stout on nitro, c02 question...

[morrillt]

If i insist on serving my stout at 43 (because I have other beers in the kegerator)

And I want to have 1.5 c02 dissolved.

According to temp charts i need between 4-5 psi c-2 to get to 1.5 dissolved c02

When i switch over to beergas....

If i do the classic 75 25 split at 30psi thats essentially putting me at like 7.5 psi's of c02, which is too much carbonation right?

I am thinking i either drop the pressure to 20, or I change my beer gas ratio to like 15% c02,

SO my question is which aprroach is better, and is my logic sound? I keep on getting over carbed nitros.....

thanks
TOdd

 

[Nate]

I'm not sure how you'll get a custom beer gas unless your gas company is somehow willing to do that for you. Quite frankly, I'm not sure you'd even want to go down that road. Basically, beer gas is beer gas.

Most of our beers have been nitro'd and my experience has been that the beer itself (how heavy it is, sugars, protein, etc), your physical keg/tap setups and your keg temperatures are going to determine what beer gas pressure you should run. We've had some beers that were beautiful on nitro and easy to setup while others simply didn't do well on nitro and I had to play with pressure quite a bit (a couple of them probably should have been setup on just CO2).

 

[day_trippr]

According to our favorite carbonation table, dispensing at 43°F would require a scant 3 psi on straight CO2 to hit 1.5 volumes of carbonation. Woof.

If you're trying to drive the typical stout faucet, you want to be running around 30 psi (I run a war club of a MicroMatic SS stout faucet at 35 psi). So you're looking at 10/90 CO2/Nitro to maintain a 1.5 volume beer with a set pressure of 30 psi.

I run what's pretty much a standard - 25/75 - which comes straight out of the tank farm at my lhbs (no blender). Finding someone that'll dial up 10/90 could be a challenge.

A pricey alternative would be a gas blender, but 10/90 is going to be a custom order on top of the exotic price...

Cheers!

 

[tgif]

I'm pretty sure stout faucets have restrictor plates to help knock gas out of solution to make a better head, so the actual volumes of CO2 in the glass should be lower than the keg.

 

[day_trippr]

I'm pretty sure stout faucets have restrictor plates to help knock gas out of solution to make a better head, so the actual volumes of CO2 in the glass should be lower than the keg.

And so ?

Cheers!

 

[tgif]

Give it a shot and see what happens anyways, you should probably be fine

 

[morrillt]

So, why do i need 10 90 and everyone else 25, is it just everyone else serves at 55 or something ? Or people are cool with higher carbonation? Over carbonation has plagued me in the past with the stout faucet going berserk, so I am tryhing a more sensitive approach this time.....

 

[morrillt]

Day tripper, I am curious how you avoid over carbing, what temp are your beers stored at? I find that dissolving 2litres of c02 seems to completely make my nitro tap a gassy mess.... no cascade just foam....

According to our favorite carbonation table, dispensing at 43°F would require a scant 3 psi on straight CO2 to hit 1.5 volumes of carbonation. Woof.

If you're trying to drive the typical stout faucet, you want to be running around 30 psi (I run a war club of a MicroMatic SS stout faucet at 35 psi). So you're looking at 10/90 CO2/Nitro to maintain a 1.5 volume beer with a set pressure of 30 psi.

I run what's pretty much a standard - 25/75 - which comes straight out of the tank farm at my lhbs (no blender). Finding someone that'll dial up 10/90 could be a challenge.

A pricey alternative would be a gas blender, but 10/90 is going to be a custom order on top of the exotic price...

Cheers!

 

[day_trippr]

25/75 beer gas at 35 psi into beer at 40°F comes out to about 2 volumes, which is a satisfying level for me. It took a little experimentation with the pressure but the stout faucet I have (the aforementioned Micro Matic war club) works great with those parameters...

Cheers!

 

[doug293cz]

Figuring out volumes of carbonation when using beer gas is not a simple as when using straight CO2. You can't just ratio the psi from a carbonation table or calculator by 4X for the 25/75 CO2/N2 mix.

Volume of carbonation depends on temperature and CO2 partial pressure. If the headspace is pure CO2 then the partial pressure is the absolute pressure in the headspace. However, the gauge only measures the difference between the headspace pressure and atmospheric pressure (14.7 psi at sea level.) This is known as gauge pressure, and is the value that is used in the charts because it is directly measurable. So, if you want 1.5 volumes at 43°F, then you need a gauge pressure of 2.4 psi when using pure CO2. This means the CO2 partial pressure required is 17.4 psi.

Now when using 25/75 beer gas, the CO2 partial pressure is only 25% of the total absolute pressure. So, for the CO2 partial pressure to be 17.4 psi, the total absolute pressure needs to be 68.4 psi. At 68.4 psi the gauge pressure would be 68.4 - 14.7 = 53.7 psi.

Now if you set the beer gas regulator to 30 psi, then the total absolute pressure is 44.7 psi, and the CO2 partial pressure (which determines equilibrium volumes of carbonation) will be 44.7/4 = 11.2 psi. 11.2 psi of CO2 partial pressure at 43°F gives 0.99 volumes of carbonation. So, your beer on 30 psi of beer gas would eventually drop to a carb level of .99 volumes. (It's actually a little more complicated than just described, and the carb level won't actually drop that much.)

Brew on

 

[day_trippr]

^That is why I work with just ones and zeroes

So it's the absolute pressure versus mix ratio that determines the carbonation level, not the relative (indicated) pressure?
I don't actually understand why that's the case but I'll take it at face value, as it helps explain why my setup actually works well...

Cheers!

 

[doug293cz]

^That is why I work with just ones and zeroes

So it's the absolute pressure versus mix ratio that determines the carbonation level, not the relative (indicated) pressure?
I don't actually understand why that's the case but I'll take it at face value, as it helps explain why my setup actually works well...

Cheers!

Close.

tl;dr: The carb level depends only on the CO2 partial pressure, but the CO2 partial pressure depends on the gas mix ratio and the total pressure. All pressures are referenced to vacuum.

It's the absolute pressure (pressure compared to total vacuum) of CO2 alone (and the temperature) that determines carb level. When mixed with other gases, the absolute pressure of the CO2 alone is known as the CO2 partial pressure. The presence of other gases does not effect the partial pressure of the CO2.

When a volume contains only CO2, the total absolute pressure is equal to the CO2 partial pressure. If you add 3X the number of N2 molecules to the volume previously containing pure CO2, the N2 has 3X the partial pressure of the CO2, and the total absolute pressure is now 1X + 3X = 4X the pressure with CO2 alone. So, the CO2 partial pressure is now 1/4 the total pressure, and the gas mix ratio is 25/75 CO2/N2.

You cannot measure partial pressures, you can only measure total pressure. And unless you have a special type of pressure gauge, you can only measure the total pressure difference between the pressurized volume and the atmosphere. You calculate absolute pressure by adding gauge pressure to atmospheric pressure (14.7 psi at sea level.) So, to determine the partial pressure of CO2 in a beer gas system, you add 14.7 to the gauge pressure and then multiply by the fraction (percentage/100) of the CO2 in the gas mix.

Now if you want to use the carbonation charts or calculators, you have to convert the CO2 partial pressure into an equivalent gauge pressure for pure CO2. You do this by subtracting the local atmospheric pressure (usually 14.7 psi) from the CO2 partial pressure.

Whew!

Brew on