How Much CO2 Pressure Lost in Pressure Transfer?

[Clint Yeastwood]

I serve my stout at 35 F on beer gas, and the pressure I use is around 32 psi. Sometimes I go up to 35 psi, so I figure about 8 to 8.4 psi come from CO2. The familiar chart says that's somewhere around 2.4 volumes. Seems awfully high, but the beer is perfect, so I don't question it.

I'm going to pressure-transfer a new stout tomorrow. I plan to use CO2 to move it, because beer gas is harder to get, and something could go wrong. I will then purge the CO2 from the serving keg and apply beer gas. I don't want a lot of CO2 in there when the pressure goes up.

So how much CO2 should be in the stout before I move it, to compensate for what I lose in the transfer? If I want 8 psi of CO2 after the transfer, where should I be before I move the beer? Right now, at about 68, it's at 10 psi. An online calculator thinks I'll be at 7.53 psi when I get down to 35 degrees if I don't add gas.

A little late to ask, I guess. I just turned the fermenting fridge down to 35 from 68, so I won't see 35 until tomorrow. Not much time to increase the carbonation.

 

[day_trippr]

Sometimes I go up to 35 psi, so I figure about 8 to 8.4 psi come from CO2.

That's not how it works. You have forgotten the lessons that @doug293cz has provided on this specific topic in an earlier thread of yours.
https://www.homebrewtalk.com/threads/how-to-carbonate-stout-on-beer-gas.725190/#post-10266911
The important, relevant bit:

"At 35°F and 32 psig beer gas at 25/75, the equilibrium carbonation level is 1.19 volumes, which is totally acceptable for a nitro stout."

As for carbonation loss from racking...I wouldn't worry about it or try to outfox it during the transfer. Just give it a little time on the back end to reach your desired equilibrium again...

Cheers!

 

[VikeMan]

That's not how it works. You have forgotten the lessons that @doug293cz has provided on this specific topic in an earlier thread of yours.
https://www.homebrewtalk.com/threads/how-to-carbonate-stout-on-beer-gas.725190/#post-10266911

@Clint Yeastwood I recommend downloading the spreadsheet @doug293cz attached to that post. There are no rules of thumb that will get the right answer when a gas mix is involved, e.g. you can't divide the PSIG by 4 and plug that into the standard CO2 carb charts.

 

[Clint Yeastwood]

Thanks for reminding me of the old thread. I just realized I have no idea whether the percentages in the gas refer to moles, volume, mass, or what.

Interesting thing: I used absolute pressures and Kelvins to get the final keg pressure at 35, and I ended up with 8.77 psi, whereas the online calculator I used out of laziness gave 7.53. I told ChatGPT to write a simple Pascal program for future use, and after maybe a dozen tries, it still hasn't written one that works. Bad news for kids using it to do their homework. It told me the final gauge pressure would be negative. That was interesting. It can't get the final temperature right to save its virtual life.

I converted Fahrenheit to Kelvins, which is not rocket science, and then I used (P/T = constant) to get the end pressure. I get an initial temperature of 293.15, a final temperature of 278.55, and I used an initial absolute pressure of 24.7 psi.

I would still like to know if anyone has information, from actual experience, about CO2 pressure loss from transfer.

 

[Clint Yeastwood]

I don't really know what a volume is. Reading up. The web says it's one liter of CO2 in one liter of beer, but it doesn't give the temperature or pressure of the CO2. I suppose this means a volume contains more or less molecules depending on the temperature and pressure. It must not be a measure of mass or moles. So engineers developed this term in order to have something that gives you the same amount of fizz at different temperatures and pressures? That sounds like engineers to me.

The volume of gas in a beer does not increase linearly with gauge pressure at the same temperature, so whatever the molar percentage of CO2 is in beer gas may be, you can't say doubling or tripling the gauge pressure will double or triple the volume or the fizz. I guess.

CO2 volume increases way faster than absolute CO2 pressure, so that's annoying. Hmm.

So this is why we like the chart, then.

 

[day_trippr]

I would assume all measurements are normalized to STP.

For calculating "volumes of CO2", the temperature of the CO2 can be assumed to be that of the beer, and the temperature of the beer and the CO2 pressure are whatever you want to plug in. Our favorite carbonation table has done all the work for you

Cheers!