Dumb question about CO2

[Sean W.]

OK, this is a really dumb question, but still...

Let's say you're gassing your brite tank with CO2 before transferring fermented beer into it. If you were to charge it up with a good deal of CO2 the night before, but not fully purge it of all oxygen, what happens? Does the CO2 settle to the bottom overnight, leaving most of the O2 on top, or do they mix to a fairly homogenous distribution?

For the purpose of practicality, this is on a 7 bbl. pub system with brite tanks in the cold room at about 35 degrees.

[dfalken]

CO2 is heavier than O2 and will settle below it. - EDIT: actually it won't layer for very long, thanks others for clarifying this.

[gitchegumee]

I respectfully disagree. Over time the two gases will form a homogenous mix. IMHO, you should always purge all oxygen from a tank before you ever put beer in it.

[KWLSD]

The answer, I suspect, is somewhere between the two already given

CO2 is heavier so will tend to settle, as it would if you simply release CO2 into a cellar (for example, which is why it is always advisable to site your CO2 alarm close to the floor).

Part of the reason for that is because air is ~80% nitrogen and there is a big difference in molecular weight (MW of CO2 = 44, MW of N2 = 28).

Oxygen (as O2) is slightly heavier than N2 (MW of O2 = 32) so is still lighter than CO2, but if you have gassed-up the tank correctly there should be very little O2 left, so the way gas molecules move about will mean there won't be a distinct O2 'layer' as such.

However I quite agree that you shouldn't look to put beer into a tank that hasn't been purged as well as it practicably possible.

[kai]

That's not a stupid question, it's an exceptionally good one.

If you partially purge a tank and leave it overnight, by morning the CO2 will have distributed all the way through the tank and you will not have a CO2 blanket of any description.

Think about it this way; CO2, O2 and N3 don't settle into layers in the air we breathe. I know we have wind and pressure cells and so on, but the innards of your BBT have temperature fluctuations and Brownian motion and all that other stuff I never learned in physics too.

If you partially purge a tank and immediately fill it, you *may* keep O2 out. That depends on a few factors, like how gently you conduct your CO2 purge, and how much headspace is left in the tank post-filling. So even if you think a tank is purged enough, it may not be...

[schmogger]

Another thing to think about is that when you do transfer the beer from the fermenter to the brite tank, I assume that you have the head spaces connected together, that there will be a layer of CO2 blanketing the beer as it is filling the brite tank. So as the brite fills the O2 left in the brite tank is then being pushed to the fermenter. I know that the gasses will blend to a certain extent but you will get the majority out and plus most likely you will be carbonating the beer once the transfer is complete.

[Sean W.]

Thanks for the thoughtful replies, folks. I'll take these thoughts into consideration! We don't link our headspaces from the brite back to the fermenter though.

[KWLSD]

Think about it this way; CO2, O2 and N3 don't settle into layers in the air we breathe.

CO2 does layer and very significantly, which is (as I said above) why it is important to fit CO2 alarms close to floor level in cellars or other places where high concentrations can occur.

It's also the reason why climbing into a vessel when it's empty isn't good idea, as any CO2 present will be at the bottom.

The only reason it doesn't do this noticeably in air is that there is a such a small percentage (~0.04%) and the inherent movement of molecules keeps it reasonably well distributed.

If you really want to know whether you've fully purged your tanks, the answer (or one of them, at least!) is to use an oxygen meter to measure the level of O2.

[gitchegumee]

Right. All good points. CO2 will layer for a while. Not long. Random gas motion will distribute the gas over time. Yes, I know CO2 is heavier. So is oxygen heavier than nitrogen in air. They don't layer. The atmospheric O2 isn't settling on the ground. And neither does the CO2 in our atmosphere. For example I have used nitrogen to "blanket" my BBT while we package. Only to keep the pressure constant for packaging purposes--NOT to keep CO2 in solution. That doesn't work. If I leave it "blanketed" over night, my beer is lifeless with nearly no carbonation. Dalton's law of partial pressures states that gases act independently. No matter the pressure of nitrogen on my BBT, it will lose carbonation if I start removing beer to package. Not quickly, but certainly. Oxygen is one of the worst enemies of beer. Keep it away as best you can. Purge your tanks completely before introducing beer. In my current brewery, I fill with clean water and push out with CO2. And then leave the serving tanks with CO2 pretty much always. I don't open them. I clean and sanitize under CO2 pressure. Keep my CO2. Everyone has their favorite ways of doing things. I'm just trying to save money, make work easier, and produce the best beer I can.

[KWLSD]

For example I have used nitrogen to "blanket" my BBT while we package. Only to keep the pressure constant for packaging purposes--NOT to keep CO2 in solution. That doesn't work. If I leave it "blanketed" over night, my beer is lifeless with nearly no carbonation. Dalton's law of partial pressures states that gases act independently. No matter the pressure of nitrogen on my BBT, it will lose carbonation if I start removing beer to package.

I wish more people understood that! The number of times I've heard it said with great authority that putting N2 top pressure on a tank has no effect on the CO2 dissolved in the beer...

Not only will it lose CO2 (until equilibrium is reached) but it will also gain N2.

And if the top pressure is on a constant N2 feed then the longer it's left, the more N2 will become absorbed.

Anyway this is straying off topic somewhat, so I'll shut up

[kai]

CO2 does layer and very significantly, which is (as I said above) why it is important to fit CO2 alarms close to floor level in cellars or other places where high concentrations can occur.

It's also the reason why climbing into a vessel when it's empty isn't good idea, as any CO2 present will be at the bottom.

The only reason it doesn't do this noticeably in air is that there is a such a small percentage (~0.04%) and the inherent movement of molecules keeps it reasonably well distributed.

If you really want to know whether you've fully purged your tanks, the answer (or one of them, at least!) is to use an oxygen meter to measure the level of O2.

Sorry, clearly I wasn't being clear enough. What I meant to write was that yes, you can form a blanket in your bright tank for a partial purge, but if you purge a tank the night before and fill it the next day, then don't expect that CO2 blanket to be there. And if you partially purge a tank and then fill it straight away, then there may well still be air in the headspace too. And if you leave that beer in that tank with that CO2 blanket, the blanket will disappear and the O2 will go into the beer.

That's what I was saying.

I also meant to type N2.

And in addition, even if you have a DO meter, don't rely on the readings you get from the sample port! There's most of a whole tank above that to consider too.

[Fullcourt]

I want to thank gitchegumee and emphasize that every brewer should be familiar and comfortable with Dalton's Law of Partial Pressures.
Gas distribution plays such an important role in the making of beer.
Way to go gitchegumee !

[Natrat]

Charles' Law and Boyle's Law are useful, too. Even Hooke's Law (a little more esoteric), Henry's Law, and Gay-Lussac's Law apply. Avogadro and Graham, anyone?

So much science with gas pressure and fluidics! Fun!

[el_mocoso]

The law I deal with mostly at the brewery is Murphy's, Second most is Blue.

[KWLSD]

Bernouilli...or the gases might not even get there