Welding Tips and Tricks

I am new to tig welding and have noticed something with my shielding gas. I have been trying to use every last drop of argon so even when my regulator shows empty I continue to weld as long as my flow meter shows the correct flow. What seems to be happening is that even though I show the proper flow I find that I am obviously loosing my shielding effect and have to change the bottle. This has happened with the last few bottles. It isn't really a big deal for me especially since I am only practicing right now and realize that if I had a critical weld to do I would make sure I was showing pressure in my tank. All the same I am curious to know what is going on. Is there other gasses in there? I understand argon is heavy which makes it even more confusing. Wouldn't argon be the last thing to leave the bottle? By the way I tried searching the forum on this subject but couldn't find anything.

With tig you definitely notice the end of cylinder. Its just how it goes. I don't think theres much you can do about it, other than cranking up the regulator/ flow metre pressure.

You should't ever let your gas bottle to get fully empty. Otherwise they may have problems at filling station when refilling the bottle. So using bottle to fully empty and risking the weld itself serves absolutely no purpose. That last remaining gas won't save you that many dollars.

In atmosphere there is about 20,9% oxygen, 78% nitrogen, 1% argon and litte bit of other gases too. Gases are always trying to get in to balance and this can happen even trough a steel and easily trough rubber gaskets etc.
So that in mind we want to have argon inside the gas bottle, so humidity won't get that easily inside the bottle. It will in time, but argon or other gases (pressure) will slow it down a bit.

Markus wrote:You should't ever let your gas bottle to get fully empty. Otherwise they may have problems at filling station when refilling the bottle. So using bottle to fully empty and risking the weld itself serves absolutely no purpose. That last remaining gas won't save you that many dollars.

unless they just swap out the bottle like they do here

doneinone,
Your regulator has a set pressure of usually around 50# and this is independent of the flow portion. Once the bottle is down to or just slightly above this pressure the flow will start to decrease and you will have to open the flow meter up more.

As to the part about using the contents down to the last ounce, everything that has been mentioned is true. It's been awhile since this has come up in a thread so I'll give a crash course in what's involved in filling a cylinder the proper way and why.

If you have a cylinder that's under 125 SCF you may or may not own that cylinder. If you take your cylinder in and wait for it to be filled and leave with your owned cylinder then they just pressure transferred from a bank of larger cylinders. If that's the case then you definitely don't want to use it down to nothing because then all the things that Markus spoke of come into play. If you exchange your cylinder for a full one, use it down as far as you want because that cylinder is going to be treated in a different manner. Cylinders that are exchanged are connected to a manifold with as many as 100 cylinders at a time and all drained and evacuated with a vacuum pump. Then they will be purged (sometimes with Nitrogen because it's cheaper and a great moisture carrier) and evacuated up to three times to remove all the moisture.
That cylinder is then filled and leak checked.

I'll add one other thing to this about the gas in a cylinder. Gas is analyzed for moisture in parts per million and if .01 parts per million applies to a full cylinder there are many parts as compared to a empty cylinder. A cylinder increases in temperature as it's filled and then returns to ambient temperature after it sets and it's moisture content condenses and drops through the gas (distillation). As you use the gas the temperature in the cylinder drops even more so the moisture doesn't always leave with the gas that's being used. There's a pressure/temperature relationship with all gases.

Len

Thanks for the replies. It all makes sense. I have a couple of owner bottles and I swap them out when they are empty. My supplier is just a depot I don't think they fill anything there. When I see the needle on empty I will just call it done to avoid any problems.