Another purging question

[whateg0]

Now that I've done some TIG welding with my setup, I want to do some more.

So, I am looking now for more of an explanation of why, not how. Maybe...

When purging a tube, for example, why is it necessary to keep the gas moving? Why can the tube not be filled with Ar and then left in such a manner that foreign gases can not find their way in?

Dave

[Flexneck]

Technically one does not really need to have flow through the tube. However, by having a slight positive pressure of argon (or other purging gas) you can ensure that any minor leaks you may have will not effect your weld quality. Once the atmosphere has been displaced from the tube (or backside of the weld) only a slight flow is really required. More flow doesn't hurt the weld, it only consumes more gas.

[Bob the Welder]

Argon is heavier than air and thus will fall through any cracks or other such openings and will be replaced by the atmosphere which contains all the gasses that you are trying to get rid of.

As you weld a joint you will have duct tape wrapped around it that is pealed back as you weld. You need a small but constant source of argon to prevent sugaring if you are welding stainless.

[Pangea]

If you seal that tube up enough to guarantee no leakage, you will get a blow out when you make a puddle.

[B_C]

What he said, (expansion )

[Rocky D]

Bruce said it well...and I would only add that the size of the entry hole should be the same size as the exit hole/s and I normally turn the gas up for the initial purge, then down to a whisper to weld.

[whateg0]

Thanks!!! That's kinda what I suspected. When I did the recent exhaust work for a buddy, I butted the tubes tight and then put masking tape over the end, leaving just a small hole so that gas could flow through. Then I did just as described above, setting the flowmeter to about 25 CFH and letting all of that go through the tubing for a bit, then closing the valve on the "T" so that there was - uh, about - well, less flow going that way than through the torch. Anyway, it worked. But it's good to know why it worked.

Dave

[papasloan]

If you are teeing of a single regulator usa a hose with a valve near the end and one at the regulator. Open the valve at the end and set the flow with the valve at the regulator to 5 cfm. Then when setting the flow at the torch add 5cfm to the flow you would normally set for the torch.
No one mentioned Solar Flux for a backing. I do exhaust where you will never see the back side so Solar Flux works great. Dont think I would use it up stream of a turbo. But all other exhaust work would be ok.
Its not good for everything but any thing that doesnt show is ok. I always make sure I dont have any on the mating surfaces as it will migrate to the top and embed itself at the edges of the weld leaving black spots that are a bear to get out.
I take a cone and section it to make it in to a curve ususlly 1 section at a time. To purge the piece for each segment would use a lot of argon. That makes Solar Flux great in my book.
You dont need to make a new batch each time. I use a small jelly jar and keep a batch mixed all the time. A couple cut off acid brushes live in the jar. Just add a little more alcohol and or Solar Flux from time to time.
Still working on my first can. It cost about the same as a bottle of Argon. And has saved me a lot in argon and time.
Just 1 more option to consider.

[Rocky D]

That's denatured alchy, guys...not Jim Beam!