Questions on beer gas (Nitrogen/CO2 mix) in a new keg

[J2W2]

Hi,

I've been running a beer gas line and stout tap for years, and never really thought about what happens to beer gas in a new keg of beer.

I just kegged a new stout and put it on beer gas last week. I recently purchased some Plaato keg scales, and I put one under my beer gas tank. I don't know what the tank weighs full or empty since I've had it a while, so I just plugged in some numbers in the Plaato app so the tank read around half full.

When I first set up the tank, Plaato said I had 2.3 pounds of gas in the tank. After several days, that had dropped to 2.2 pounds, and now, after eight days, it reads 2.16 pounds. Depending on how accurate the scale is, I could be down a little over 2.5 ounces of beer gas in eight days. Is that acceptable for a new keg, or do you think I have a leak? I should also mention I purged the keg several times after I installed it, and I did not recheck the keg weight as I probably should, so I'm sure the starting weight was less than 2.3 pounds due to lost gas in the purges.

As I said, I've never really pondered the reaction of beer gas and beer. It's my understanding that the CO2 will be dissolved into solution, but the Nitrogen will not. So it seems like a new keg, with limited headspace, would absorb the available CO2, causing a little more gas to enter the keg, but eventually the headspace would be all Nitrogen. That also makes me believe there would be more CO2 available as the beer level drops, causing the remaining beer to have a higher level of CO2 than the beer in the full keg. Does that make sense?

Finally, I use the "set it and forget it" method of carbonation, and I follow the same process with my beers on beer gas. So does it really take a week or two before beer gas beers are ready to serve?

Thanks for your input!

 

[Vale71]

Depending on how small your headspace and how much you would like to increase your beer's carbonation it could actually take forever.

The problem with carbing with beer gas is that the regulator is only pushing a CO2/N2 mixture at a fixed rate. As CO2 is absorbed the actual N2 content of the headspace increases and the partial pressure of CO2 keeps decreasing until equilibrium is reached. Unfortunately equilibrium will be reached at (possibly much) lower carb level than what you were aiming for. Since this is a stable configuration carbonation will never increase until you start serving the beer and more CO2 can be pushed into the headspace, starting the process again.

Worst case your beer might reach the desired carbonation level long after the keg has actually kicked...

 

[J2W2]

Depending on how small your headspace and how much you would like to increase your beer's carbonation it could actually take forever.

I have a 10 pound CO2 tank on my five-tap keezer, and a 5 pound CO2 tank from when I started with a two-tap kegerator. I'll usually start carbing a new keg on the smaller tank, before moving it to the keezer, to watch for leaks and to spread out my cleanup somewhat. In the past, I had put my stout kegs on CO2 at low psi for a week before moving them to the keezer. For whatever reason, I got it in my head that that was a bad idea (messed up the beer gas "ratio" or something), so this time I just put it on beer gas right off the bat.

Long story short, I poured a pint last night, after nine days on beer gas, and it was flat as could be - no cascade whatsoever. I still drank it; it tasted good enough I didn't want to waste it. And now I'm opening the keezer back up tonight, releasing the nitro out of the keg, and hooking it up to 5 psi of CO2 for a week or so. I guess the good news is I now have a little more headspace in the keg!

 

[odie]

interesting. I have a RIS on nitro. the keg was carbed at very low psi and set in storage for several months. I think it came out of the 60' cellar at 5 psi. When I finally tapped it with nitro at 25psi it poured pretty good. nice head/foam...but lately the pours are rather flat...like an old English hand pump...

not sure what to do so I switch back to CO2 and burst the keg headspace with about 30-40 psi and then sealed it. After a few days for the CO2 to absorb it went back on nitro. Haven't had a chance to sample it yet...at 14% it's an occasional sipper.

 

[J2W2]

interesting. I have a RIS on nitro. the keg was carbed at very low psi and set in storage for several months. I think it came out of the 60' cellar at 5 psi. When I finally tapped it with nitro at 25psi it poured pretty good. nice head/foam...but lately the pours are rather flat...like an old English hand pump...

not sure what to do so I switch back to CO2 and burst the keg headspace with about 30-40 psi and then sealed it. After a few days for the CO2 to absorb it went back on nitro. Haven't had a chance to sample it yet...at 14% it's an occasional sipper.

Are you serving it on pure nitro or beer gas? I'm no expert, since I can't even get mine right , but I wonder if pure nitro somehow forces the absorbed CO2 out over time - maybe forcing it more into the beer at the bottom of the keg (first served) or something? Again, this is purely a guess on my part.

 

[odie]

pure nitro would just serve flat beer. I think mine is 70/30. pretty sure when someone says "nitro" they are implying a beer gas mix. Does anyone carb on CO2 and serve on pure nitro? Is that even possible? Or would that actually be preferable?

Sorry, I have more questions than answers :O

 

[Vale71]

pure nitro would just serve flat beer. I think mine is 70/30. pretty sure when someone says "nitro" they are implying a beer gas mix. Does anyone carb on CO2 and serve on pure nitro? Is that even possible? Or would that actually be preferable?

Sorry, I have more questions than answers :O

You could serve on pure nitro provided you finished the keg in a single session (a few hours). The moment you dilute the CO2/N2 mix in the headspace CO2 will start coming out of solution and the beer will start losing carbonation.

 

[J2W2]

You could serve on pure nitro provided you finished the keg in a single session (a few hours). The moment you dilute the CO2/N2 mix in the headspace CO2 will start coming out of solution and the beer will start losing carbonation.

That's why I asked if it was pure nitro, as it might explain odie's loss of carbonation.

I know pure nitro is available; I've seen a few member's mention using it to push wine to a tap without carbing it.

 

[odie]

beer gas is normally served like 20-30 psi? I kinda think if you were carbed with just CO2 at normal volumes that the higher nitro serving pressure would keep the co2 in the beer? Or does the CO2 mix evenly though out the keg...the beer and the head space? And as the volume drops you need additional co2 coming in along with the nitro to keep the proper co2 volume in the beer? I think that's how I recall the nitro discussion once?

 

[J2W2]

beer gas is normally served like 20-30 psi? I kinda think if you were carbed with just CO2 at normal volumes that the higher nitro serving pressure would keep the co2 in the beer? Or does the CO2 mix evenly though out the keg...the beer and the head space? And as the volume drops you need additional co2 coming in along with the nitro to keep the proper co2 volume in the beer? I think that's how I recall the nitro discussion once?

I run mine at 35 psi. My understanding is that the concentration of CO2 should actually increase as the keg is emptied. For instance, I'm currently carbing my new keg of stout to 5 psi of CO2. Once it's back on beer gas, whatever additional CO2 is in the headspace should be forced into the beer. So, the more headspace, the more CO2 available to saturate into the beer. Once all the available CO2 has been absorbed, the headspace would be pure nitrogen.

I could be wrong on this, but I would think if I were to frequently purge the headspace, allowing fresh beer gas in to replace the CO2 depleted gas, that eventually I would end up with a keg of stout carbonated to 35 psi of CO2.

 

[Vale71]

I could be wrong on this,

Unfortunately you are. Gases work independently of each other, even in a mixture. One gas (in this case N2) never "pushes" another gas (in this case CO2) into solution. All that matters is each gas' individual partial pressure.

 

[J2W2]

Unfortunately you are. Gases work independently of each other, even in a mixture. One gas (in this case N2) never "pushes" another gas (in this case CO2) into solution. All that matters is each gas' individual partial pressure.

Are you saying that if I had 25%/75% CO2/N2 at 32 psi, the CO2 concentration would max out at 8 psi (25% of 32)?

If so, since I run at 35 psi, should I pre-carb my stout to 9 psi to start out with the maximum CO2 concentration?

 

[Vale71]

Are you saying that if I had 25%/75% CO2/N2 at 32 psi, the CO2 concentration would max out at 8 psi (25% of 32)?

If so, since I run at 35 psi, should I pre-carb my stout to 9 psi to start out with the maximum CO2 concentration?

Both spot on.

 

[DuncB]

The other way to think about it is what vols of CO2 do you want in your Stout. Read the psi for the temp you serve at then multiply by 4 for the beer gas reading. As J2W2 has surmised. Might want to add a few psi if non return valves in the gas lines.
With this in mind I pressure ferment the stout so that it has my aimed for vols of CO2 when crashed to serve temp.
Then put it on the beer gas and its good to go straightaway.
One other thing to watch out for is different ratios on the beer gas, some are filled 30 % CO2 and 70% N2, this will affect your settings as well.

The other problem with CO2 tanks being filled with beer gas is you can't get very much in there, the CO2 and Nitrogen are both in the cylinder as gas, wheras the CO2 only cylinders have liquid. My keg filling guy struggles to get 400g of CO2 and 1200g of Nitrogen into a 5kg cylinder.

Only just set mine up and boy has it made a difference to the stout.

Finally as the system is running at higher pressure than a CO2 only system it will expose any leaks more so it's really worth keeping a close eye on the cylinder when you change kegs etc.

 

[odie]

CO2 is liquid. Beer gas is gaseous. where I'm at you can old get the tall bottles of gas. About 4-5ft tall.

Anyway, my assumption was to carb to desired vol of CO2...usually pretty damn low for a nitro stout from what I gather. Maybe 5 psi or less at 32'. Basically almost just the natural CO2 volume.

When you tap it set your nitro reg based on your mix. 25/75 would be a 4x factor. 20/80 mix would be a 5x factor. Whatever your normal psi for your serving temp, crank the nitro 4x or 5x or whatever based upon your nitro mix.

 

[DuncB]

About 1.3 vols of CO2 according to the official Guinness stuff I read and then just multiply up related to your beer gas for the reg pressure.

 

[Ayzala]

Maybe off topic but I've been questioning, If beer gas is gaseous and allowed to sit, wouldn't the two gasses separate in the bottle? And as CO2 is heavier than N2, if you went straight for beer gas (assuming your bottle sat for a while) you would be charging your keg with just N2. Or am I way off on this?

 

[day_trippr]

No, because gas molecules are bouncing all around inside volume which eventually results in homogenization....

Cheers!

 

[DuncB]

If you went to the bottom of a well you wouldn't suffocate even though the air is still. In a mine fresh air is pumped in and stale air out to get rid of toxic gases and the accumulated CO2 from metabolism.
If you got the cylinder super cold to take all the energy out of the molecules ie near nitrogen freezing point they probably would in time stratify more but there would still be some mix at the interface.
Genuinely the CO2 heavier sitting below the nitrogen isn't a reality at our temp and pressures, it's a brewing myth like airplanes exploding when window is shot and the cabin is pressurised and decompresses ( see mythbusters for this) .
Reality the rapid air loss in flight occurs due to venturi effect from the hole in the window as the plane is flying along and that causes the pressure difference and problems ( mythbusters didn't test that aspect ).

 

[Vale71]

Or am I way off on this?

Yes.

 

[J2W2]

Thank you very much everyone for all the great information.

I've got my stout on CO2 at 8 psi. I'll let it sit for a few more days and then switch it back to beer gas next week.

As a side note, the talk of various beer gas mixes got me curious. I pulled out the receipt from my last tank swap in 2020 and it states "25% Carbon Dioxide Balance Nitrogen", so at least I'm correct on the ratio I'm running.

Thanks again!

 

[doug293cz]

A lot of misunderstanding of how partial pressure works in the above posts. To get the partial pressure of each gas in a mixture if you know the ratio of gases, you have to start with the absolute pressure, not the gauge pressure. To get absolute pressure you have to add atmospheric pressure to the gauge pressure. Thus if you have 75/25 (N2/CO2) beer gas at 30 psi gauge pressure, the absolute pressure is then 30 psi + 14.7 psi = 44.7 psi. The CO2 partial pressure is 0.25 * 44.7 = 11.175 psi. If this were pure CO2 in the headspace, the gauge pressure would be 11.175 - 14.7 = -3.525 psi. The equilibrium carbonation level at 45°F would be about 0.94 volumes.

Brew on

 

[J2W2]

A lot of misunderstanding of how partial pressure works in the above posts. To get the partial pressure of each gas in a mixture if you know the ratio of gases, you have to start with the absolute pressure, not the gauge pressure. To get absolute pressure you have to add atmospheric pressure to the gauge pressure. Thus if you have 75/25 (N2/CO2) beer gas at 30 psi gauge pressure, the absolute pressure is then 30 psi + 14.7 psi = 44.7 psi. The CO2 partial pressure is 0.25 * 44.7 = 11.175 psi. If this were pure CO2 in the headspace, the gauge pressure would be 11.175 - 14.7 = -3.525 psi. The equilibrium carbonation level at 45°F would be about 0.94 volumes.

OK, you've lost me on some of this.

I follow your explanation on partial pressure. Since I'm in Lincoln Nebraska, I assume I would substitute 14.7 psi (psi at sea level) with 30.0 psi (an average of our atmospheric/barometric pressure). My beer gas ratio is 75/25, and I run it at 35 psi. Using your formula, that gives me an absolute pressure of 65 psi (30 psi + 35 psi), and a CO2 partial pressure of 18.75 psi (0.25 * 65).

But then I get lost on your negative psi. Using your formula, with pure CO2 my gauge pressure would be -11.25 (18.75 - 30). What exactly does the negative psi represent? I'm also not sure what volumes of CO2 this would be. I keep my beer at 39°F, but I can't find a carbonation chart that goes below 1 volume.

And my bottom line question to you would be: What pure CO2 psi setting would you recommend I use to "pre-carb" my stouts, based on my other information?

Thank you very much for your help!

 

[doug293cz]

Atmospheric pressure for Lincoln, NE is about 14.1 psi, not 30 psi. So, 35 psi gauge is 49.1 psi absolute. That means your CO2 partial pressure is 0.25 * 49.1 = 12.275 psi, and the N2 partial pressure is 0.75 * 49.1 = 36.825 psi.

The carbonation charts are all based on gauge pressure, but carbonation actually depends on partial pressure. To convert the partial pressure to an effective gauge pressure, you have to subtract atmospheric pressure (14.1 psi at your elevation) from the partial pressure, so you effective CO2 gauge pressure is 12.275 - 14.1 = -1.8 psi. Since none of the carbonation charts show negative CO2 gauge pressures, you have to use an equation to determine the equilibrium volumes of CO2.

Normal pressure gauges measure the difference in atmospheric pressure and the pressure in a vessel. A negative gauge pressure indicates that the vessel is at a partial vacuum - the internal pressure is less than the atmospheric pressure. When talking about partial pressures, gauge pressures are not really useful, since a pressure gauge can only measure the combined pressures of all of the gases present.

A good equation for determining volumes of CO2 vs. CO2 partial pressure is:

V = P*(0.01821+0.090115*exp(-(T-32)/43.11))-0.003342​

Where P is the partial pressure of CO2, not the gauge pressure, and T is in °F​

​

To calculate the CO2 partial pressure required to get a specific number of volumes of carbonation, you solve the above equation for P, which gives you:

P =(V+0.003342)/(0.01821+0.090115*exp(-(T-32)/43.11))​

​

To get the gauge pressure for carbonating with pure CO2 you have to add local atmospheric pressure to the partial pressure calculated above. In your case atmospheric pressure is about 14.1 psi. If the required gauge pressure comes out negative, then you will need to carbonate at a higher temperature, so the gauge pressure becomes positive.

The temperature and CO2 pressure you should use to carbonate depends on the carbonation level you want in your beer. You also may need to adjust your beer gas pressure so that you maintain your desired carbonation level throughout the serving life of your keg.

Say you wanted 1.2 volumes of CO2 in your beer, and you wanted to carbonate at 5 psi gauge pressure. You would then have to carbonate at 62°F.

If you want to store and serve you beer at 50°F, then you would need a partial pressure of CO2 of 15.51 psi, and a total absolute pressure of 75/25 beer gas of 15.51/0.25 = 62.1 psi absolute, or 48.0 psi gauge. If you want to store and serve at 45°F, then the partial pressure of CO2 needs to be 14.18 psi and the beer gas gauge pressure needs to be 42.6 psi.

Brew on

 

[day_trippr]

Say you wanted 1.2 volumes of CO2 in your beer, and you wanted to carbonate at 5 psi gauge pressure. You would then have to carbonate at 62°F.

This is exactly what I was going on about in a different but related "beer gas" thread wrt nudging a stout from its intrinsic post-fermentation carbonation level (roughly between .6 and .8 volumes depending on the warmest post-fermentation temperature seen) to the ~1.2 volumes I like for dispensing at 35 psi using 70/30 beer gas...

Cheers!

 

[Gozie Boy]

OK, you've lost me on some of this.

I follow your explanation on partial pressure. Since I'm in Lincoln Nebraska, I assume I would substitute 14.7 psi (psi at sea level) with 30.0 psi (an average of our atmospheric/barometric pressure). My beer gas ratio is 75/25, and I run it at 35 psi. Using your formula, that gives me an absolute pressure of 65 psi (30 psi + 35 psi), and a CO2 partial pressure of 18.75 psi (0.25 * 65).

But then I get lost on your negative psi. Using your formula, with pure CO2 my gauge pressure would be -11.25 (18.75 - 30). What exactly does the negative psi represent? I'm also not sure what volumes of CO2 this would be. I keep my beer at 39°F, but I can't find a carbonation chart that goes below 1 volume.

And my bottom line question to you would be: What pure CO2 psi setting would you recommend I use to "pre-carb" my stouts, based on my other information?

Thank you very much for your help!

^This is why I had to leave Lincoln, NE. I just couldn't stand the pressure!

 

[DuncB]

Isn't a keg a closed system so how does the atmospheric pressure act on it?
The pressure in your cylinder or keg shouldn't change if it was in a vacuum would it?
The atmospheric pressure does act on the beer / beverage when poured.
The temperature does effect the solubility though.
I am confused.

 

[day_trippr]

Gauges read differently, maybe?

 

[doug293cz]

Isn't a keg a closed system so how does the atmospheric pressure act on it?
The pressure in your cylinder or keg shouldn't change if it was in a vacuum would it?
The atmospheric pressure does act on the beer / beverage when poured.
The temperature does effect the solubility though.
I am confused.

The atmosphere does not act on what's inside the keg. It's all about pressure gauges. Common pressure gauges measure the difference between atmospheric pressure and the pressure inside the keg. What's important for all physical chemistry effects is the absolute pressure (which a normal gauge can't read) of the gas in the keg headspace, and in particular the partial pressures of the individual gases in the headspace. CO2 is the most important headspace gas, and O2 is the second most important.

Brew on

 

[DuncB]

The atmosphere does not act on what's inside the keg.

So the gauge on our keg in space would be off the upper end of the scale if " carbed " to 1 psi on earth and retested in space and the whole system would most likely fail in a big way.

Will have to go back to the physics A level books a darkened room and an alcohol free coffee.

 

[doug293cz]

So the gauge on our keg in space would be off the upper end of the scale if " carbed " to 1 psi on earth and retested in space and the whole system would most likely fail in a big way.

Will have to go back to the physics A level books a darkened room and an alcohol free coffee.

No, the difference is only 14.7 psi sea level vs. outer space. But it does need to be accounted for when doing calculations.

Brew on

 

[Hwk-I-St8]

Wow, that's a lot of math....more than I want to do.

Aren't the carbonation charts based off of gauge pressures already?

 

[day_trippr]

If they are you can bet the charts/tables are set for sea level...

Cheers!

 

[doug293cz]

Wow, that's a lot of math....more than I want to do.

Aren't the carbonation charts based off of gauge pressures already?

Yes.

If they are you can bet the charts/tables are set for sea level...

Cheers!

And, yes.

The tables assume that the headspace gas is 100% CO2, and they have already done the adjustment for absolute vs. gauge pressure (but only for sea level.) The charts are useless as soon as you start talking about beer gas (mixed N2 and CO2.) It's not difficult to build a spreadsheet that does elevation correction, gauge vs. absolute correction, calculates partial pressures of both CO2 and N2, and spits out gauge pressure for beer gas at any N2/CO2 ratio.

Brew on