[BigD]

Hey guys,

I've been doing more stainless lately and I've been experimenting with different approaches to purging, as we all know each situation is pretty much a bespoke solution unless it's the same pipe every time. I know Argon is heavier than air and should build up and displace air bottom to top in the container. But I haven't had any luck purging by depending only on that. For example, a cylindrical container with a cap being welded on the end with good fitup, set it on the cap and flow argon from the top and not sealing the top. Physics seems to dictate that the Argon should go to the bottom and all the air should float on top. But no matter how long (within reason) I let the purge run, the back of the weld didn't look all that great. But take the same idea and wrap the top with foil and after only purging for 10-20 seconds, the back of the weld was nice and shiny silver and straw. I'm purging through a dual flow meter, running about 10CFH for the purge.

I've also noticed on other people's videos on youtube, they will seal the entire container and add vent holes even when the joint is not the highest point in the structure.

Am I doing something wrong with the open purge approach and it SHOULD work or in practice, do you guys also find you need to seal up the container?

Thanks!

[Oscar]

Argon is heavier than air, and will attempt to settle at the bottom of the container, but the problem is that Argon atoms are not bound to each other like a liquid. In gaseous form, argon will eventually start mixing with the air after it's had enough time. It just doesn't "clump" together like a thick heavy molasses because: it is a gas. It has no surface tension or anything else like that which would help keep it "together" like most liquids. Think about it---they get argon from the very air we breathe, so why doesn't all the argon just fall to the ground in our open atmosphere because it is denser than air? Because, you guessed it, it's a gas. Same reason C25 does not 'settle' in your MIG tank. It's always good to go, fully mixed as 75% argon/25% CO2. Now if you feed [the container] enough argon, then it displaces the air in the container, but with an open container you'd have to pump a lot of it to keep it separated from the atmosphere. Hence sealing and purging with vent holes.

So no, you're not doing anything wrong, in fact you did everything right: you used the scientific method to test your hypothesis by running an experiment [that is pertinent to your application] several times with repeatable results and in the end you falsified your hypothesis with a valid conclusion: relying on solely the density of argon gas in our atmosphere does not produce sufficient weld-backing/shielding. Don't worry about the physics part "seeming to dictate". Physics is very tricky with it comes to invisible gases. Did you know the mean-velocity of argon atoms at 32°F and atmospheric pressure is 380 meters/second? That's faster than the speed of sound (345 m/s). In college we used the Boltzman Law for kinetic energy of gases to calculate stuff like this. So trust me, those single argon atoms are racing all over the place and knocking into other air molecules all the time. The average velocity for Helium under the same conditions is about 1200 m/s! This is partly the reason why Helium flow rates need to be very high. Yes it has to do with density as well, but molecular average velocity has a lot to do with how quickly it "wants" to dissipate into our atmosphere.

[Poland308]

Slowing down your flow rate to 2-3 cfh can help some too. But tapping over the sections of a joint is the most reliable method. I use painters tape and just peel back a few inches at a time.

[BigD]

Oscar, as always you blow me away with how much time you're willing to take to explain everything fully, I hope it doesn't get old telling you how much I appreciate it!

Josh, do you not find that you cook the tape? I find unless I go only a little bit at a time and let things cool, I start charring the tape. So I try to use foil as much as I can.

[LtBadd]

depending on how close the weld is to the tape, i do have some high temp tape, but still it won't stand up to the arc, but is better then painters tape...again only if it's going to be in a high temp area otherwise a seal is a seal.

[BigD]

Yeah I use PET tape for holding things together to tack but even then, especially with smaller parts where things are close together, I end up cooking it a bit. At least it comes off clean once it cools unlike other thinks like aluminum hvac tape.

[Poland308]

I just peel it as I go. It burns some but usually works good enough.