how do blended gases behave in solution? A beergas question.

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Lost

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I know, the title is about as clear as mud but it's the best I could come up with.

I'm wondering specifically about beer gas - which is about 70/30 blend of nitrogen and CO2. I have a nitro setup and have used beer gas and get the idea (serve at high pressure for the stout faucet while not over carbing the beer).

But I'm wondering why we don't just carb the beer to 1.5 volumes of CO2 and then put it on straight nitrogen to serve. I expect nitrogen is cheaper and, practically speaking, I'm having a hard time finding beer gas since I moved.

Say I did carb with CO2 and serve with nitrogen, will the CO2 come out of solution even with the nitrogen pressurizing the keg to 30psi? Is the volume of CO2 that stays dissolved in the beer simply a function of pressure or does it have to be CO2 gas that's applying that pressure?
 
Say I did carb with CO2 and serve with nitrogen, will the CO2 come out of solution even with the nitrogen pressurizing the keg to 30psi? Is the volume of CO2 dissolved in the beer simply a function of pressure or does it have to be CO2 gas that's applying that pressure?

Yeah, CO2 will come out of the beer. Even though there's nitrogen pressure, from a physics perspective, the headspace in the keg is a vacuum as far as the CO2 is concerned. Say you had 4.8 gallons of beer that had been carbed to 2.2 vol CO2 and put inside a 5gal corny. The 0.2 gal of empty space is completely devoid of CO2, so the beer has to off-gas in order to reach equilibrium.

You have 2.2 vol CO2 x 4.8gal = 10.56gal CO2 total. The total volume in the keg is 5gal, so 10.56 gal CO2 ÷ 5gal volume = 2.1 vol CO2 in the entire volume (2.1 dissolved in the beer, and 2.1 now in the headspace). As you dispense the beer, say a gallon, you now only have 3.8 gallons of beer and that 0.2 gallons of headspace @ 2.1 vol, and a gallon of "empty" space. The CO2 will equalize across the whole volume, and now you're left with 4gal x 2.1 volumes = 8.4gal CO2 total. Divided across the 5gal total volume, your beer and headspace is now at 1.68 vol. As you can see, your beer will go flat quite quickly as you dispense.

This is the same reason why the beer slowly goes flat throughout the night if you use the hand-pumped "party taps" from the liquor store. You're pumping mostly nitrogen & oxygen into the keg.
 
Each gas in a mixture behaves (ideally) as if no other gasses are present. Thus if you carbonate with CO2 but serve under nitrogen the CO2 in the beer sees a partial pressure of CO2 of 0 in the headspace and CO2 will migrate to the headspace until the partial pressure of CO2 in the headspace is the same as that in the beer. The total pressure (what you would read on an attached pressure gauge) will be the sum of the partial pressures of nitrogen and CO2 so that in this case the gauge reading would increase over time.
 
Well that's what I was afraid of. I appreciate the timely and thoughtful responses.
 
The CO2 will equalize across the whole volume,

It doesn't work quite like that but the general concept is correct. The CO2 will distribute so that its chemical potential in the beer and in the headspace is the same so that the larger the headspace the less the volumes in the beer at equilibrium. It would be messy (calculus) to compute what happens as the beer is drawn but it is quite simple to compute what the equilibrium headspace partial pressure of CO2 and thus equilibrium volumes in the beer would be if one takes a nitrogen filled keg and fills partially with beer charged to a given number of volumes. I've done that for beer initially at 12 psig and 40 °F (about 2.5 vols). The resultant partial pressures and volumes are plotted vs. fraction of volume which is headspace at http://www.pbase.com/image/139057170. Remember that the pressures are gauge so that there is nothing funny about negative psig (unless they are more negative than 14.697).
 
If you can't find blended gas you could always use a gas blender with a seperate Nitrogen tank and co2 tank. Kind of expensive to set up, but If you can't get beer gas or if it is very expensive like it was around here it may be an alternative. I have been using one for a year or so and wouldn't go back.

I now get both tanks filled for less than 25% the cost of 1 beer gas tank and it lasts 3x as long.

There is one currently pretty cheap on e-bay(not mine). Cheap being relative since they sell for $600-$800 new.

http://www.ebay.com/itm/McDantim-Tr...765?pt=LH_DefaultDomain_0&hash=item2568aaef2d
 
Is the volume of CO2 that stays dissolved in the beer simply a function of pressure or does it have to be CO2 gas that's applying that pressure?


I guess this fundamental question never really got answered. What controls the carbonation level is the partial pressure of carbon dioxide. If you have a 20% CO2 blend with the regulator set for 30 psig the absolute total pressure is 30 + 14.7 = 44.7 psia. Twenty percent of that is 6.94 psia which corresponds to 8.94 - 14.7 = -5.76 psig partial pressure of CO2 which would give you about 0.8 volumes. Suitable for a stout and some ales but not much else.
 
I have a related question. Is there any reason why you cannot just carb your beer with beer gas? From what I understand, nitrogen does not want to go into solution with beer as readily as carbon dioxide does.

If you carb with beer gas (i.e. just hook up beer gas after transferring to keg and let it sit for two weeks), will you get probably carbed beer? Also, will some nitrogen go into solution and create a different carbonation "feel" to the beer?

I am in the process of setting up my first nitro stout in my kegerator at home. I primed my stout with sugar in the keg and I'm letting it sit at RT for 2 weeks. Then I'm going to hook up beer gas to it, let it sit for a day and try my first pour. Would this approach result in a different carbonation than if I just hooked up to beer gas and waited?

Sorry for hijacking this thread, but I thought my question was related. . . .
 
It is related. Yes, you can carb with beer gas but you must keep in mind that the level of carbonation will be determined by partial pressure of the CO2 as in the example in #8 and that partial pressure is obtained by multiplying 14.7 + the gauge reading by the percentage CO2. If that is converted back to psig (noting that it can be negative for low gauge pressures and low percentage) you can use the ordinary Zahm and Nagle (ASBC) table to compute the number of volumes.

You can go to McDantim's website (they are the people that make gas blenders) where they have temperature vs pressure vs volumes curves for various common CO2/N2 blends.

There are really only 2 situations in which one might wist to do this:

1. Stouts and some ales where you want low carbonation and want to push the beer hard through small holes in a "sparkle plate" to get the tiny bubbles and creamy head which are so much a part of the stout experience

2. Where you have really long lines or large vertical rise such that the pressure required to move the beer to the tap would result in way over carbonation if it were supplied solely by CO2. Nitrogen here, as is the case in 1, is there to push. That's all.
 
Thanks for the answer ajdelange. So, if I'm understanding this correctly, nitrogen is assumed to have zero impact on carbonation in these formulas?

The spreadsheet calculators on the McDantim website are helpful in getting an answer, but not necessarily understand the concept.

In my case, I've carbed with sugar, so I have a set amount of carbonation in my beer before I attach beer gas. Let's say I have 2 volumes of CO2 if I hit my target. Now, I introduce beer gas (75% N2/25% CO2) at 30 PSI in my kegerator set to 44 F. According to my the calculator, that will result in 0.96 volumes CO2. I got the 30 PSI/44 F/25% CO2 values from what is officially recommended by Guinness.

So does this mean over time the beer will flatten until it reaches an equilibrium between the 2 volumes and the 0.96 volumes?

Is Guinness recommending to serve their stout at 0.96 volume?

I appreciate any advice as I'm trying to mimic Guinness as much as possible with my N2 setup on my first try.
 
So, if I'm understanding this correctly, nitrogen is assumed to have zero impact on carbonation in these formulas?

Yes, that's the assumption. Of course some nitrogen does dissolve (that's why divers get the bends) but it is little compared to the CO2.


In my case, I've carbed with sugar, so I have a set amount of carbonation in my beer before I attach beer gas. Let's say I have 2 volumes of CO2 if I hit my target. Now, I introduce beer gas (75% N2/25% CO2) at 30 PSI in my kegerator set to 44 F. According to my the calculator, that will result in 0.96 volumes CO2. I got the 30 PSI/44 F/25% CO2 values from what is officially recommended by Guinness.

So does this mean over time the beer will flatten until it reaches an equilibrium between the 2 volumes and the 0.96 volumes?

That is what would happen if you purged the headspace with the 75/25 mix over and over again or if you put flat beer in a keg and applied the mix at 30 psi from day 1. Because you already have "extra" CO2 in the beer some will leave to enter the headspace. The amount depends on the relative volumes of the headspace and the beer as discussed in an earlier post. As you draw more and more beer the headspace mixture approaches 75/25 more closely and the level of cabonation approaches 0.96 more closely but as you started with more CO2 than 0.96 vols. there will always be extra CO2 and 0.96 vols. will never be reached.

Is Guinness recommending to serve their stout at 0.96 volume?
Relying on memory (a dangerous thing to do) that seems about right. 30 psig in a 75/25 mix does give a nice pour but a stout faucet (i.e. one with a sparkler plate) is required. The creamy head comes when the lightly carbonated beer is vigorously forced through the small holes in the sparkler plate.

I appreciate any advice as I'm trying to mimic Guinness as much as possible with my N2 setup on my first try.

I guess I'd cool the keg and bleed it down to atmospheric. Then wait a day and do it again. And then perhaps again on a 3rd day. At this point you should be reasonably close to 1 vol. Then apply the 30 psig blend. That should get you pretty close.
 
Guinness recommends serving at 1.2 volumes. The recommended serving temperature for Guinness Draft is 38 degrees. At sea level this is approx 25% co2 mix at 38Deg with a pressure of 35 PSI.

If you want to leave temp at 44 deg, you would need to bump pressure to about 41 PSI.
 
Thanks for the responses. I do have a stout faucet, so I expect to be able to get the creamy head that I'm looking to mimic.

I think I understand this much better now, but have two last questions if you still have some patience to answer . . .

1. So if I had just forced carbonated with beer gas, then I could set it and forget it like I do with straight CO2 and end up with calculated volumes of CO2 in my finished beer. As long as this beer was kept under constant pressure with beer gas, the CO2 volumes would stay constant throughout the life of the beer, because the ratio of CO2 volumes in the headspace would be equal to the CO2 volumes in the beer throughout the life of the keg, right?

By priming with sugar, I have put some initial CO2 in solution, and that will not leave unless I bleed it out over time. So, once I add beer gas on top of that, I will automatically end up with a proportionately higher CO2 level then if I had just used beer gas on flat beer. Makes sense!


2. Why do you recommend cooling the keg before bleeding down to atmospheric? I thought cold beer more readily accepts dissolved CO2. Are you saying that cold beer will more readily dissolve air to replace the CO2?


zazbnf, 38 degrees seems much too cold for my liking. I'm surprised Guinness recommends this cold of a temperature for a stout. I'll probably start with 30 psi and then increase until I get the desired end product.
 
Obviously I'm not one of the physics experts in this thread but, as a practical matter, I've noticed that CO2 levels are not super critical. If the beer is about 1-2 volumes of CO2 the pour will be good. It's about experimenting with the serving pressure and this too will be affected by the line length (more line = greater restriction = less apparent pressure at the faucet).

So I wouldn't purge the tank, or maybe do it once. I don't think you'll notice much difference either way. If the beer is all foam and takes forever to settle then you've got too much CO2 in solution.

And you can carb using beer gas but it's so expensive I would rather not.

Lastly, I hope you're seating the lid on the corny using some pressure from the tank. Otherwise you have no way of knowing if the lid is sealed and if your beer is actually carbonating or if the CO2 is just venting out the lid.
 
Lost, yeah, I seated the lid by pressurizing with some beer gas from the cylinder. I then bled some of the gas off once the lid was seated correctly to get some of that extra pressure out of there.

My beer line is only about 3 feet, so not much restriction there.

I'm going to play around with the pressure and see what works, but I was hoping to get a better theoretical understanding of what was going on before I started experimenting.

This thread has been a huge help. Can't wait to try and pour a nitro stout in 2 weeks.
 
1. So if I had just forced carbonated with beer gas, then I could set it and forget it like I do with straight CO2 and end up with calculated volumes of CO2 in my finished beer. As long as this beer was kept under constant pressure with beer gas, the CO2 volumes would stay constant throughout the life of the beer, because the ratio of CO2 volumes in the headspace would be equal to the CO2 volumes in the beer throughout the life of the keg, right?

That's correct. As far as the beer is concerned the only thing that counts is the partial pressure of CO2. It will become and stay carbonated to the number of volumes corresponding to that partial pressure.

By priming with sugar, I have put some initial CO2 in solution, and that will not leave unless I bleed it out over time. So, once I add beer gas on top of that, I will automatically end up with a proportionately higher CO2 level then if I had just used beer gas on flat beer. Makes sense!

We're assuming you will have extra CO2. If your priming were such that the carbonation of the beer were at the level implied by the desired final storage temperature, gauge pressure and blend (% CO2) then there would be no extra CO2.


I don't share lost's opinion that dissolved volumes are not that important. The amount of carbonation in a beer can have quite a pronounced effect on the way it is perceived. A weizen served at 1

2. Why do you recommend cooling the keg before bleeding down to atmospheric? I thought cold beer more readily accepts dissolved CO2. Are you saying that cold beer will more readily dissolve air to replace the CO2?

I was just describing how to get to about 1 volume at 44 °F (which corresponds to about 1 atmosphere headspace pressure) from some higher volume level and pressure. If you chill the beer to 44 °F and bleed it until no more gas leaves the keg the pressure in the headspace will be 1 atmosphere. If the beer is highly carbonated gas will leave the beer and enter the headspace and if you bleed again the headspace pressure will return to 1 atmosphere. Keep doing this and eventually the beer will be at equilibrium with the 1 atmosphere in the headspace and no more gas will leave the beer.

Yes, it is true that cold beer holds more CO2. The description assumes a fairly hefty charge of CO2 at the outset i.e. before cooling.

I don't share lost's opinion that CO2 level is not that important. Carbonation level can have quite an effect on the mouthfeel and flavor perception associated with a given beer. Consider a Weizen carbonated to only 1 volume or, more to the point here, an over carbonated stout. Stout needs the creamy head and just enough spritz left on the tongue to give the beer a little life. Commercial brewers are quite insistant about the gas pressure applied to their beers in order to insure that the carbonation level is proper.
 
That's a thoughtful response. To clarify, my point isn't that carbonation doesn't matter or is imperceptible. It is.

My point is that this is less so with beers served through a stout faucet. The restrictor plate effectively degasses most of the co2 out of solution anyway. I've experimented with my setup and found that the most perceptible consequence of co2 changes (I carb before hooking the beergas up) is in the time it takes the cascading head to settle. The beer doesn't taste noticeably different to my unrefined palette at least.
 
So if I had just forced carbonated with beer gas, then I could set it and forget it like I do with straight CO2 and end up with calculated volumes of CO2 in my finished beer.

One flaw in this approach: since the N2 doesn't go into solution, it eventually dominates the headspace. Just make it easy assume you have 500 ml of headspace. Fill it with 25/75 beer gas. Most of the CO2 goes into solution making room for another 125 ml of beer gas (times pressure, of course). Repeat, 30, 7.5 ... I'm 40 years away from the math to tell you what the final carbonation level will be, but it won't be what you wanted.

If you have N2 and CO2 and don't want to mess with a blender, carbonate with CO2 to your desired volumes, dispense with N2 and every four or five pints attach your CO2 and dispense one. Kind of a pain, but it works.
 
One flaw in this approach: since the N2 doesn't go into solution, it eventually dominates the headspace. Just make it easy assume you have 500 ml of headspace. Fill it with 25/75 beer gas. Most of the CO2 goes into solution making room for another 125 ml of beer gas (times pressure, of course). Repeat, 30, 7.5 ... I'm 40 years away from the math to tell you what the final carbonation level will be, but it won't be what you wanted.

If you have N2 and CO2 and don't want to mess with a blender, carbonate with CO2 to your desired volumes, dispense with N2 and every four or five pints attach your CO2 and dispense one. Kind of a pain, but it works.

You know, that's a cheap and simple solution. 25/75 mix would imply 1 of every 4 pints should be served using CO2 at the same pressure as the nitrogen. Just add a second regulator so you have your low pressure CO2 for the regular beer tap and a high pressure CO2 for the nitro stout. I bet you could even put a y in the line and feed both into the disconnect, just use a valve to switch between the 2.
 
David, things just got more confusing. . . .

So you're saying the CO2 level will keep creeping up every time you remove liquid from the keg and replace it's equivalent volume with beer gas? and essentially you're re-establishing equilibrium by putting pure CO2 in after every 4 cycles? I'm going to sit down and do the math at some point, but I wonder what overall change in volumes you would see if you drained a 5 gallon keg 1 pint at a time?

I can't imagine a brew pub would do this all the time, but I guess you're saying they use a blender to maintain the right balance?
 
dermotstratton said:
David, things just got more confusing. . . .

So you're saying the CO2 level will keep creeping up every time you remove liquid from the keg and replace it's equivalent volume with beer gas? and essentially you're re-establishing equilibrium by putting pure CO2 in after every 4 cycles? I'm going to sit down and do the math at some point, but I wonder what overall change in volumes you would see if you drained a 5 gallon keg 1 pint at a time?

I can't imagine a brew pub would do this all the time, but I guess you're saying they use a blender to maintain the right balance?

No, he is advocating for the use of straight nitrogen in place of beer gas. Then top up occasionally with co2.
 
A pub would buy the beer with the proper CO2 charge from the brewery and then apply mixed gas at the appropriate pressure such that the partial pressure of CO2 would be in equilibrium with the carbonation in the beer. No CO2 would exchange between the mix and the beer. As beer is drawn off it is replaced by the mix.
 
Best practice, IME, is to look at the McDantim website and download their excel calculator. Based off that knowledge, use co2 to carb the beer at room temps (to make it manageable to carb to ~1 volume). Then when ready, hook up to your nitro mix, and you will be happily pouring lovely cascading nitro beers.

This is essentially what ajdelange above is referring to-- pubs get the kegs pre-carbed to the proper volume and never alter their nitro mixture blend.

:mug:
 
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