Pressure ~ still versus carbonated beverage, room temp versus chilled

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Cider Wraith

Well-Known Member
Aug 11, 2022
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Looks like the Vols to PSI @ temp charts are intended as guidance on carbonating still beverage. For example, in a five gal corny at room temp with one to two gals headspace over still beverage, if tank pressurizing to about 30 PSI, after a being chilled a couple of days, CO2 will have been absorbed into solution and pressure will drop into single digits. So, headspace pressure over still beverage can drop by more than two-thirds given time and temperature. Multiple things are going on, liquid and headspace volume, temperature, time and I appreciate CO2 will move into or out of solution over temperature, time to achieve stasis.

Next, with similar conditions, if pressure fermenting the beverage will have already absorbed maximum CO2 at given pressure, temperature, time. So if pressure fermenting to about 18 PSI observation seems after being chilled headspace pressure drop is slight, maybe only a few PSI to around 12 ~ 14. Seems to make sense given that the beverage is already saturated with CO2, the difference being only the change necessary for equilibrium at the new temperature. If so, then seems one could simply pressure ferment to a few PSI over serving?

If what's described above seems correct, do charts or formulas exist that show headspace pressure change based upon temperature change alone in sealed systems at equilibrium?

You folks would have described this far better, maybe the above assumptions are all wrong. Feedback / words of wisdom appreciated
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The standard carbonation chart can be used diagonally: If you pressure fermented, the temp and pressure on that chart should tell you where you are at... as temperature drops, if you don't apply additional pressure there will be some CO2 loss, but I'm not sure how much. As I'm not certain which vessels you're referring to, I'll suggest this: Look up the carb level on the standard chart:
carbChartAND Quickstructions_1_0.jpg

..Adjust your pressure for your desired volume to the current temperature (wether in fermenter or keg) and, following the chart diagonally, it will maintain that volume level as it cools to your target temperature.
If you're pressure fermenting at room temps, unless your pressure is in the high 20-30PSI range, you still will need a bit more time, but the standard chart, when used diagonally will inform you of volume decrease with a temperature drop.
Thanks for the reply ... ok working on this ...

18 PSI at 65 F showing 1.87 Vols

Going to look for the closest to that is:

1.86 Vols at 34 F showing PSI 3

So in a sealed system, at stasis, a temperature drop from 65 F to 34 F results in a pressure drop of 15 PSI? Seems high, sounds right?

If so, with everything else being equal, in a sealed vessel (like a commercial bottle of beverage) a cool down from room temp to fridge temp causes a five-sixths pressure decrease?

Edit - went back and looked at that chart and the title includes, "Volumes of CO2" ... so maybe it's strictly for how gas would perform in a vessel without liquid or taking into account gas moving into and out of solution?
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Got some results. Three batches pressure fermented at about 65 F to about 18 PSI and given CO2 saturation into beverage, when refrigerated down to 30s F seems to have experienced a pressure drop of about 2 PSI, down to about 16 PSI. So not much, and again, that's given a large head space volume equivalent to beverage volume, about 2.5 gals each. Another way to have approached the question might have been to have moved a refrigerated keg at serving pressure to room temp for a few days and wouldn't that resulting PSI be the target fermenting pressure.

Next effort might be to just guesstimate pressure fermenting to about 14 PSI refrigerates down to serving.

What have you had success with - feedback welcomed -
Very curious...
I'm finishing a cider in the keg by spunding and it is currently sitting at 12-13 psi (in fermentation fridge at 21 oC)
In a day of what, I intend moving keg + spunding valve to the fridge and observe what the pressure drop will be.
I'll report back
So in a sealed system, at stasis, a temperature drop from 65 F to 34 F results in a pressure drop of 15 PSI? Seems high, sounds right?
The "at stasis" part is important. When a fermented bev is spunded during fermentation, it's reasonable to assume the carbonation level (measured in volumes of CO2) is at equilibrium per the chart based on the liquid temperature and gauge pressure. That is to say that when fermentation is done and no more pressure is being added, as long as the temperature remains stable, the pressure and carbonation level will remain the same.

When liquid is still and not fermenting and you apply a constant head pressure (not just pressurize and disconnect), the chart's carbonation level based on temperature and pressure, is where it will EVENTUALLY reach equilibrium. How fast it arrives at that level is primarily based on the ratio of surface area between the liquid and CO2 headspace and the total volume of liquid. This rate can be increased significantly by blowing CO2 through a fine stone or by agitation to create tiny bubbles. I.e. increasing surface area. In a typical keg, without any of the rushing tactics, that equilibrium can take more than 2 weeks.

One way to know if a beverage has taken all the CO2 it's going to, at a given temperature and pressure, is to disconnect the CO2 source and simply replace the gas supply hose with a pressure gauge. If the headspace pressure doesn't drop over a couple hours, it's at equilibrium.

In a closed system, the pressure is predicable based on the chart but it gets tricky because the headspace will warm up or cool down a lot faster than the liquid so pressure measurements will be artificially high or low depending on whether you're in the process of heating up or cooling down.
Now for the emperical test :)
Sample size of 1

I've moved my cider with spunding valve to the fridge
14 psi at 21 oC (which would mean 1.6 volumes CO2)

The pressure is indeed dropling, but not (yet) as low as expected.
The fridge is off at night so also not yet as cold as should be.
I'll keep observing

And with a little luck (carbonation wise), there is still some yeast activity to bring it back up to the required volume CO2.

I don't know how much yeast activity is reduced by increased pressure and decreased temperature.

I'll keep observing
The "at stasis" part is important. When a fermented bev is spunded during fermentation, it's reasonable to assume the carbonation level (measured in volumes of CO2) is at equilibrium per the chart based on the liquid temperature and gauge pressure......
Thanks for this thoughtful reply -
I think it is more important to focus on volume of CO2 rather than the pressure as pressure is bound with temperature. So you would say you carbonated to 1.87 vol rather than 18 PSI etc... The chart comes into play when you know your Volume target but want to match current or future temperatures.