Appreciate all the responses. Always like to learn.
I was looking at this on the Physics Stack Exchange. Lots of back and forth on the matter.
Gas molecules in air are moving very fast, like average speed of 500 meters/sec (more than the speed of sound). The molecules are tiny (angstroms) and far apart of average compared to their size but there are an incredible number of them in a liter of air (10**22). So, they don’t move very far before they undergo a collision, like 1000 angstroms on average. So, individual molecules are rapidly undergoing collisions.
Those collisions are what cause gases to set up what we measure as temperature (related to average kinetic energy) and to thermalize if we mix gases of different temperatures. They also mediate the movement of different components of a gas as they diffuse through each other and set up a constant mixture if they were initially separated.
Gases take some time to mix because they undergo an incredible number of random collisions during the process of mixing.
If you could remove the Earth’s atmosphere and shoot a single oxygen molecule straight up with the 500 m/sec velocity mentioned above, it would travel something like 10 km before it stopped and started to fall. This is why the molecular weight of different gases doesn’t have much affect on the way they mix over distances we care about (even ignoring all the convective effects).
-Al
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No need to be sorry Alan. I’ve never utilized Argon as a protective layer in anything other than an enclosed and protected tank. I thought was implied, but makes sense that it might not be taken that way. It is why I mentioned that any slight turbulence would challenge the gas layer as well (including adding more Argon or CO2 to the tank).
There is little doubt that even in more open spaces CO2 can displace significant amounts of oxygen, as during harvest in many wineries, if tye doors aren’t open, the amount of CO2 produced can be dangerous to small dogs while adult humans are unaffected (I’ve seen this). It doesn’t mean there’s no oxygen lower down, just not as much as the dogs need.
CO2 released from fermentation should be at very close to the temp of the ferment, even if the gas coming out of solution might lower the temp slightly. It still layers to a dangerous level fairly quickly.
When you have an active source of CO2, it does fall initially. There are demonstrations to make soap bubbles that are mostly filled with CO2, and they fall. A blob of CO2 molecules acts similarly until it starts mixing which takes surprisingly long. In an winery with active ferments, there is a steady source of CO2 dropping down where it can affect small dogs. If one could shut off the source (ferments) it would eventually mix, but the dog needs to breath more frequently than that process allows without some active ventilation.
-Al
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Yes, as Al said (and I may not have made clear), a source of high concentration CO2 (or any gas) can displace an equivalent volume of surrounding air without much mixing. In which case it will act just like a volume of cooler air, and sink because of its higher density. Between convection and diffusion, it will mix with the surrounding air quickly enough (minutes to many minutes, depending on the volume in question), but in that time it would definitely be enough to incapacitate someone.
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The temp factor is what initially made me question Alan’s first reply. I read it to be that the main reason CO2 was falling was because it was colder or coming from a colder container. I’ve mostly known it from breweries coming out of secondary fermentation when beer is moved out of the tanks. Tank temps are cooler than primary but pretty close to the temp of the brewery air. No doubt it falls to the ground at that point.
Sorry if my explanation was unclear. I was focusing on the idea (which I think some people do have) that the heavier components of a gas will sink, and the lighter comonents will rise. That’s not the case, as over time everything will mix uniformly (speaking of normal conditions in a room, tank, or bottle).
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No problem. I appreciate you taking the time to explain.
I’m mostly of the belief that given a long enough timeline everything will truly mix. It’s all one 
I got to say, this turned out to be much more complex and interesting thread than I imagined! Thanks everyone.
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Yes, I wasn’t aware of the large effect dissolved CO2 has on oxygen transfer.
-Al
Alas, I can confirm this.
Well, we have had some discussions where words I chose to make my argument came across as overly strong. I’m trying to avoid that, while still giving my interpretation of how I would recommend thinking about this little corner of science 
You can see that there is a lot of nuance, and even I have to back up and consider that certain conditions can make things confusing lol.
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As a bystander, I am very thankful for the extra nuances & thoroughness to this thread. And being mindful of each other 
.
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All well and good but what was the question/issue?
There’s a famous case of this from my former field of geology: the Lake Nyos disaster of 1986 in Cameroon. Sudden overturning of the lake released a couple hundred thousand tons of (mostly) carbon dioxide that flowed down the flanks of the volcano, killing almost 2,000 people and even more animals. Horrifying stuff.
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I’ve read about the Lake Nyos disaster, another similar event in a lake nearby.
-Al
I’m with the others in appreciating you taking the time to explain the science more thoroughly.
Yeah, Lake Monoun in ‘84. The only two in recorded history. Freaky.