Cold Crash Suckback Issue - Resolved

[Zippy123]

The Problem: Cold crashing creates a vacuum in the fermentation vessel. This vacuum creates "suckback" through the airlock or blowoff tube which may introduce one or more undesirable components into the ferm vessel: airlock fluid, O2, or spoilage organisms. I have tried every suckback solution I could find and none of them ever worked satisfactorily so I developed my own. With that said, if this solution has already been posted somewhere then I claim it is impossible to find. So hopefully this solution is easier to find!

The Solution: From a sealed fermentation chamber, run a blowoff hose to a sealed "accumulation vessel" (my made-up name!). Run another blowoff hose from the accumulation vessel into the airlock bucket (water or sanitizing solution). During active fermentation CO2 flows from the ferm vessel into the accum vessel and then through the airlock water where it is released into the atmosphere. When cold crash is activated, the suction created by the crash creates a vaccum in the accum chamber. This vacuum pulls water from the airlock bucket into the accum vessel. Because our friend Gravity is always turned on, there is absolutely no chance for the airlock water to get into the ferm chamber blowoff hose. The water sucked back from the airlock bucket into the accum chamber falls to the bottom of the accum chamber, and CO2 from the fermentation process is "returned" to the fermentation vessel. When the entire ferm and crash process is complete the OUT port from the ferm vessel is connected to a keg and CO2 is connected to the IN port. The ferm chamber is gently pressurized and finished product flows gently into the keg. A completely closed system from pitching to kegging, without touching anything in the process.

A few more details: The ferm vessel has two sealed bulkhead connectors: an OUT port connected to a Flotit floating dip tube and an IN port with just a standard short tube. If you're looking at the pics, the third "thing" on the lid is a sealed bung with a temperature probe that is part of my DIY fermentation chamber control system. The accumulation vessel is a 3-gallon Fermonster with sealed bulkead connectors for IN and OUT. The port connected to the airlock side has a 4-inch long piece of tube (cut from an unused dip tube). So when water is sucked into the accum chamber it drips down starting 4" away from the blowoff tube connection inside the accum vessel. Since these pics were taken I have switched the airlock liquid to a 2-gallon vessel as during one of my "test runs" the 1-gallon vessel almost ran dry.

Evolution of the issue for me: For a year and a half now I have been using the Fermonster vessel. After reading this thread on making a closed transfer system for the fermonster, I set up my fermonster for just that. However I still had issues dealing with suckback. I tried everything from a balloon to a really long blowoff hose to vodka in the airlock and a few other techniques that did not work. For me the problem came to a head after I designed and built my own custom fermentation chamber. (I'll have to post a thread in the DIY form on that project; it is actually quite amazing if I do say so myself!) I programmed a "fast crash" mode that crashes beer from around 70 deg to 35 deg in about 16-18 hours. So suckback became a bigger issue than before when i just moved the Fermonster into my garage fridge and it took a few days to crash. With this system, once I pitch the yeast and put the lid on the fermenter I don't touch it until the beer is finished and ready to transfer to the keg. And it is completely closed, and there is no suckback.

 

[Whisky River]

https://www.norcalbrewingsolutions.com/store/CO2-Carbon-Dioxide-Harvester-Kit.html

https://www.brewhardware.com/product_p/ccguardianv3.htm

 

[pers]

An alternate method would be to use the keg you're planning to transfer the beer into as the "accumulation" vessel. This will clear the keg of almost all oxygen. Then before cold crashing unhook all the connections, keg and fermentor. Attach a co2 line to the fermentor and put a small amount of pressure on during cold crash, 2psi should be fine. You'll have to put another hole in top of fermentation chamber. Also a carb cap, on outside, connected to a quick disconnect inside to connect to fermentor.

This method eliminates the suck back of liquid entirely and only sucks in co2 from a tank. You also get an almost entirely oxygen free keg went transferring the beer.

 

[mac_1103]

An alternate method would be to use the keg you're planning to transfer the beer into as the "accumulation" vessel. This will clear the keg of almost all oxygen. Then before cold crashing unhook all the connections, keg and fermentor. Attach a co2 line to the fermentor and put a small amount of pressure on during cold crash, 2psi should be fine.

Sounds like you aren't using the accumulated CO2 during the cold crash at all. You can put 2 psi on your fermenter before cold crashing whether you purged a keg with fermentation gas or not.

I purge a keg and fill a cold crash guardian.

 

[pers]

Sounds like you aren't using the accumulated CO2 during the cold crash at all. You can put 2 psi on your fermenter before cold crashing whether you purged a keg with fermentation gas or not.

I purge a keg and fill a cold crash guardian.

Correct, not using any collected co2. Was just giving OP an alternate idea with setup outlined in 1st post and Items probably on hand, other than a carb cap, that gives a purged keg as well.

Other methods could be done to use collected co2 as the suck back source but add cost.

 

[mac_1103]

Other methods could be done to use collected co2 as the suck back source but add cost.

You can DIY a krausen catcher/CO2 harvester for the cost of a few gromets, plastic barbs, and some tubing. OP basically already did a DIY CO2 harvester, apparently without spending much.

With that said, if this solution has already been posted somewhere then I claim it is impossible to find.

DIY krausen catcher/CO2 harvester.

 

[Zippy123]

I failed to mention two goals for this project, which I have achieved:

(1) Completely hands-off from pitching to kegging. My DIY fermentation chamber monitor will run the entire process, without me touching a button or hose, from pitching to kegging. In "auto" mode it maintains ferm temp and starts d-rest at a specified gravity. I have complete control over the d-rest SG start setpoint, ramp parameters, and d-rest final temp. When the FG has been stable for a specified period of time, I get a text notification that fermentation is complete. I can then log into the system remotely via the web and start cold crash. I can also do it right at the controller on the ferm chamber but it's so much more fun to log in from my office and click a few boxes.

(2) No buying more stuff. I don't think this needs explanation.

An alternate method would be to use the keg you're planning to transfer the beer into as the "accumulation" vessel. This will clear the keg of almost all oxygen. Then before cold crashing unhook all the connections, keg and fermentor. Attach a co2 line to the fermentor and put a small amount of pressure on during cold crash, 2psi should be fine. You'll have to put another hole in top of fermentation chamber. Also a carb cap, on outside, connected to a quick disconnect inside to connect to fermentor.

An alternate method would be to use the keg you're planning to transfer the beer into as the "accumulation" vessel. This will clear the keg of almost all oxygen. Then before cold crashing unhook all the connections, keg and fermentor. Attach a co2 line to the fermentor and put a small amount of pressure on during cold crash, 2psi should be fine. You'll have to put another hole in top of fermentation chamber. Also a carb cap, on outside, connected to a quick disconnect inside to connect to fermentor.

This method eliminates the suck back of liquid entirely and only sucks in co2 from a tank. You also get an almost entirely oxygen free keg went transferring the beer.

I thought about using a keg for the accumulator but then I would need to move around hoses and pressurize the system before crashing, else water would be sucked into the keg which would be counterproductive.

You can DIY a krausen catcher/CO2 harvester for the cost of a few gromets, plastic barbs, and some tubing. OP basically already did a DIY CO2 harvester, apparently without spending much.


DIY krausen catcher/CO2 harvester.

Somehow I missed or did not follow the thread you posted. Lots of great ideas there, from simple and cheap to complex and possibly spendy. There may be as many possible solutions to the problem as there are homebrewers. I am not sure how a few pint mason jars (or even quart jars) is enough capacity for the suckback. I nearly drained a 1-gallon bucket...

 

[Climb]

Another option is to use the your dispensing keg as your accumulation vessel to purge it and put a spunding valve on the output of the serving keg. Best to add the spunding valve after the first couple days of fermentation.

 

[Zippy123]

Another option is to use the your dispensing keg as your accumulation vessel to purge it and put a spunding valve on the output of the serving keg. Best to add the spunding valve after the first couple days of fermentation.

This would work for sure during fermentation. Not sure how well it would work during cold crash, based on my experience pressurizing the fermonster. I have experimented with disconnecting the blowoff from the fermonster and slightly pressurizing it with CO2 before starting cold crash. I tried that probably 3-4 times but each time I needed to add more pressure several times to prevent the vessel from collapsing on itself. I did not feel comfortable pressurizing it so much that I would not have to repressurize later. So I was constantly concerned about the pressure or vacuum in the vessel. Perhaps with the additional volume of the keg, I could slightly pressurize the keg and fermonster and not have the fermonster collapse. However that would not achieve one of my objectives: Completely hands-off from pitching to kegging. Not only would there be hoses to disconnect and move around, but my experience with spunding valves suggests it would require some fiddling to get the relief pressure correct.

So many ideas, so little time for experimentation!

 

[Zippy123]

Another option is to use the your dispensing keg as your accumulation vessel to purge it and put a spunding valve on the output of the serving keg. Best to add the spunding valve after the first couple days of fermentation.

This triggered a thought: I could simply add the keg in my existing setup between the ferm vessel and the accumulator. I would have a purged keg ready for filling and any suckback from the airlock would drop into the accumulator, not into the keg. Dang that might work. Achieves both my goals of completely hands off and no new parts to buy, and I save some CO2 since I don't need to purge the keg.

 

[Jim R]

Maybe I am missing something. Why do you need to cold crash in your fermenter? Why wouldn't you just transfer the beer to your keg after fermentation and the cold crash in the sealed keg? Why do you need to deal with an airlock after fermentation when you are lowering the temperature of the fermented beer? Again maybe I am missing something.

 

[mac_1103]

Why do you need to cold crash in your fermenter?

To keep the yeast and other gunk out of the keg. It's great if you don't find it necessary, but lots of people do it.

 

[Jim R]

To keep the yeast and other gunk out of the keg. It's great if you don't find it necessary, but lots of people do it.

I can see where that would be a theoretical reason. I do a pressure transfer from my fermenter with a floating dip tube and then cold crash in the keg. When I eventually tap the beer, the first couple ounces may be minimally cloudier than the rest of the glass. It would be a non-issue for me. It is probably is worth experimenting with both techniques to just avoid the dilemma.

 

[Zippy123]

I can see where that would be a theoretical reason. I do a pressure transfer from my fermenter with a floating dip tube and then cold crash in the keg. When I eventually tap the beer, the first couple ounces may be minimally cloudier than the rest of the glass. It would be a non-issue for me. It is probably is worth experimenting with both techniques to just avoid the dilemma.

It seems to me that "crashing after kegging" is essentially the same thing as "transferring from ferm vessel to keg then putting keg in kegerator". I have done that and eventually I get the same beer as if I crashed before kegging. But it seemed to take waaaay longer to get clean, clear beer that way. It took a few weeks or longer for my desired clean, clear beer if I didn't crash in the ferm vessel. I know, counterintuitive, but that's what I found if I crashed in the keg.

Now that I have a fully automated ferm chamber it makes sense for me to crash before kegging. I put the ferm vessel into my ferm chamber (a 5 CF freezer), pitch the yeast, hook up the blowoff tube, close the lid, program the fermentation parameters into my DIY ferm chamber monitor, and click the Monitor button. A few weeks later I have fermented, d-rested, and cold crashed beer ready to keg. Clear and clean. If it's a lager it sits in the coldest spot in my basement for 3-4 weeks at a minimum; if it's an ale and there's a spot in my kegerator it goes in there right away and is carbonated, clear, and drinkable in 4-5 days. I used to use gelatin to clear my beer but since I've been following this process it's clear in a week and crystal clear in 2.

If your ferm vessel is NOT GLASS and if you can add a small amount of pressure to it, you can do a simple crash in the ferm vessel by slightly pressurizing when you put it in the refrigerator, but you need to check it every 12 hours at least and add more CO2 to keep the pressure up, so it doesn't collapse on itself. This is what I did before I completed my automated ferm chamber project. It's "fussy" but it works. I prefer the "set it and forget it" approach which is why I came up with my solution here.

 

[Climb]

I have experimented with disconnecting the blowoff from the fermonster and slightly pressurizing it with CO2 before starting cold crash. I tried that probably 3-4 times but each time I needed to add more pressure several times to prevent the vessel from collapsing on itself.

I think the reason your fermonster is depressurizing is because the CO2 is going into solution. With a spunding valve, the CO2 has plenty of time to equalize in the head space and the beer.
Another option is to use a low pressure regulator like those used for propane tanks on bar-be-ques along with your CO2 tank; but why not use the free CO2 from fermentation. Spunding valves aren't that expensive and will free up a lot of room in your fermentation chamber.

 

[doug293cz]

If your ferm vessel is NOT GLASS and if you can add a small amount of pressure to it, you can do a simple crash in the ferm vessel by slightly pressurizing when you put it in the refrigerator, but you need to check it every 12 hours at least and add more CO2 to keep the pressure up, so it doesn't collapse on itself. This is what I did before I completed my automated ferm chamber project. It's "fussy" but it works. I prefer the "set it and forget it" approach which is why I came up with my solution here.

I think the reason your fermonster is depressurizing is because the CO2 is going into solution. With a spunding valve, the CO2 has plenty of time to equalize in the head space and the beer.
Another option is to use a low pressure regulator like those used for propane tanks on bar-be-ques along with your CO2 tank

Yep, if you lower the temperature the CO2 pressure will drop a little from the temp change, but the really big effect is that at lower temps more CO2 will dissolve into the beer at a constant pressure, and the CO2 absorption will really drop the pressure quite a bit if there is no make-up gas available. A low pressure CO2 regulator will make compensating for the additional absorption automatic.

Brew on

 

[Zippy123]

Yep, if you lower the temperature the CO2 pressure will drop a little from the temp change, but the really big effect is that at lower temps more CO2 will dissolve into the beer at a constant pressure, and the CO2 absorption will really drop the pressure quite a bit if there is no make-up gas available. A low pressure CO2 regulator will make compensating for the additional absorption automatic.

Brew on a

Yeah I still can't wrap my brain around CO2 absorption. Add a few psi and it will be absorbed so the 2 psi you put in there is still in there but somehow the pressure has dropped back to zero. Where did the pressure go? The CO2 you put in there, is in there, but the pressure is not. But for the want of a single chemistry class in HS or trade school...

 

[doug293cz]

Yeah I still can't wrap my brain around CO2 absorption. Add a few psi and it will be absorbed so the 2 psi you put in there is still in there but somehow the pressure has dropped back to zero. Where did the pressure go? The CO2 you put in there, is in there, but the pressure is not. But for the want of a single chemistry class in HS or trade school...

Yes the CO2 in the keg is still there, but once it is absorbed into the beer, it no longer contributes to the pressure. Only CO2 in the headspace contributes to the pressure.

Brew on

 

[Zippy123]

Yes the CO2 in the keg is still there, but once it is absorbed into the beer, it no longer contributes to the pressure. Only CO2 in the headspace contributes to the pressure.

Brew on

So there's CO2 under pressure in the headspace, it makes itself cozy with the beer and vanishes from the headspace, and therefore no more pressure. Witchcraft I say! Witchcraft!

 

[mac_1103]

Witchcraft I say! Witchcraft!

What do you think happens when you force carbonate a keg of beer?

 

[doug293cz]

So there's CO2 under pressure in the headspace, it makes itself cozy with the beer and vanishes from the headspace, and therefore no more pressure. Witchcraft I say! Witchcraft!

Pretty much. Not all of the CO2 in the headspace gets absorbed by the beer. An equilibrium gets established between the CO2 in the headspace and in the beer, and that equilibrium point changes with changes in temperature. If the headspace CO2 pressure is higher than what would be in equilibrium with the concentration of CO2 in the beer, then some of the CO2 from the headspace goes into solution until the headspace pressure drops to the point where the beer and headspace are in equilibrium again. On the other hand, if the headspace CO2 pressure is less than what would be in equilibrium with the CO2 in the beer, then CO2 will come out of solution and enter the headspace, raising the pressure, until CO2 pressure and CO2 level in the beer are back in equilibrium.

The tendency of a system not in equilibrium to change in a way that brings it closer to equilibrium is known as Le Chatelier's Principle which is something that is taught in basic chemistry courses. So the previous paragraph is your "Intro to Chemistry" lesson for today.

How much the pressure inside a sealed (no source for, or leak of CO2) container will change with a temperature change can be calculated, and I have a spreadsheet that does that for kegs (and similar vessels.)

Brew on

 

[Zippy123]

What do you think happens when you force carbonate a keg of beer?

I believe Henry's Law applies here. If I recall it is something like C=kP. The solubility of a gas in a liquid at a given temperature is directly proportional to the partial pressure of that gas above the liquid. Dissolution, equilibrium, blah blah blah.

Blaming it on "missing a chemistry class" and Witchcraft was apparently a bad attempt at a joke. And possibly offensive to the Education Occultist communities.

 

[mac_1103]

I think it was "I still can't wrap my brain around CO2 absorption" that threw us off. You need to remind your writers that the setup is just as important as the joke.

 

[anteater8]

Some of these solutions seem way overcomplicated to me. I've been using mylar balloons that cost 36 cents a piece on amazon, fill them with a little co2 from the tank (it doesn't take much at all), and then tape it to a hose and connect to the fermentation vessel.

https://www.amazon.com/dp/B07DHKTVVZ?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_1&th=1

Even easier for anyone who uses the modified fermonster set up is to just pressurize the carboy a little before or while you are cold crashing. Again, it does not take much co2 at all to counteract the temperature change.

 

[kohalajohn]

I"m not familiar with fermzilla PET fermenters. I am guessing the reason you don't seal it during a cold crash is that the depressurization could collapse it?

I ferment in corny kegs. I seal them off during the crash and just let the depressurization happen.

Perhaps my o rings would take some strain, but so problems so far.

 

[Climb]

Do you think you still have more than ambient pressure in your corny keg fermenters during the cold crash or are you sure the pressure is less than ambient during the cold crash?
Are using a spunding valve or sealing the corny fermenter with many gravity points before FG is achieved? Or adding CO2 to the corny fermenter via a CO2 tank before cold crash?

 

[kohalajohn]

I have been assuming that the crash will reduce inner keg pressure to below outside pressure. As Doug says, that cold beer will suck in a lot of co2.

But I don't know. I could put a gauge on it and find out.

When fermentation is finished and I'm about to crash the temperature, I remove the sounding valve, as I don't think it is as robust as a post to block outside air pressure. I don't add any co2. I just let the pressure drop.

I have also been assuming that during the crash, the beer is not affected by these relatively small changes in pressure.

After the crash I let the kegs warm up to their previous temperature and pressure, so that I can do a closed transfer.

So I have not thought it mattered to me. The kegs start at 55f, are sealed, drop to 36f then rise back up to 55f, all while sealed. It's like the drop in pressure never happened.

I have a cold crash coming up soon. I'll put a gauge on it and see.

But more importantly, does anybody have any thoughts on whether a pressure drop and rise, in a sealed vessel, has any effect on anything? I'd be interested to learn more.

 

[mac_1103]

Kegs and keg components are not designed to handle a vacuum and *might* be damaged over time. Probably not a big deal in this use case. Then again, putting a couple of PSI on the keg before crashing isn't a big deal either.

 

[IslandLizard]

[...] putting a couple of PSI on the keg before crashing isn't a big deal either.

Exactly! It may need to be "topped up" a few times depending on headspace volume.

Or use a Cold Crash Guardian or simply a mylar balloon filled with CO2. They do take up space, though.

 

[kohalajohn]

Good idea. I was thinking about the health of the yeast. But yeah, there is also the health of the keg itself.

@IslandLizard said the success of topping up, depends on headspace volume. And he's right, so that got me thinking about changing my setup. I can increase my headspace volume by changing where I put the spunding valve.

Right now I have my spunding valve between the fermenter and the dispenser.

But if move the spunding valve to the dispenser, and then connect the two kegs with a gas/gas jumper, then I have one big connected keg, a huge volume of headspace.

I make three gallon batches in five gallon cornies. So combined headspace would be seven gallons. With a bit of a co2 bump, that there would lots of gas volume to balance out the cold shrinkage. I would be using my dispensing keg as a Cold Crash Guardian. Maybe the inner pressure would be a touch below outer pressure, but it would be much less strain on the keg than what I am doing now.

I think I'll do that. In a few days I'll try it with my crash.

 

[IslandLizard]

I have been assuming that the crash will reduce inner keg pressure to below outside pressure. As Doug says, that cold beer will suck in a lot of co2.

Cold crashing in a closed container will indeed reduce the pressure inside. This is partly due to lower temperatures decreasing the pressure, but mostly by the beer (or water, etc.) absorbing more CO2 at lower temps until a new pressure equilibrium has been formed between dissolved gas (in the liquid) and free gas in the headspace at the prevailing temps.

I make three gallon batches in five gallon cornies. So combined headspace would be seven gallons. With a bit of a co2 bump, that there would lots of gas volume to balance out the cold shrinkage. I would be using my dispensing keg as a Cold Crash Guardian. Maybe the inner pressure would be a touch below outer pressure, but it would be much less strain on the keg than what I am doing now.

You could calculate (or just estimate) how much CO2 will be dissolved in the beer and how much will remain as gas in the entire 2-keg system.

Since you only have 3 gallons in a 5 gallon keg (having an actual volume of about 5.25 gallons), you should be able to use only the single keg, with ~12 psi of pressure in the ~2.25 gallon of headspace, keeping it from potentially imploding (or sucking air).
The 12 psi of pressure will carbonate your beer a little, so when transferring to your serving keg put a similar amount of (counter)pressure in it and slowly bleed it off while transferring (filling) to prevent excessive foaming.

 

[mac_1103]

This is partly due to lower temperatures decreasing the pressure, but mostly by the beer (or water, etc.) absorbing more CO2 at lower temps until a new pressure equilibrium has been formed between dissolved gas (in the liquid) and free gas in the headspace at the prevailing temps.

And most of us don't cold crash nearly long enough to reach equilibrium.

You could calculate (or just estimate) how much CO2 will be dissolved in the beer and how much will remain as gas in the entire 2-keg system.

Our resident beer math nerd has a spreadsheet for that but I can't seem to find the link right now.

 

[doug293cz]

And most of us don't cold crash nearly long enough to reach equilibrium.

Our resident beer math nerd has a spreadsheet for that but I can't seem to find the link right now.

You might find it down below:

Brew on

 

[Bobby_M]

A corny keg will definitely suck air in through the lid under a mild vacuum. It's made to seal under pressure but the bale handle is just there to hold the gasket in the seat so you can get the pressure started. It's not strong enough to hold the lid in place under vacuum.

 

[Bobby_M]

Yeah I still can't wrap my brain around CO2 absorption. Add a few psi and it will be absorbed so the 2 psi you put in there is still in there but somehow the pressure has dropped back to zero. Where did the pressure go? The CO2 you put in there, is in there, but the pressure is not. But for the want of a single chemistry class in HS or trade school...

I like practical analogies. Think of the headspace of a keg as the lobby at a concert venue and the beer as the actual performance space. 1000 people are lined up outside. They let all the people into the lobby all at once an hour early so they are all crammed in banging elbows constantly. They open the doors and people start filing in but it takes time because the "interface" (door) between the two rooms is small/limited..

I don't know, it sounded better in my head than it does reading it back. The pressure never actually goes to zero. It's just that proportionally the headspace is such a small amount of the total space so 2 psi will equalized down to a tenth of that or so.

 

[Climb]

Hey @doug293cz - Do you have a similar spreadsheet that incorporates volume of CO2 generated from gravity points in addition to everything else that is already in the spreadsheet you attached above? I am thinking of a scenario where a spunded fermenter is sealed before fermentation is complete.

 

[mac_1103]

I am thinking of a scenario where a spunded fermenter is sealed before fermentation is complete.

Wouldn't "End of Fermentation Pressure" account for that?

 

[Climb]

@mac_1103 Yes, I believe it would, but that is after the fact. What I am asking for is a tool that would help you determine at what gravity to seal up the fermenter based on expected FG and desired volume of CO2 or end pressure at a specific temperature. I have found the following in a post from from Kai T. from May 2010, "each gravity point gives you about 0.5 volumes of CO2". So I suppose I could run with that. Sorry, but I kind of slept through chemistry class, so anything dealing with stoichiometry, mols and Avogadro's number takes me outside my comfort zone.
Since fermentation is on going in the sealed fermenter, the amount of CO2 is more of a dynamic system. Carbonation charts don't account for new CO2 being generated. They are meant for a closed system after fermentation is complete.

 

[mac_1103]

What I am asking for is a tool that would help you determine at what gravity to seal up the fermenter based on expected FG and desired volume of CO2 or end pressure at a specific temperature.

I guess I'm still confused. Doesn't the spunding valve make calculating this superfluous? Calculate the pressure needed for the desired volumes of CO2, then set it and forget it. Are you afraid of spunding too soon? If so, why?

Carbonation charts don't account for new CO2 being generated. They are meant for a closed system after fermentation is complete.

The physics is the same. 15 PSI at 68F gives 1.7 volumes of CO2 at equilibrium whether the pressure comes from a tank or from fermentation. If you're looking for pre-equilibrium numbers, then I think the calculation is a lot more complicated.

 

[Climb]

I can understand your confusion. I was trying to hide a lot of details that most might not be interested in. Let me start by answering your questions. I believe you are thinking of a different use case that what I am. The use case I have is similar to carbonating a keg using gravity points instead of priming sugar or external CO2. I am not afraid of spunding too soon. I am not looking for pre-equilibrium numbers.
Now let me provide some background for my request and details on my fermentation system/process and my current predicament.
My fermentation system/process - My typical batch is 10.5 gallons and I ferment in a half barrel sanke. I chain the half barrel to two sixtels in serries with a blowtie spunding valve at the output of the second sixtel; i.e. at the end of the chain. I add the blowtie after the first couple days of fermentation. This allows me to purge the sixtels and spund at the same time. After fermentation, I do a closed transfer into the two purged sixtels. I have one blowtie spunding valve that has a pressure gauge that only goes up to 15 psig. I typically spund between 5 and 10 psig and finish carbonation after kegging with a CO2 tank using the set and forget method.
My current predicament - My last batch I repitched some old yeast slurry without making a starter. This is not my normal process. I was being arrogant, overconfident and lazy. So I had either an underpitch and/or very low vitality yeast; likely both. I have never taken gravity readings to verify that fermentation has ended and that the FG is close to prediction. I just let things ferment for a few extra weeks beyond normal and then keg to the sixtels. With this batch, I kegged before fermentation was complete. My FG reading happens during transfer. The FG was way high (1.021, predicted FG is 1.007) and the sample was super sweet. At this point I knew I was in trouble and decided to finish kegging anyways. So now I have slowly fermenting beer in the sealed sixtels. The gravity has dropped and has been at 1.013 (via hydrometer) for the last month. The current temp of the sixtels is about 75 F. I am trying to push fermentation to complete with this temperature. When I connect the blowtie to the sixtel the needle on the blowtie pressure gauge pegs out, so I don't know exactly what their pressure is. Anyways, this experience has convinced me that I need to start taking gravity measurements to verify that fermentation is complete. Shame on me for not making this standard operating procedure before now. And while I was reading this thread and looking at doug293cz's spreadsheet, it seemed to me that adding in calculations for CO2 generated from yet to be fermented sugars would be a nice addition to what he has already done. It would be useful for those that want to get some carbonation from gravity points instead of using priming sugar or external CO2.