On the Welding Tips and Tricks podcast I've heard people mention that the laminar gas flow with a gas lense causes instability to the arc compared to welding with a standard cup. I think Mark Winchester said it first and then Brad brought it up too in his first appearance on the podcast, both really great episodes by the way.
Was just wondering if anyone could give me a more detailed explanation of why this is the case. I use standard cups on aluminium myself because they are robust and cheap and definitely get good results but I can't say I actually understand why the arc should be less stable on AC with a gas lense.
Thanks

This is a great "rabbit hole" to explore.
It is my understanding that on AC, the larger gas coverage from a gas lense will allow the cleaning action to go further than with a standard collet. I believe the preference for standard collets and small cup size is to reduce the etching zone outside of the weld bead.
I've had the pleasure of nerding out with Aren Jenkins about what the "cleaning action" is actually doing.
(please chime in and correct me if i'm wrong)
The electrode positive cycle is forcing previously oxidized chemicals to reduce, or deoxidize by using some of the electrical potential from the arc.(Like recharging a battery) At these extreme temperatures the effect would be short lived if it were not for the inert shielding gas. Though we can only see its effects on the surface, I am told this effect permeates well below. When the weld metal becomes flooded with electrons (during the EP cycle) the metal no longer needs the oxygen to be stable and becomes liberated...and very conductive. The arc now jumps to the electrode through the path of least resistance which happens to be the spot we just deoxidized.

I should probably go look this up because I may be talking out of my butt.
Will return with the correct answer.
Welcome to the club Josef. Great questions!
Brian V

In Lamens terms the arc follows the gas path, the bigger cup the more the arc is gonna tend to wander outwards. Try to size up your weld to the size of your cup. 5 will allow you to safely make a weld bead the same diameter as the cup, need a bigger weld, use a bigger cup. So for like 3/8 material i use a 6std cup (bigger weld). You can make welds on AC with larger gas lense cups, no doubt but why fight a wandering arc if you dont have to. hope that made some kind of sense....best wishes guys.

Thanks for the responses.
Brian that's pretty interesting. I just found a research paper where they looked at AC arcs on aluminium and explored the development of the cleaning area depending on current, frequency and balance. It's pretty heavy reading, here's the link:
https://s3.us-east-1.amazonaws.com/WJ-www.aws.org/supplement/wj-2010-01-s001.pdf
Interestingly, they said that the actual physical mechanism of the cleaning action is still not understood (they wrote this in 2010) and they proposed a couple of theories. I hope I can dig out some more recently written stuff on this topic.
Was also interesting that the cleaned area grows in diameter over time but basically stops when it gets to certain size. I'd bet if they used different size cups they'd get different final sizes of the cleaned zone, just like Brad says and showed in his video. Unfortunately in the paper they described most of the settings they used but didn't mention cup size.
Brad, do you think there's any significant different between using a standard vs gas lense of the same size on AC? So for example if I'm getting good results with a standard 5, is there going to be any noticeable difference when using a #5 gas lense? In terms of arc stability, cleaning action, penetration etc. Thanks and hope you're doing well after having covid and shingles.

There is a difference between a #5 Gas lense and a #5 collet. The Laminar Flow will fight the alternating Flow of Electrons and will leave wider etching which means less Penetration.

The biggest Argument for my Students and Customers:

A lense is expensive when you play Tungsten Diver and blast molten Aluminium on it. A Standard collet body is cheap..

Das ist ja super das meine Videos dir geholfen haben, vielen Dank für das Kompliment :)

To anwers for everyone:

This are just my 2 Cents…
When the Machine cycles Electrode positive the electrones (Energy) go from the workpiece into the Tungsten. This is also why you find Oxide and Stuff like that on your Tungsten after some time welding.

So Energy (Electrons) and dirty stuff is flying towards your Tungsten and a laminar Flow of Gas Runs against it.

A standard collet already provides a turbulenced Gas Flow which helps the Strange process of AC.

Rush Kane is a well known Professional who uses gas lenses but he also uses active Gas Mixtures.

Now just think of the turbulences caused by a standard collet Body just like adding some very very little oxygen from the air into the process

I Took this discussion on my Instagram page

I had an interesting experience relating to this a couple of days ago. In a bicycle workshop where I volunteer we have a cheap made in China TIG welder. It does AC but the arc extinguishes constantly on aluminium, like every 3 to 10 seconds. I think the arc is rectifying and the machine just doesn't have the right electronic components to prevent it.
Anyway what I realised is that the arc extinguishes way less frequently when using a gas lense! No idea why but it made a big, big difference. So I can say I'm typically a #5 standard guy but when I'm using a crap machine I'm a fan of the gas lense ;-)

Josef, i may get torched for sying this but i honestly have found that the gas lense is a little more forgiving and easier to dial in, but once you learn to dial in a 5std cup you wont ever look back. I really think new welders should start with a gas lense personally.

I thought I might point something out. All of the moving parts, electrons, ions, air, shielding gas, etc., all have mass. With that in mind, it stands to reason that they can influence, and even disrupt each other as they flow and travel. I hadn't considered that much before. Since they each have mass, and different mass relative to one another, they also have inertia. Maybe that can add something to the picture of what's going on.

Just wanted to share something I realised yesterday whilst experimenting at work. Had a lot of parts to do so I was playing around with different tungstens, cups, gas flow etc. These were short autogenous outside corner welds on sheet aluminium.
The width of the etching line appears to be dependent on gas flow rate. Perhaps this is stating the obvious for some but I really had no idea. I have always been playing with balance and cup size to try to control the etch width. I have been playing with gas to try to get welds shinier, reduce peppering, help arc stability (not too much gas) and as Jody has said several times too much gas flow can chill the puddle and reduce penetration.
What I hadn't thought is that if you reduce the gas flow rate, with the same size cup, you get a smaller etching line. I was welding these outside corners with a #4 gas lense (sorry not sorry!!!) and turned my flow down to about 6 CFH. The etching line came right down to the toes of the welds and was really consistent. Gas flow back up and the etching flared back out.
Have any of you observed this before?
By the way Brad I definitely agree that a gas lense is more forgiving than a standard cup. I think it's to do with stickout and visibility. Guys like you and Mark Winchester have such minimal stickout and short arc length that you can often only see the leading edge of the puddle and everything else (the tungsten, most of the arc) is hidden by the cup. I think this requires a lot of hood time to be able to trust that the leading edge of the puddle is giving you all the information you need and to be able to read it. Beginners seem to want to be able to see everything and I think the gas lense allows that.
Cheers!