[Richard]

There was a recent thread about purging of kegs where the stratification of CO2 in kegs was questioned. One post says that we would all be dead if CO2 sank down and formed a blanket below oxygen. Here is a link to an article in Science magazine which describes exactly that in a geologically active region in ancient Rome.

http://www.sciencemag.org/news/2018/02/roman-gate-hell-killed-its-victims-cloud-deadly-carbon-dioxide

Here is an excerpt:
 Pfanz and his colleagues measured the CO2 concentration in the arena over time. During the day, the sun’s warmth dissipates the gas. But at night the gas—slightly heavier than air—billows out and forms a CO2 “lake” on the sheltered arena floor. It is particularly deadly at dawn, when the CO2 concentration 40 centimeters above the arena floor reaches 35%, enough to asphyxiate and kill animals or even people within a few minutes, Pfanz says. But concentrations fall rapidly with height.

CO2 stratification is real and can be deadly (or useful, if you are careful).

[Robert]

The problem with these types of "evidence" (and my quotes aren't suggesting this isn't real) has been touched on in these threads.  CO2 doesn't stratify perfectly, resulting in discrete layers of pure gases. (Those pesky Dalton and Henry fellows again.)  In purging a keg we want the gas in the keg, or at the interface with the beer, to be 100% CO2 and 0% O2.   In the situations variously cited involving burping lakes or other natural phenomena,  the CO2 concentration only has to rise above 3% to be lethal.  That may be in the neighborhood of the best you can get with stratification-based keg-"purging" strategies for all I know, though I don't have actual figures; suffice it to say, it is a concentration below 100%.  So yes, stratification is real, but a red herring with respect to the topic of keg purging.

[The Beerery]

The problem with these types of "evidence" (and my quotes aren't suggesting this isn't real) has been touched on in these threads.  CO2 doesn't stratify perfectly, resulting in discrete layers of pure gases. (Those pesky Dalton and Henry fellows again.)  In purging a keg we want the gas in the keg, or at the interface with the beer, to be 100% CO2 and 0% O2.   In the situations variously cited involving burping lakes or other natural phenomena,  the CO2 concentration only has to rise above 3% to be lethal.  That may be in the neighborhood of the best you can get with stratification-based keg-"purging" strategies for all I know, though I don't have actual figures; suffice it to say, it is a concentration below 100%.  So yes, stratification is real, but a red herring with respect to the topic of keg purging.

Amen!

[klickitat jim]

I've heard that CO2 can pool in derelict oil tankers too. So maybe one could get one of those pretty cheap to use instead of urging by pushing sanitizer with CO2.

[Richard]

So yes, stratification is real, but a red herring with respect to the topic of keg purging.

I don't think it is completely a red herring. The equations that say it takes 30+ purging cycles seem to be assuming complete mixing of the gases in some equilibrium state. The truth is somewhere in between, so it takes more than 1 but less than 30 purging cycles. Stratification definitely helps, but it isn't a magic solution (except for the Romans).

[tommymorris]

No one has addressed how long it takes CO2 to stratify. It is not instantaneous. When you fill a keg with CO2 from a bottle, the CO2 entering the keg is like a raging river. It should mix thoroughly with any gas already in the keg. My guess is stratification will take a relatively long time, minutes or even hours. When I purge a keg, I don’t wait that long before I fill it with beer.

So, I don’t think you can fill a keg with CO2 then immediately push beer in and expect the CO2 to have stratified such that all other gasses are pushed out when filling with beer.

[Robert]

But the gas laws tell us that over that same time stratified gases will mix.  It's a no win situation, you will never get discrete strata of pure gases.  Just let go of the homebrew myth, and blow your sani out of the keg.

[hopfenundmalz]

Your lungs work when the CO2 comes out of the blood to the gas in the lungs. Don’t confuse some lethal level of CO2 with what is O2 free. A 10% concentration will kill you in about 2 minutes. That is roughly 19% O2, 71% N2 and 10% CO2.

I will srick by my statement that if CO2 would sink out of the atmosphere we would all be dead. Local sources, like Lake Nyos or Richard’s example are exceptions, as the concentration can be higher than the lethal level.

On a side note. I have seen some videos of brewers going into fermentation cellars with a candle (a few from PU). I realized this little candle doesn’t light the way, if it goes out, turn around and get your butt out of there.