Force carbonation - changing the temp half way through the process

[suitbrewing]

Greetings to all

I have started to force carbonate two kegs while in my fridge @ 35f. My schedule is as follows:
- 24 hours @ 30psi
- De-gas and reduce to 12psi (will only be kept here for 36 hours)

I was meant to keep it @ 12psi for another few days (say 10 days) to stabilise the carbonation and then to disconnect from CO2 and to store until serving.

As I need the fridge to ferment a new brew the kegs will be taken out to room temp which is currently 68F.

Once the kegs reach room temp, should I rump up the pressure to whatever the calculators give me for 2.5 volumes @ 68F (27.6 psi) or would 12psi be sufficient.

The kegs will be placed in my kegerator and will be cooled to 35f prior to serving. This will probably happen in a couple of months.

 

[mongoose33]

Why don't you just finish force-carbing them cold and be done with it? To get to 12 psi I've been doing 36psi for about 24 hours with the beer at 32 degrees. It'll be close, maybe a little shy, but close.

I'd put them on 36 psi for another 4-6 hours and then see how they are, i.e., drop to serving pressure and (assuming you have one) use a picnic tap to check the carbonation.

 

[suitbrewing]

Many thanks for the feedback

The only reason why I don't go over 30psi is that I don't know how to calculate the rate at which "time needed to carb" is reduced as a function of higher pressure. So in fear of over carbonating I stick to what works.

I was also with the impression that in order to get a balanced keg (not under or over carbonated keg) you have to spend some time at the serving/"set and forget" pressure.

I hope my terminology makes sense.

If you were to crank up to 36psi and then remove from fridge, would you degas/ drop the pressure?

 

[mongoose33]

Many thanks for the feedback

The only reason why I don't go over 30psi is that I don't know how to calculate the rate at which "time needed to carb" is reduced as a function of higher pressure. So in fear of over carbonating I stick to what works.

I was also with the impression that in order to get a balanced keg (not under or over carbonated keg) you have to spend some time at the serving/"set and forget" pressure.

I've never read nor heard that. I just force carbed a beer last week; my usual thing and it was just fine.

I think too much attention is paid to hitting exactly some specified volume of CO2 in beer. If you're trying to win a competition, maybe, but otherwise it's just not that crucial. If it's a little high for your taste, release some pressure from the PRV a few times, or leave the PRV open for an hour or two, then see where you are.

It just seemed like you were doing all sorts of backflips to figure out something that isn't that crucial or that hard.

I hope my terminology makes sense.

If you were to crank up to 36psi and then remove from fridge, would you degas/ drop the pressure?

It depends on whether I'm drinking it now or later. If now, depressurize (I don't say "degas" because all you're doing w/ the PRV is releasing headspace pressure, not reducing the the volumes of CO2 in the beer itself). If later, I'd probably leave it alone.

 

[MagicMatt]

Not sure where you're getting your carbing info but I think it's a little off. You mentioned 35°F, 2.5vols, and 12psi.......but 2.5vols @ 35°F would be between 9-10psi. 12psi at that temp would give you about 2.75vols.

Not that it's a huge difference, but just wanted to make sure you had your numbers correct. As mongoose said, it's not incredibly important as long as you're in the ballpark.

In regard to your initial questions, yes you should be able to just pull the gas line from the kegs when removing them from the kegerator (don't vent the pressure). Let the kegs warm to room temp, and then you can hook it back up @ ~28-30psi to get your desired 2.5vols. Note it will take longer to reach a fully carbed state at the warmer temps. When it's time to move it back to the kegerator, let it chill for about a day before venting and putting the gas back on at serving pressure.

 

[doug293cz]

Not sure where you're getting your carbing info but I think it's a little off. You mentioned 35°F, 2.5vols, and 12psi.......but 2.5vols @ 35°F would be between 9-10psi. 12psi at that temp would give you about 2.75vols.

Not that it's a huge difference, but just wanted to make sure you had your numbers correct.

In regard to your initial questions, yes you should be able to just pull the gas line from the kegs when removing them from the kegerator (don't vent the pressure). Let the kegs warm to room temp, and then you can hook it back up @ ~28-30psi to get your desired 2.5vols. Note it will take longer to reach a fully carbed state at the warmer temps. When it's time to move it back to the kegerator, let it chill for about a day before venting and putting the gas back on at serving pressure.

The highlighted point is frequently stated, and believed to be true by many (most?) brewers. While it is well known that beer absorbs more CO2 at lower temps at a given pressure, I have never seen an authoritative reference that said rate of absorption was faster at lower temperatures. I believe the instantaneous rate of absorption is proportional to the difference between the headspace CO2 partial pressure and the equilibrium CO2 partial pressure for the current level of CO2 in the beer at the current temperature. But this says nothing about rate variation as a function of temperature. This means that cold beer will absorb CO2 faster at 12 psi than warm beer at 12 psi, but not that cold beer at 12 psi will absorb CO2 faster than warm beer at 30 psi. To put this in an equation:

Carbonation_Rate = C * (P[CO2] - P[eq])
P[CO2] = Headspace CO2 partial pressure
P[eq] = Equilibrium CO2 partial pressure for current carbonation level @ current temp​

We know how P[eq] varies with temperature, but do we really know how C varies with temperature?

Brew on

 

[suitbrewing]

Thank you both for your the informative answers.

I ended up taking the kegs out of the fridge and keeping them @ 12psi.
Its been 3 days now so I could proceed to crank up the pressure to 28psi and keep it there for a while!

If my understanding is correct once the beer absorbs the 2.5vols, keeping it @ 28psi will not make the beer over carbonate (i.e. the system becomes balanced)?? (the same way that 10-12 psi will not overcarb my cold beer in the kegerator).

I know I sound completely OCD about this thinks but my experience tells me that correcting an over carbed beer kept in a Sanke keg can be a real pain.

 

[doug293cz]

Thank you both for your the informative answers.

I ended up taking the kegs out of the fridge and keeping them @ 12psi.
Its been 3 days now so I could proceed to crank up the pressure to 28psi and keep it there for a while!

If my understanding is correct once the beer absorbs the 2.5vols, keeping it @ 28psi will not make the beer over carbonate (i.e. the system becomes balanced)?? (the same way that 10-12 psi will not overcarb my cold beer in the kegerator).

I know I sound completely OCD about this thinks but my experience tells me that correcting an over carbed beer kept in a Sanke keg can be a real pain.

Yes, you can up the pressure to 28 psi now. If you had a keg pressure gauge, you would probably find the headspace pressure has increased quite a bit over the 12 psi it was when cold (if the beer was already close to full carbonation, the pressure would be close to 28 psi.)

Your understanding is correct. If you set the pressure at the equilibrium pressure for the desired carb level for the current temperature of the beer (i.e. the chart value), you cannot over carbonate. What happens physically it this. If the beer carbonation level is below the equilibrium value for pressure and temp, then the beer continues to absorb more CO2. If the carbonation level is above the equilibrium value then the beer will lose CO2. In a sealed container (bottle or detached keg) when you warm the beer up, the beer then has more than the equilibrium level of carbonation, so CO2 escapes to the headspace. This raises the pressure in the headspace, which increases the equilibrium carbonation level of the beer. CO2 will come out of the beer until the carb level and pressure are again in equilibrium, and no CO2 is lost. The reverse happens when you cool the beer back down. If you know both the beer volume and headspace volume, it's possible to calculate how much the carb level of the beer and headspace pressure change on heating and cooling.

28 psi is the correct pressure to obtain 2.5 volumes of CO2 at 68˚F, at 70˚F you need 29 psi, and at 72˚F you need 30 psi.

Brew on

 

[chudsonvt]

Once the carb level is correct, if the CO2 is disconnected and then the keg warms, the total CO2 in the keg will remain the same. Some will come out of solution to increase the pressure of the head-space to equilibrium at that temperature. (DO NOT PURGE THAT PRESSURE). If there is very little head-space, very little will come out of solution and all should be well. If you did have a lot of head space (enough to cause a noticeable drop in the dissolved CO2), returning to the cold and allowing time for it to be re-absorbed should result in the same amount carbonation you started at. In either case, there is no harm in connecting the CO2 and setting the pressure for that temperature, but this should not even be be necessary if you did not purge the pressure to begin with.

 

[chudsonvt]

One more thing to note, if you have significant head-space (key word significant) and you set the pressure at a warm temp, you will end up with a little higher level of carbonation after cooling and letting things equalize than when there is very little head-space. This is simply because you can have more CO2 to absorb before you will reach equilibrium.

 

[doug293cz]

Once the carb level is correct, if the CO2 is disconnected and then the keg warms, the total CO2 in the keg will remain the same. Some will come out of solution to increase the pressure of the head-space to equilibrium at that temperature. (DO NOT PURGE THAT PRESSURE). If there is very little head-space, very little will come out of solution and all should be well. If you did have a lot of head space (enough to cause a noticeable drop in the dissolved CO2), returning to the cold and allowing time for it to be re-absorbed should result in the same amount carbonation you started at. In either case, there is no harm in connecting the CO2 and setting the pressure for that temperature, but this should not even be be necessary if you did not purge the pressure to begin with.

I think there was some uncertainty about whether or not OP's kegs had reached equilibrium carb level when they were taken out of the cooler. So CO2 at pressure was applied to be sure that carbonation does finish. If carbonation was complete, then you are correct, there was no need to hook up to CO2 out of the cooler, but it wouldn't hurt anything either.

Brew on

 

[doug293cz]

One more thing to note, if you have significant head-space (key word significant) and you set the pressure at a warm temp, you will end up with a little higher level of carbonation after cooling and letting things equalize than when there is very little head-space. This is simply because you can have more CO2 to absorb before you will reach equilibrium.

True. Now ima haffa do the calcs to quantify this, just so we all know how big the effect can be.

Brew on

 

[chudsonvt]

True. Now ima haffa do the calcs to quantify this, just so we all know how big the effect can be.

Brew on

I am interested in the result!

 

[MagicMatt]

The highlighted point is frequently stated, and believed to be true by many (most?) brewers. While it is well known that beer absorbs more CO2 at lower temps at a given pressure, I have never seen an authoritative reference that said rate of absorption was faster at lower temperatures. I believe the instantaneous rate of absorption is proportional to the difference between the headspace CO2 partial pressure and the equilibrium CO2 partial pressure for the current level of CO2 in the beer at the current temperature. But this says nothing about rate variation as a function of temperature. This means that cold beer will absorb CO2 faster at 12 psi than warm beer at 12 psi, but not that cold beer at 12 psi will absorb CO2 faster than warm beer at 30 psi. To put this in an equation:

Carbonation_Rate = C * (P[CO2] - P[eq])
P[CO2] = Headspace CO2 partial pressure
P[eq] = Equilibrium CO2 partial pressure for current carbonation level @ current temp​

We know how P[eq] varies with temperature, but do we really know how C varies with temperature?

Brew on

That's a great point. The solubility of a gas in a liquid is directly proportional to the pressure of that gas surrounding (above) the surface of the solution, so it would make sense that the time to absorb would be the same.

 

[doug293cz]

That's a great point. The solubility of a gas in a liquid is directly proportional to the pressure of that gas surrounding (above) the surface of the solution, so it would make sense that the time to absorb would be the same.

I don't know if C is relatively constant with temperature, increases at higher temperature, or decreases at higher temperature. Since C is a rate constant, and most rates increase with temperature, a simple answer would be that C increases with temperature, but then simple answers are often wrong.

And C isn't really a constant, as it contains some system dependent factors. Specifically:

C = K* A / V

A = Surface area of beer exposed to the headspace
V = Volume of beer
K = the actual rate constant with units of mass / (area * time * pressure)​

So, the question is: What kind of temp dependence does K have?

I have an idea of how to determine this experimentally (at least with water, as I don't have a lot of beer to experiment with.) And I finally have the equipment needed to conduct the experiment. Unfortunately the experiment will take weeks to run, so don't anyone hold their breath.

Edit: Since Carbonation_Rate should have units of "volumes of CO2" / time, and "volumes of CO2" has units of mass / volume, then the Rate should have units of mass / (volume * time). So if:

mass / (volume * time) = K * area * pressure / volume, then
K must have units of mass / (area * pressure * time)​

Brew on

 

[govner1]

The highlighted point is frequently stated, and believed to be true by many (most?) brewers. While it is well known that beer absorbs more CO2 at lower temps at a given pressure, I have never seen an authoritative reference that said rate of absorption was faster at lower temperatures. I believe the instantaneous rate of absorption is proportional to the difference between the headspace CO2 partial pressure and the equilibrium CO2 partial pressure for the current level of CO2 in the beer at the current temperature. But this says nothing about rate variation as a function of temperature. This means that cold beer will absorb CO2 faster at 12 psi than warm beer at 12 psi, but not that cold beer at 12 psi will absorb CO2 faster than warm beer at 30 psi. To put this in an equation:

Carbonation_Rate = C * (P[CO2] - P[eq])

P[CO2] = Headspace CO2 partial pressure

P[eq] = Equilibrium CO2 partial pressure for current carbonation level @ current temp​

We know how P[eq] varies with temperature, but do we really know how C varies with temperature?



Brew on

The P Chem rule is that the solubility of a gas is inversely proportional to the temperature.

 

[doug293cz]

The P Chem rule is that the solubility of a gas is inversely proportional to the temperature.

Yeah, and there empirical formulas that account for the non-idealities, specifically for CO2 in beer. But that affects P[eq], and says absolutely nothing about rates.

Brew on

 

[doug293cz]

Ok, here's how changing temp on a fully carbed keg affects the headspace pressure and carb level.

A typical ball lock corny has a total volume of about 5.35 gal. If you put 5 gal of beer in the keg, you are left with 0.35 gal of headspace. If you only put 2.5 gal of beer in, then you have 2.85 gal of headspace.

If you carb 5 gal of beer at 70˚F and 29 psi, you will get 2.42 volumes of carbonation. If you then disconnect the CO2 and chill the keg to 40˚F, the headspace pressure will drop to 12.11 psi, and you will have 2.49 volumes of carbonation (after everything comes to equilibrium.) In this case 2.53 g of CO2 is absorbed by the beer from the headspace.

If you carb only 2.5 gal of beer in a 5 gal keg at 70˚F and 29 psi, you will get 2.42 volumes of carbonation. If you then disconnect the CO2 and chill the keg to 40˚F, the headspace pressure will only drop to 18.20 psi, and you will have 3.06 volumes of carbonation (after everything comes to equilibrium.) In this case 11.88 g of CO2 is absorbed by the beer from the headspace.

So, headspace to beer volume ratio has a pretty big effect on changes in carb level and headspace pressure when changing temperature. Just for reference, a full 12 oz bottle has about the same headspace to beer ratio as a full 5 gal keg (about 6% headspace.)

Brew on

 

[PlinyTheMiddleAged]

Edit: Since Carbonation_Rate should have units of "volumes of CO2" / time, and "volumes of CO2" has units of mass / volume, then the Rate should have units of mass / (volume * time). So if:

mass / (volume * time) = K * area * pressure / volume, then
K must have units of mass / (area * pressure * time)​

Brew on

Hmm. I thought volumes was unit-less. Or actually it's volumes per volume. So if I had a nice pint of IPA in front of me that was carbed to 2.1 volumes, it would contain 2.1 pints of CO2.

 

[doug293cz]

Hmm. I thought volumes was unit-less. Or actually it's volumes per volume. So if I had a nice pint of IPA in front of me that was carbed to 2.1 volumes, it would contain 2.1 pints of CO2.

Turns out that one "volume" of CO2 is 1.9768 g/L. One liter of CO2 at STP (0˚C, 1 atmosphere) has a mass of 1.9768 grams.

Brew on

 

[PlinyTheMiddleAged]

Yes. CO2 has mass. But when I look at a chart that calls out "volumes" at some combination of pressure and temperature, that has units of volumes per volume not grams per volume. It is clear that you can convert volumes to mass per volume.

 

[doug293cz]

Yes. CO2 has mass. But when I look at a chart that calls out "volumes" at some combination of pressure and temperature, that has units of volumes per volume not grams per volume. It is clear that you can convert volumes to mass per volume.

A "volume" of a gas is a nebulous concept. If I tell you I have 1 liter of CO2, how much do I actually have? A given amount (mass) of gas will take up the full volume of the container in which it is constrained. You don't actually know the mass of gas you have unless you know the volume, temperature and partial pressure of the gas. The only useful measure of the amount of a gas is the mass.

If I tell you I have 1 volume of CO2 in 10 liters of liquid, what I am saying is that the liquid contains as much CO2 as would be in a 10 L container at 1 atm pressure and 0˚C if the CO2 were in a gaseous form. How much CO2 is that? It's 19.768 g. No matter what volume of liquid I have that contains 1 volume of CO2, it contains CO2 at a concentration of 1.9768 g/L. You can't do any real calculations with "volumes" you need to use something in the form of mass/volume, which you can convert back to "volumes" when you're done with your calculations.

Brew on

 

[PlinyTheMiddleAged]

Doug,

Yes, "volumes" of gas are strange units to use, but I didn't make them up. It's what's in the charts that folks use to set carb levels at given temperatures and pressures.

And just as you did in your example above where you set pressure and temperature to figure out the weight in a given volume, if you tell me the weight of a gas, I can tell you the volume given the temperature and pressure - folks often use standard temperature and pressure.

I'm just trying to avoid confusing people into thinking that the carb volume charts numbers mean grams of CO2 per liter of beer. The chart that is often linked on this site has units of volumes of CO2 (at standard temperature and pressure) per volume of beer.

And nice example on the effect of headspace on carb levels when changing temperatures. Never really thought of that!