What's the best % mix for nitro/beer gas?

[TAK]

Title says it all.

I have a 20 lb nitro tank that I picked up from one of the welding supply shops in town. It's beergas mix, but I have no idea what the percentages of Nitro to CO2 are. I've never had it refilled, but it's about out. I get my CO2 refills from a competing weld supply shop, and they'll exchange the tank I have. They'll also do whatever percentage mix I want.

If I knew what was in the tank I've been using, I may just replicate that. Nonetheless, it seems like a good opportunity for me to dial in my nitro setup, I just don't know what an ideal mix is for a kegorator situation. I'm pretty sure beergas mixes usually range between 70-80% N2 & 20-30% CO2.

Any advice on the butter zone would be greatly appreciated.

 

[doug293cz]

You want the mix to give you a CO2 partial pressure that is in equilibrium with your carb level and serving temp (from the carb chart or calculator) while the total pressure is at your serving pressure. The calculation is a follows:

CO2 partial pressure = 14.7 psi + table/calculator value
CO2 percent in mix = 100% * CO2 partial pressure / (serving pressure + 14.7 psi)
N2 percent in mix = 100% - CO2 percent in mix​

The addition of 14.7 psi is to convert gauge pressure to absolute pressure.

Brew on

 

[TAK]

You want the mix to give you a CO2 partial pressure that is in equilibrium with your carb level and serving temp (from the carb chart or calculator) while the total pressure is at your serving pressure. The calculation is a follows:

CO2 partial pressure = 14.7 psi + table/calculator value
CO2 percent in mix = 100% * CO2 partial pressure / (serving pressure + 14.7 psi)
N2 percent in mix = 100% - CO2 percent in mix​

The addition of 14.7 psi is to convert gauge pressure to absolute pressure.

Brew on

I'm not disputing the math, but it doesn't seem to jive with my experience so far. I always try to carb lightly, somewhere between 1 - 2 volumes. Say I'm serving at 44F (I'd love to have a setup that allowed me to serve nitro stuff in the 50's, but I digress), Nitro asside, that come's in at 4 PSI of CO2 pressure. I assume that's the "table/calculator value" in your math.

I typically push my beergas between 25-30 PSI. The formulas above say, to keep that in equilibrium, I should have about 42-47% CO2 in my mix. I'm pretty confident that I had no more than 30% in the tank I've been using. If the math above is correct, I'd suspect that most of my kegs would have basically lost all carbonation as they equalized.

Are you sure that 14.7 constant is accurate? With that in there, "table value" goes to zero at 30% CO2 and about 34 PSI. In other words, with a 70/30 N2/CO2 mix served at 34 PSI, it's saying that wouldn't maintain any carbonation.

Does this make sense, or am I misinterpreting your formulas?

 

[doug293cz]

I'm not disputing the math, but it doesn't seem to jive with my experience so far. I always try to carb lightly, somewhere between 1 - 2 volumes. Say I'm serving at 44F (I'd love to have a setup that allowed me to serve nitro stuff in the 50's, but I digress), Nitro asside, that come's in at 4 PSI of CO2 pressure. I assume that's the "table/calculator value" in your math.

I typically push my beergas between 25-30 PSI. The formulas above say, to keep that in equilibrium, I should have about 42-47% CO2 in my mix. I'm pretty confident that I had no more than 30% in the tank I've been using. If the math above is correct, I'd suspect that most of my kegs would have basically lost all carbonation as they equalized.

Are you sure that 14.7 constant is accurate? With that in there, "table value" goes to zero at 30% CO2 and about 34 PSI. In other words, with a 70/30 N2/CO2 mix served at 34 PSI, it's saying that wouldn't maintain any carbonation.

Does this make sense, or am I misinterpreting your formulas?

3 psi gauge pressure of CO2 gives 1.52 volumes of carb @ 44˚F, so let's do the math with that. The gauge measures the difference in pressure between atmospheric pressure (14.7 psi) and the pressure in the keg. Therefore, the pressure in the keg is 14.7 psi + 3 psi = 17.7 psi. So, 17.7 psi is the partial pressure of CO2 you need in the keg to maintain 1.52 volumes of carb @ 44˚F. At 30 psig beer gas, the absolute pressure in the keg is 14.7 psi + 30 psi = 44.7 psi. The partial pressure of a gas in a mixture is just equal to the volume fraction of that gas in the mixture times the total pressure, or the required volume fraction equals the desired partial pressure divided by the absolute pressure. For our case that is:

Percent CO2 in mix = 100% * 17.7 psi / 44.7 psi = 40%​

If you used a 30% mix of CO2, then @ 30 psig (44.7 psia) the CO2 partial pressure would be 0.30 * 44.7 = 13.4 psia (-1.3 psig.) Since the CO2 gauge pressure is negative (but not the partial pressure) you can no longer use the charts, because they don't cover negative gauge pressures. So, you have to use a carbonation calculator (like the one here) and iterate to figure out the equilibrium volumes of carb. In this case the equilibrium carb level works out to 1.15 volumes.

Edit. The table value of pressure going to 0 doesn't mean there is no carbonation. It just means there is 14.7 psi absolute pressure in the keg, and since the tables are for pure CO2, the CO2 partial pressure is 14.7 psi (partial pressures are always absolute pressures, there are no gauges than measure only the pressure of one gas in a mixture.) Some level of carbonation is maintained for any CO2 partial pressure above 0 (partial pressures cannot go negative.)

Brew on

 

[Stillraining]

I'm not even close to following either of you so please pardon my ignorance on this subject..

but why mess with Atmospheric pressure in your equations in the first place.. When I fill my truck tires to 45 PSI that is 45 psi above atmospheric pressure insist it? My gauge never says 14.2 just sitting on the bench. My Barometer measure that exchange but non of my pressure gauges do.

 

[doug293cz]

I'm not even close to following either of you so please pardon my ignorance on this subject..

but why mess with Atmospheric pressure in your equations in the first place.. When I fill my truck tires to 45 PSI that is 45 psi above atmospheric pressure insist it? My gauge never says 14.2 just sitting on the bench. My Barometer measure that exchange but non of my pressure gauges do.

You mess with atmospheric pressure corrections because the gauge on your CO2 or beer gas tank doesn't measure absolute pressure, which is what matters for gas absorption/desorption by liquids. The CO2 only cares about the CO2 pressure in the keg (that's the physics.) It doesn't care about anything outside the keg, or any other gases inside the keg, but the gauge measures the difference between inside and outside. Your tires do care about the air pressure outside the tire, because they need a certain pressure above the atmospheric pressure to have the proper load supporting ability. If you don't correct for the fact that the gauge does not measure absolute pressure, you get the wrong answer. The common carbonation charts and calculators all take care of the atmospheric pressure corrections "under the covers" because they know what kind of gauge you have, and they know you don't want to deal with the corrections. (Although if you live at high enough altitudes, you have to deal with the atmospheric pressure corrections anyway.)

Brew on

 

[Stillraining]

So your saying that a beer gas regulator on a CO2 tank can/will only measure CO2 gas? Wow interesting.
I have bottles in my shop of Argon, Helium, Oxygen, Acetylene and Argon /CO2 mix, and even hook up and cut with propane occasionally... and I swap regulators on them all the time welder to welder, bottle to bottle . They all just give me pressure readings in the tank above atmospheric pressure which existed in the tank prior to it being pressurized with anything..

So I still dont get it, but that's one smart beer gauge I guess. And the reason I have no interest in physics or algebra...It hurts my head.

Carry on.

 

[day_trippr]

Swung right through that one...

 

[doug293cz]

So your saying that a beer gas regulator on a CO2 tank can/will only measure CO2 gas? Wow interesting.
I have bottles in my shop of Argon, Helium, Oxygen, Acetylene and Argon /CO2 mix, and even hook up and cut with propane occasionally... and I swap regulators on them all the time welder to welder, bottle to bottle . They all just give me pressure readings in the tank above atmospheric pressure which existed in the tank prior to it being pressurized with anything..

So I still dont get it, but that's one smart beer gauge I guess. And the reason I have no interest in physics or algebra...It hurts my head.

Carry on.

No, I did not say that a gauge on a beer gas regulator measures only the CO2 pressure, because it doesn't. No mechanical pressure gauge can measure the pressure of a single gas in a mixture. Not sure how you interpreted what I wrote to mean the opposite. I reread my posts, and can't see any way that they could be interpreted to say a gauge could measure just the CO2 in a mixture.

What I did say is that only the pressure exerted by CO2 affects the level of carbonation. If there is nitrogen, oxygen, or any other gas present in a mixture with CO2, each gas will have its own contribution to the total pressure. But those other gases won't affect the carbonation level. The CO2 contribution to the total pressure (known as the CO2 partial pressure) depends only on the volume of the headspace, temperature, and number of CO2 molecules present in the headspace. The CO2 partial pressure is unaffected by the presence of other gases. The other gases have their own partial pressures, and they also are independent of the other gases present. The total, or absolute pressure is the sum of the partial pressures of all of the gases present in the headspace. The gauge pressure is the difference between the total pressure and the air pressure outside the keg (i.e. atmospheric pressure.)

Brew on

 

[Stillraining]

OK.................I can sleep now...

 

[TAK]

3 psi gauge pressure of CO2 gives 1.52 volumes of carb @ 44˚F, so let's do the math with that. The gauge measures the difference in pressure between atmospheric pressure (14.7 psi) and the pressure in the keg. Therefore, the pressure in the keg is 14.7 psi + 3 psi = 17.7 psi. So, 17.7 psi is the partial pressure of CO2 you need in the keg to maintain 1.52 volumes of carb @ 44˚F. At 30 psig beer gas, the absolute pressure in the keg is 14.7 psi + 30 psi = 44.7 psi. The partial pressure of a gas in a mixture is just equal to the volume fraction of that gas in the mixture times the total pressure, or the required volume fraction equals the desired partial pressure divided by the absolute pressure. For our case that is:

Percent CO2 in mix = 100% * 17.7 psi / 44.7 psi = 40%​

If you used a 30% mix of CO2, then @ 30 psig (44.7 psia) the CO2 partial pressure would be 0.30 * 44.7 = 13.4 psia (-1.3 psig.) Since the CO2 gauge pressure is negative (but not the partial pressure) you can no longer use the charts, because they don't cover negative gauge pressures. So, you have to use a carbonation calculator (like the one here) and iterate to figure out the equilibrium volumes of carb. In this case the equilibrium carb level works out to 1.15 volumes.

Edit. The table value of pressure going to 0 doesn't mean there is no carbonation. It just means there is 14.7 psi absolute pressure in the keg, and since the tables are for pure CO2, the CO2 partial pressure is 14.7 psi (partial pressures are always absolute pressures, there are no gauges than measure only the pressure of one gas in a mixture.) Some level of carbonation is maintained for any CO2 partial pressure above 0 (partial pressures cannot go negative.)

Brew on

I follow the math, and you are certainly knowledgeable on the subject matter, but I still can't seem to reconcile it with my experience and common practice in the homebrew scene. If you search online for any information about running a nitro setup, you'll find information/directions along these lines...

• Carb lightly to somewhere between 1 - 2 volumes CO2
• Beergas mix is usually 25% CO2, maybe +/- 5%
• Push that beergas at 25 - 30 PSI

I think these are fair assumptions for an average setup...

• Middle of the line carbonation at 1.5 volumes CO2
• Serving temps between 40F - 50F
• Middle of the line temps puts us at about 3 PSI CO2 pressure
• Walk into a gas shop and ask for beergas mix, you'll probably get 25% CO2

From these assumptions, if I work backwards for serving pressure...

Serving Pressure = [CO2 Partial Pressure / CO2 in Mix] - 14.7
= [17.7 / 0.25] - 14.7
= 56.1​

I can play around with any of these inputs within conventional ranges, it always puts the last variable outside of said conventional ranges.

Do you run a stout faucet with a nitro setup? If so, what do you spec it at?

 

[doug293cz]

I follow the math, and you are certainly knowledgeable on the subject matter, but I still can't seem to reconcile it with my experience and common practice in the homebrew scene. If you search online for any information about running a nitro setup, you'll find information/directions along these lines...

• Carb lightly to somewhere between 1 - 2 volumes CO2
• Beergas mix is usually 25% CO2, maybe +/- 5%
• Push that beergas at 25 - 30 PSI

I think these are fair assumptions for an average setup...

• Middle of the line carbonation at 1.5 volumes CO2
• Serving temps between 40F - 50F
• Middle of the line temps puts us at about 3 PSI CO2 pressure
• Walk into a gas shop and ask for beergas mix, you'll probably get 25% CO2

From these assumptions, if I work backwards for serving pressure...

Serving Pressure = [CO2 Partial Pressure / CO2 in Mix] - 14.7
= [17.7 / 0.25] - 14.7
= 56.1​

I can play around with any of these inputs within conventional ranges, it always puts the last variable outside of said conventional ranges.

Do you run a stout faucet with a nitro setup? If so, what do you spec it at?

I do not run nitro or a stout tap. All of my writing on carbonation is based on well established principles of physical chemistry. If you find any errors in my physical chemistry, please let me know, so I can correct any errors. I suspect many folks run lower than they realize carb levels on their nitro systems. If they are happy with the results, then no problem. I'm not a stout expert or aficionado, but what I have read indicates that they are often served at quite low carb levels.

Brew on

 

[TAK]

Okay, no offense, but... I appreciate the formulas, however I feel that my question hasn't really been answered, and instead my thread was derailed by a technical discussion without reference to practical application. I don't dispute the math, but it still doesn't reconcile to my experience. With the tank I had, whatever that mix may have been, if I took any beer that only has the carbonation left over from ferments (which will still be a little more than equilibrium with atmospheric pressure), it won't really pour with a head at all from my stout tap. However, if I carb it to 1.something, it will pour with a good head. Also, if I put an undercarbed beer on my nitro tap, it eventually equalizes to whatever partial pressure of CO2 I had in that tank, and eventually it also pours with the same good head as if it'd been precarbed.

I'm confident that my tank had 20-30% CO2. If I had my tank filled based on the math we've discussed, I'm confident a tank of 40-45% CO2 isn't going to give me a nice nitro pour. I don't know how to reconcile the math to this practical experience, but that's a moot point.

I'll re-pose the question. For those running a beergas mix on a nitro tap who get a known mix of beergas, what mix has worked best for you?

 

[YeastMode]

75% nitro 25% co2.

I fill the headspace and purge with co2. Then I chill to serving temp and carb with beer gas mix until desired carbonation is reached. Then I serve through a nitro tap. I don't know if this correct but it works for me.

 

[TAK]

75% nitro 25% co2.

I fill the headspace and purge with co2. Then I chill to serving temp and carb with beer gas mix until desired carbonation is reached. Then I serve through a nitro tap. I don't know if this correct but it works for me.

Thanks for the feedback. What PSI do you push at, and what temp?

 

[YeastMode]

Thanks for the feedback. What PSI do you push at, and what temp?

The serving temperature is at my usual beer serving temperature of ~38* to 40* (give or take how accurate my controller is that particular day) and the PSI for the nitro set up is between 22 and 30 depending on the balance of the system. Honestly, I do it more by feel than anything else, but that is always where I start and it gets me close to where I need to be.

and @doug293cz answer would be more technically correct, because it accounts for more variables and compensates fro them. its math, and there is only one answer in math, which was his. To use an analogy, his explanation would get you the right answer every time, where mine is basically giving the whatever face and going "close enough."

Now that he explained it, I will probably run the calculations for my new rolling keezer set up because math, and as my dad used to say, "I may talk out of my a$$, but my a$$ is very knowledgeable."

EDIT: I just read through doug's explanation again, and I want to edit to point out that I try to carb my stouts on the very low side, like 1.4 or 1.5 volumes. Remember that the nitrogen doesn't really want to stay soluble, so you need the beer to be cold and the pressure to be high. If I carbed to lets say 3 volumes of Co2, and then served at 30 with beer gas, I would probably end up with a foamy mess. The way I understand it, the entire point of nitro is to get that small bubble release when poured so you have that creamy texture.

 

[55x11]

The serving temperature is at my usual beer serving temperature of ~38* to 40* (give or take how accurate my controller is that particular day) and the PSI for the nitro set up is between 22 and 30 depending on the balance of the system. Honestly, I do it more by feel than anything else, but that is always where I start and it gets me close to where I need to be.

and @doug293cz answer would be more technically correct, because it accounts for more variables and compensates fro them. its math, and there is only one answer in math, which was his. To use an analogy, his explanation would get you the right answer every time, where mine is basically giving the whatever face and going "close enough."

Now that he explained it, I will probably run the calculations for my new rolling keezer set up because math, and as my dad used to say, "I may talk out of my a$$, but my a$$ is very knowledgeable."

EDIT: I just read through doug's explanation again, and I want to edit to point out that I try to carb my stouts on the very low side, like 1.4 or 1.5 volumes. Remember that the nitrogen doesn't really want to stay soluble, so you need the beer to be cold and the pressure to be high. If I carbed to lets say 3 volumes of Co2, and then served at 30 with beer gas, I would probably end up with a foamy mess. The way I understand it, the entire point of nitro is to get that small bubble release when poured so you have that creamy texture.

doug's math is correct. and so is TAK's calculation:
"Serving Pressure = [CO2 Partial Pressure / CO2 in Mix] - 14.7
= [17.7 / 0.25] - 14.7
= 56.1
"

I suspect most nitro stouts are carbonated at maybe only 1 volume of CO2 or so - Guinness Draught is carbonated at about 1.1-1.2 volumes.
Because most of the nitrogen will come out of the solution as well, that will add to the CO2 head, so you really need to carb it very lightly.

Also don't forget that a beer fermented at around 68 deg F will naturally contain about 0.75 volumes of CO2 already from yeast.

Typically beer gas is 25% CO2 /75% Nitrogen or 30%/70%.

Assuming you need 1.1 volume carbonation at 45F gives you about 37 psi of 25/75 beer gas.

 

[TAK]

So, it turns out that the place that said they'd order whatever mix I need doesn't make it so easy to do so. I ended up going back to the same place I got my beer mix from before, and this time I was able to get more info. It's 25% CO2 / 75% N2.

Incidentally, I stumbled upon a good deal for a second keezer, and took the opportunity to set it up to pour my nitro from a more moderate temp than my kegorator. My Kegorator is usually hovering just above 40F, and I set up my keezer to be at or just above 50F. I've been pouring a pale ale off it, and it is great to have a quaffable beer at that temp.

But I digress. At the higher temp, I noticed at first that my pours were more foamy than normal. Which doesn't surprise me. I probably slightly over-carbed. After a while, the keg quickly took a turn in the other direction, and my pours have much less foam. I'm sure this is because the once over-carbed beer equalized to the lower partial pressure from my beergas mix. The math above seems to match this practical experience. In any case, this gets me to my new question...

How easy is it to mix my own beer gas from a CO2 tank and an N2 tank. Is it as easy as using a Y-splitter between the respective regulators and the keg, like so?....

 

[day_trippr]

[...]How easy is it to mix my own beer gas from a CO2 tank and an N2 tank. Is it as easy as using a Y-splitter between the respective regulators and the keg, like so?....

Not that simple, you actually need a blender...

http://www.micromatic.com/draft-keg-beer-edu/dispensing-beer-blended-gases-aid-119.html

Cheers!

 

[55x11]

So, it turns out that the place that said they'd order whatever mix I need doesn't make it so easy to do so. I ended up going back to the same place I got my beer mix from before, and this time I was able to get more info. It's 25% CO2 / 75% N2.

Incidentally, I stumbled upon a good deal for a second keezer, and took the opportunity to set it up to pour my nitro from a more moderate temp than my kegorator. My Kegorator is usually hovering just above 40F, and I set up my keezer to be at or just above 50F. I've been pouring a pale ale off it, and it is great to have a quaffable beer at that temp.

But I digress. At the higher temp, I noticed at first that my pours were more foamy than normal. Which doesn't surprise me. I probably slightly over-carbed. After a while, the keg quickly took a turn in the other direction, and my pours have much less foam. I'm sure this is because the once over-carbed beer equalized to the lower partial pressure from my beergas mix. The math above seems to match this practical experience. In any case, this gets me to my new question...

How easy is it to mix my own beer gas from a CO2 tank and an N2 tank. Is it as easy as using a Y-splitter between the respective regulators and the keg, like so?....

This won't work. If N2 pressure is higher it will quickly close the check valve on CO2 side (since pressure on the other side is higher than CO2).

 

[TAK]

(sad face)

That's a toy I probably shouldn't buy, as much fun as it sounds. Oh well, makes sense.

Short of that, maybe I'll just get in the habit of bleeding the beergas mix from the headspace part way through the keg and supplementing with some extra CO2 before re-applying the beergas. Certainly not an exact science, but that should help mitigate it re-equalizing to an undercarbed level if my beergas mix isn't rich enough in CO2.

The extra dose of CO2 wouldn't equalize back to the beergas tank, right? I believe at rest the check valve is in a closed state. So, a higher proportion of CO2 in the keg wouldn't equalize backwards, unless I'm wrong.

 

[ChelisHubby]

No, I did not say that a gauge on a beer gas regulator measures only the CO2 pressure, because it doesn't. No mechanical pressure gauge can measure the pressure of a single gas in a mixture. Not sure how you interpreted what I wrote to mean the opposite. I reread my posts, and can't see any way that they could be interpreted to say a gauge could measure just the CO2 in a mixture.

What I did say is that only the pressure exerted by CO2 affects the level of carbonation. If there is nitrogen, oxygen, or any other gas present in a mixture with CO2, each gas will have its own contribution to the total pressure. But those other gases won't affect the carbonation level. The CO2 contribution to the total pressure (known as the CO2 partial pressure) depends only on the volume of the headspace, temperature, and number of CO2 molecules present in the headspace. The CO2 partial pressure is unaffected by the presence of other gases. The other gases have their own partial pressures, and they also are independent of the other gases present. The total, or absolute pressure is the sum of the partial pressures of all of the gases present in the headspace. The gauge pressure is the difference between the total pressure and the air pressure outside the keg (i.e. atmospheric pressure.)

Brew on

Thanks that dumbed it down to my level, now I have some understanding. After all this has been said what mix would you suggest and please remember I am about done with my second bomber. What are the advantages to using more or less co-2 in the beer gas mix.

 

[doug293cz]

Thanks that dumbed it down to my level, now I have some understanding. After all this has been said what mix would you suggest and please remember I am about done with my second bomber. What are the advantages to using more or less co-2 in the beer gas mix.

The "best" mix ratio of CO2/N2 depends on the desired volumes of CO2 in the beer, the desired serving temperature, and maintaining a reasonable serving pressure.

Let's assume you want 1.2 volumes of carb, and want to serve at 50˚F. First determine the partial pressure of CO2 you need to maintain 1.2 volumes at 50˚F. The chart says you need ~1 psi gauge pressure of pure CO2, which converts to a required CO2 partial pressure of 1 psi + 14.7 psi = 15.7 psi. The total absolute serving pressure is then equal to the CO2 partial pressure divided by the percentage of CO2 in the gas mix. If we have a 25% CO2/75% N2 mix, then the absolute pressure would be 15.7 / 0.25 = 62.8 psi. The gauge pressure needed for serving would then be 62.8 psi - 14.7 psi = 48.1 psi. This is kind of a high serving pressure. However, if the mix was 40% CO2/60% N2, then the absolute pressure would only need to be 15.7 psi / 0.40 = 39.25 psi, or 39.25 psi - 14.7 psi = 24.5 psi gauge pressure. This would be a much more reasonable serving pressure.

Now if you wanted to serve beer with 1.2 volumes at 38˚F, you need a CO2 gauge pressure of -2.24 psi, or 14.7 - 2.24 = 12.46 psi partial pressure. With 25% CO2 mix you would need a serving pressure of 12.46 / 0.25 = 49.84 psi absolute, or 49.84 - 14.7 = 35.14 psi gauge. This is a high but manageable serving pressure.

So, low CO2 ratio beer gas mixes are better for very low carb levels and/or low serving temp situations. High CO2 ratio mixes are better for low to medium carb levels and/or warmer serving temperatures.

All of the above assumes the beer starts at the desired carb level, and that you want to maintain the carb level at serving temp and pressure.

Brew on

 

[Yeastieboy]

Mine is 75% Nitro / 25% CO2

 

[Gustatorian]

The "best" mix ratio of CO2/N2 depends on the desired volumes of CO2 in the beer, the desired serving temperature, and maintaining a reasonable serving pressure.

Let's assume you want 1.2 volumes of carb, and want to serve at 50˚F. First determine the partial pressure of CO2 you need to maintain 1.2 volumes at 50˚F. The chart says you need ~1 psi gauge pressure of pure CO2, which converts to a required CO2 partial pressure of 1 psi + 14.7 psi = 15.7 psi. The total absolute serving pressure is then equal to the CO2 partial pressure divided by the percentage of CO2 in the gas mix. If we have a 25% CO2/75% N2 mix, then the absolute pressure would be 15.7 / 0.25 = 62.8 psi. The gauge pressure needed for serving would then be 62.8 psi - 14.7 psi = 48.1 psi. This is kind of a high serving pressure. However, if the mix was 40% CO2/60% N2, then the absolute pressure would only need to be 15.7 psi / 0.40 = 39.25 psi, or 39.25 psi - 14.7 psi = 24.5 psi gauge pressure. This would be a much more reasonable serving pressure.

Now if you wanted to serve beer with 1.2 volumes at 38˚F, you need a CO2 gauge pressure of -2.24 psi, or 14.7 - 2.24 = 12.46 psi partial pressure. With 25% CO2 mix you would need a serving pressure of 12.46 / 0.25 = 49.84 psi absolute, or 49.84 - 14.7 = 35.14 psi gauge. This is a high but manageable serving pressure.

So, low CO2 ratio beer gas mixes are better for very low carb levels and/or low serving temp situations. High CO2 ratio mixes are better for low to medium carb levels and/or warmer serving temperatures.

All of the above assumes the beer starts at the desired carb level, and that you want to maintain the carb level at serving temp and pressure.

Brew on

Sorry to just on this forum months later – was redirected by other members of the page.

So I have a beer gas mix of 75%N2/25%CO2. I initially quick-carbed my stout to 1.6 volumes CO2 and I'm looking to maintain that CO2 level after I put the beer gas tank on. If I'm pouring beer at 40ºF, what would my tank need to be set at in order to maintain that. If I'm doing the math above, it looks to be around 52.1 PSI on the beer gas regulator. That seems extremely high to me, as the guy who filled the tank told me to keep the regulator set at 30-32 PSI...

 

[doug293cz]

Sorry to just on this forum months later – was redirected by other members of the page.

So I have a beer gas mix of 75%N2/25%CO2. I initially quick-carbed my stout to 1.6 volumes CO2 and I'm looking to maintain that CO2 level after I put the beer gas tank on. If I'm pouring beer at 40ºF, what would my tank need to be set at in order to maintain that. If I'm doing the math above, it looks to be around 52.1 PSI on the beer gas regulator. That seems extremely high to me, as the guy who filled the tank told me to keep the regulator set at 30-32 PSI...

I get 54.1 - 54.2 psig to maintain 1.6 volumes of carb at 40ºF with 75/25 N2/CO2 gas mix. If you use 32 psig, your beer will eventually equilibrate at about 1.1 volumes.

Brew on

 

[Gustatorian]

I get 54.1 - 54.2 psig to maintain 1.6 volumes of carb at 40ºF with 75/25 N2/CO2 gas mix. If you use 32 psig, your beer will eventually equilibrate at about 1.1 volumes.

Brew on

Thanks for figuring that out, do you mind showing me the math?

 

[doug293cz]

Thanks for figuring that out, do you mind showing me the math?

Based on the following references, we can develop two equations: one for CO2 volumes as a function of temperature and pressure, the other for pressure as a function of volumes and temperature.

https://www.homebrewtalk.com/showpost.php?p=7564238&postcount=2
http://www.wetnewf.org/pdfs/Brewing_articles/CO2%20Volumes.pdf​

For pressures given as psi gauge pressure and temps in ˚F, we have the following:

V =(P+14.695)*(0.01821+0.090115*exp(-(T-32)/43.11))-0.003342
V+0.003342 = (P+14.695)*(0.01821+0.090115*exp(-(T-32)/43.11))
(V+0.003342)/(0.01821+0.090115*exp(-(T-32)/43.11)) = P+14.695
P =(V+0.003342)/(0.01821+0.090115*exp(-(T-32)/43.11))-14.695​

For pressures given as psi absolute pressure and temps in ˚F, we have the following:

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

When working with mixed gases you have to use the absolute pressure equations until the very end, and then convert to gauge pressure by subtracting 14.7 psi (more accurately 14.695 psi), or whatever the proper atmospheric pressure is for your elevation.

First step is to calculate the CO2 pressure needed to get your desired carb volumes at your target serving pressure. Then you divide the CO2 pressure by the fraction of CO2 in the gas mix (divide by 0.25 in your case) to get the total pressure required in the keg. Then subtract 14.7 psi to get the gauge pressure needed for your beer gas mix.

To figure out the equilibrium volumes of CO2 for a particular beer gas gauge pressure: Add 14.7 psi to the gauge pressure to get the absolute pressure. Then, multiply the absolute pressure by the fraction of CO2 in the mix to get the CO2 partial pressure. Finally put the CO2 partial pressure and temperature in the equation for volumes.

All of the above is a lot easier if you build a spreadsheet to do the various calculations.

Brew on

 

[Gustatorian]

Based on the following references, we can develop two equations: one for CO2 volumes as a function of temperature and pressure, the other for pressure as a function of volumes and temperature.

https://www.homebrewtalk.com/showpost.php?p=7564238&postcount=2
http://www.wetnewf.org/pdfs/Brewing_articles/CO2%20Volumes.pdf​

For pressures given as psi gauge pressure and temps in ˚F, we have the following:

V =(P+14.695)*(0.01821+0.090115*exp(-(T-32)/43.11))-0.003342
V+0.003342 = (P+14.695)*(0.01821+0.090115*exp(-(T-32)/43.11))
(V+0.003342)/(0.01821+0.090115*exp(-(T-32)/43.11)) = P+14.695
P =(V+0.003342)/(0.01821+0.090115*exp(-(T-32)/43.11))-14.695​

For pressures given as psi absolute pressure and temps in ˚F, we have the following:

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

When working with mixed gases you have to use the absolute pressure equations until the very end, and then convert to gauge pressure by subtracting 14.7 psi (more accurately 14.695 psi), or whatever the proper atmospheric pressure is for your elevation.

First step is to calculate the CO2 pressure needed to get your desired carb volumes at your target serving pressure. Then you divide the CO2 pressure by the fraction of CO2 in the gas mix (divide by 0.25 in your case) to get the total pressure required in the keg. Then subtract 14.7 psi to get the gauge pressure needed for your beer gas mix.

To figure out the equilibrium volumes of CO2 for a particular beer gas gauge pressure: Add 14.7 psi to the gauge pressure to get the absolute pressure. Then, multiply the absolute pressure by the fraction of CO2 in the mix to get the CO2 partial pressure. Finally put the CO2 partial pressure and temperature in the equation for volumes.

All of the above is a lot easier if you build a spreadsheet to do the various calculations.

Brew on

The equations are a bit over my head, but based on the CO2 chart that I have, maintaining 1.6 volumes of CO2 at 40ºF would take 2PSI of pure CO2 (this is based on the CO2 chart given to me at a LHBS - I assume it's universal). If I take that 2 PSI, add 14.7 atmospheric pressure, I get 16.7. 16.7 divided by 0.25 (the amount of CO2 in my beer gas) yields 66.8 PSI, which I remove 14.7 from and get 52.1 PSI. This seems unbelievably high and, by your calculations, you're getting an even higher 54.1?

 

[schatzke]

While you guys are on the subject, can beer gas go into a regular CO2 tank or does it have to be a nitrogen tank? I have a spare CO2 regulator so if I could just fill a spare CO2 tank I wouldn't need a new regulator.