# Question about Co2 and fermenting under pressure

Discussion in 'Beginners Brewing Forum' started by KenBrew, Aug 9, 2020.

1. ### KenBrew New Member

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#1
Hey there!

My question is basic, how much CO2 does a beer fermented under pressure at 5 PSI at 68 degrees Fahrenheit produce? Does the fact that it is under pressure increases the residual level of carbon dioxide?

Thanks !

2. ### thunderwagn Well-Known Member

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#2
I'd say 1 vol or a bit less even. If you're looking for a good place to start with, I'd recommend setting your spunding valve at 12 psi. I'm over 5000ft above sea level and have mine set at 15. My beer is almost perfectly carbonated once fermentation is done.
You won't be increasing the residual level just because you're fermenting under pressure. You are still, or should be releasing pressure in excess of what you have your spunding pressure set to. Hope that makes sense or helps.

3. ### KenBrew New Member

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#3
The idea is to obtain the co2 level through the conditioning in the bottle. So I want to do the calculation of the amount of OZ of corn sugar .. thanks, what I really wanted to know was that if fermentation under pressure increases the residual level of carbon dioxide .. thanks!

4. ### Hawkbox Well-Known Member

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#4
It will increase CO2 cause that is what causes the pressure. The math is figuring out how much.

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5. ### thunderwagn Well-Known Member

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#5
How do you plan on bottling? I ask because your normal method of transferring beer to a bottling bucket and using a bottle filler wand is not going to work if you've fermented under pressure. As soon as you release the pressure from the vessel, you've got a foam volcano coming. You could transfer to keg under pressure and assuming your beer is properly carbonated, you can use a beer gun to bottle.
I don't think what you're looking to do is going to work, if your plan is to carb under pressure to a certain level and then add sugar to get to the finishing level of carbonation.

6. ### Nosybear Well-Known Member

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#6
The same amount as fermentation at normal pressure. Every 342g of maltose (1 mole) produces 176g of carbon dioxide (4 moles). The residual CO2 is dependent on pressure but I don't have that table handy.

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7. ### KenBrew New Member

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#7
That is correct, it's exactly what I want to do.. I just wanted to know the residual level of carbon dioxide after fermentation so I don't overprime the beer before bottling...

8. ### thunderwagn Well-Known Member

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#8
Best advice I can give is that if you want to add sugar and bottle, skip fermenting under pressure, and just use a blow off tube. Unless you can fully carb and use a beer gun like I mentioned above, you're going to have a mess on your hands.

9. ### Hawkbox Well-Known Member

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#9
Yeah if you're planning to bottle anyway there is no value in trying to pressure ferment as well.

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