allow the solder to wick in, but no luck.

Then, I inverted the tube, dropped some flux inside and some small cut up pieces of solder, and heated it again. I assume that it got hot enough for the solder to melt, because when I inverted it again, nothing fell out.

I'm not really happy with the way this went, as I have no real way to know if the solder did anything or was enough. I can't see inside the tube. However, so far it seems to be gas tight. It's been holding 40 PSI pressure in a keg for the last hour with no leaks. For all I know, the solder didn't have any effect, and instead the barb just swaged the tube and is forming a metal-metal seal itself.

Even though it seems to be technically working, I'd really like to fill in that gap with some solder in order to help keep it sanitary. This will be used in a fermenter, so there will be plenty of opportunities for infection if it harbors microbes.

I'm looking for thoughts or advice here. I never seem to have luck with soldering things for homebrew purposes... Once I tried to solder some brass fittings onto my copper wort chiller, and the exact same thing happened there.
Do you need to leave a lot more room for the solder to wick in? Perhaps the fittings were just too dirty? Not enough flux?

Any tips are appreciated. Thanks!

 

[IanMC]

Damn.... it just started leaking. Guess I definitely do need to figure out this soldering thing.

 

[IanMC]

So I cleaned it up as best I could. Wire wheel, Barkeepers Friend. Tried to solder it again with extra flux this time. Nope... by the time the metal gets hot enough for the solder to melt, the flux is all burnt off, and the surface won't wet. I tried resting some flux atop the joint and just letting it melt in on it's own - it just formed a blob on top, and didn't wick into the gap.

Finally, I just ground down the rest of the protruding bit so that it was all flush, which looked fairly promising actually, then just to be safe I dumped a bunch of flux and solder into the tube and heated it to oblivion. No solder dripped out, so I assumed it was all melted. There was more than enough to span the width of the tube... probably way too much. It all looked like this:

Stuck it in the keg - started leaking immediately.

Well, at this point I'm demoralized and have wasted way too time and much good/expensive solder and flux on such a small and insignificant job... Time to have a beer and do something else

Paging all silver solder masters (@Bobby_M for one), any ideas? Thanks!

 

[Gruel]

You need really aggressive flux to solder stainless, which then could cause corrosion later if not cleaned off properly. At work we have been using La-Co N-3, but not for long term or inaccessible connections. (We laser weld. Some jewelers might have the equipment for small laser welds.)
Palmer has some advice on soldering, brazing and welding stainless: https://www.realbeer.com/jjpalmer/Welding.txt

How about sealing what you have from the inside with epoxy or polyurethane, maybe with some pressure behind it to force it into all leaks?

 

[etk29321]

You want this https://www.brewhardware.com/product_p/silversolder.htm

Works great to solder stainless