welding gas question

[digr]

No way that I can turn my bottles upside down, I can no longer lift them

[Shade Tree Welder]

Originally Posted by digr

No way that I can turn my bottles upside down, I can no longer lift them

Ok, I am going to assume you saw the post from Pakistan, Dubby and I
discussed that member, he is now banned as he posted a link to a site that
proposed selling equipment, but did not have any contact information. We
also think he was using an AI to clean up his language and look very literate
in english. Maybe we banned a legit guy, but oh well. In this case not worth
the risk.

Originally Posted by Lew Hartswick

That what I told them a few post back. Don't bother a "welder" with a scientific explanation. :-) :-)
...lew..

Originally Posted by Rob65

If you want a scientific explanation as to why gases mix and do not separate look up ‘Brownian motion’.

Brownian motion is the physical observed effects of Kinetic Theory or
Statistical Mechanics.

First when we model gases we assume that they are Newtonian in their
behavior; meaning the gasses (molecules or atoms) do not interact with each
other or with their container. These are considered ideal gases and the ideal
gas law applies, PV=nRT.

In welding common gases like Argon and Helium are single atoms in the gas,
other gases are molecules, such as Oxygen (O2), Carbon dioxide (CO2) or
hydrogen (H2)

So think of gases as small balls bouncing around in a container where all
collisions are 100% elastic. The average kinetic energy is going to be the
same every molecule or atom in that mix. And the temperature of the gas
is dependently related to that kinetic energy of the gas. The link below does
a better job explaining the math than I do.

But the key take away is, if you have a mixed gas in a cylinder and the
pressure and temperature are held constant and for sake of argument at
room temperature, 70F or 18C. These gases will never separate, they will
bounce around in the cylinder indefinitely. And therefore cannot separate.

Now if the system changes in state, meaning there is a change of pressure or
temperature or something else that can interact (chemical reaction) then all
bets are off.

So lets say we cool our cylinder a lot, -150°C at that temperature and
pressure, Argon is still a gas, but you will have solid CO2. Any gas at that
point removed from the cylinder will be nearly pure Argon, their will always
be a little CO2, due to solubility. We call it partitioning. But we influenced
the system by massively cooling it. So you can get separation when you
change the state of the system. That is how we purify chemicals.

Here is another way of thinking about it. If you have a container, and you
have a divider in that container that divides it in half and in one half you have
pure Argon, and in the other half you have pure CO2. Also the container is at
a constant temperature and pressure, let's say 70°F and 14.7 psi.

Now the divider is a magical divider and can be removed with out influencing
the gases in any way, other than the barrier is removed. So atoms of Argon
and molecules of CO2 that were heading toward the barrier are no longer
going to bounce off of it and will head into the opposite gas. Eventually they
will bounce of argon, or another CO2 that also moved beyond the barrier or
off the containers wall, since all gases are 100% elastic in their interactions
no energy is lost in the system, and they continuously bounce around forever
mixing.


https://en.wikipedia.org/wiki/Kinetic_theory_of_gases

https://en.wikipedia.org/wiki/Statistical_mechanics

[Shade Tree Welder]

Now back to Clive's OP. Argon has an atomic weight of ~40 AMU and CO2
has an molecular mass of ~44 AMU. So they are very close respectively.

So lets look at my Ar/CO2 example above and change out the Argon for
Hydrogen instead. So half the container, contain Hydrogen gases which
has a molecular mass of ~2 AMU. So a significant difference in mass.
But since the CO2 and Hydrogen are both at 70°F they have the same
kinetic energy, so as they bounce off each other and the container, they
will retain that same kinetic energy and will bounce around forever mixing.

Again to get separation you have to change the state of the system.

[Shade Tree Welder]

Now the other thing that people bring up is trying to explain how CO2 or propane
can build up in a ditch or in a basement etc.

Both propane and CO2 have similar molecular weights, ~44 amu; where as the
average MW for air is ~ 29. Not a huge difference.

But what happens in a system where CO2 or propane can build up to dangerous
levels? How do you explain that... In that case you have a source of either gas
that is leaking or generating the gas in a confined or semi-confined system
where the introduction of the gas is faster than the diffusion of the gases at the
interface with the atmosphere. So the concentration can build up locally as the
system is dynamic. However, if you secure the leak or whatever is generating
the gas and allow enough time the gases will dissipate eventually.

[Lew Hartswick]

Well done Professor Shade. I no longer have the patience to do that.
...lew...

[Shade Tree Welder]

Originally Posted by Lew Hartswick

Well done Professor Shade. I no longer have the patience to do that.
...lew...

I was on a long boring video conf, that should have been an email...

[astronut]

Originally Posted by Shade Tree Welder

I was on a long boring video conf, that should have been an email...

Kind of like this?

[Scotts]

Thanks for posting all that Shade. Very interesting, and I'll have a to read the links a time or 3.

Scott

[Shade Tree Welder]

Originally Posted by astronut

Kind of like this?

I just mute myself and turn my camera off, most of my co-workers do as well.

I did hear someone one time snoring. LOL.

[USMCPOP]

Originally Posted by Shade Tree Welder

...But the key take away is, if you have a mixed gas in a cylinder and the
pressure and temperature are held constant and for sake of argument at
room temperature, 70F or 18C. These gases will never separate, they will
bounce around in the cylinder indefinitely. And therefore cannot separate.

Now if the system changes in state, meaning there is a change of pressure or
temperature or something else that can interact (chemical reaction) then all
bets are off.

So lets say we cool our cylinder a lot, -150°C at that temperature and
pressure, Argon is still a gas, but you will have solid CO2. Any gas at that
point removed from the cylinder will be nearly pure Argon, their will always
be a little CO2, due to solubility. We call it partitioning. But we influenced
the system by massively cooling it. So you can get separation when you
change the state of the system. That is how we purify chemicals.
...

So you're saying I could get rid of my wife if I turn the thermostat way down? Now that I think about it, she disappeared for a couple days one time when we had a 3 day power outage on Christmas eve. Left me and the boys and the cats to chill out in a tent in the living room.