Aluminum Intake pipe, TIG welded with He/Ar mix

[jakeru]

So Jake, from what you're saying, I'm having trouble understanding. I would be under the impression that the flow in cfh would be a direct product of the pressure driving it. That would mean, in theory, that the argon pushing, say 15 cfh, meeting up with the He pushing, say 2.5 cfh, would have the argon pushing the He back to the regulator, thereby stopping flow. Now, this is my common sense theory, and, since I consider you as a "mentor" of sorts through your posts on this forum, I don't for a second doubt that you have it working. I'm just having trouble understanding how the pressures interact. when coming from two different sources. I just had a eureka moment to use as a new project. I will be starting a new thread. But in the mean time, please explain how this works without one backing up into the other. (Pictures might help). Thanks Again, Chris

Hey Chris -

Let's start with a basic description of the inert gas system's components in a TIG welding setup. Here is what things are doing when the TIG machine's solenoid is opened and gas is flowing (i.e., when one is in the middle of performing a TIG weld):


Here is what the same system looks like after the solenoid closes. Note that the pressure in the space between the needle valve and solenoid rises to the flowmeter's internally regulated pressure.


When the solenoid next opens, this pressure can suddenly vent out of the TIG torch cup, with a sudden "whoosh" sound. (But usually after a fraction of a second, it calms down and a stable arc can be struck.)

Now, let's say we're going to "Y" in a second flowmeter into the system at the hose entering the TIG machine (between needle valve and solenoid.) If both flowmeters have the same internally regulated pressure (e.g., both are factory pre-set to 30 psi internally regulated pressure), the system pressures when the solenoid is closed are not going to be any different than if there was just one flowmeter in the system. And when the solenoid starts, a flow is going to start back up smoothly and proportionally through each of the two regulators.

However if the regulated pressure of the second flowmeter is higher than the regulated pressure of the first flowmeter (e.g., 50 psi on the second and 30psi on the first), the higher pressure is going to cause a backflow through the first flowmeter's needle valve after the solenoid closes. If the first flowmeter's floating ball gauge is not functional as a check valve (it is likely the ball could impede a reverse flow, while not completely stopping it), there can be a reverse flow through the first flowmeter's floating ball gauge as well. And then, if the regulated pressure of the second flowmeter (e.g., 50psi) exceeds the pressure at which the first flowmeter's pressure relief valve opens (e.g., 40psi), then the first flowmeter's pressure relief valve will open and vent the contents of the second flowmeter's cylinder. (especially if it is left overnight without remembering to shut off the cylinder valves.)

Even if the pressure relief valve (aka "overpressure safety valve") doesn't vent any gas to atmosphere, having unmatched flowmeters is unpreferable to matching flowmeters, for maintaining as consistent as possible of a gas mix as the solenoid goes through periods of remaining open, and periods of movement. Generally, when the solenoid stays open (IE: welding long seams) the gas mix is going to be biased towards the lower pressure gas, while as the solenoid cycles more frequently (IE: doing a sequence of tack welds spaced out far enough to trigger the solenoid to close) the mix is going to be biased towards the higher pressure gas.

Although I have yet to go through a complete tank of He yet, I haven't noticed much problems as long as I have the flow set high enough so the ball of the He flow gauge isn't "bouncing" up and down off its seat. Also, there is probable a certain degree of Ar/He gas mixture variability that can be tolerated when TIG welding without issues.

[Powertig]

Thank you for sharing I had similar concern but now understand that pressure is a completely different measurement than volume. Thank you for explaining how the two interact in a flow meeter. Thank you for sharing the lower He and Ar flow rates, I have had similar experiences and was worried I was just being cheap...

[Jason]

Looks great as usual. I really need to make a trip to the welding supply store to get some new sized cups. Maybe grab a little helium too for my aluminum projects.