Force Carbonation & Head Space

[Mardoo]

I'm wondering whether there's a relationship between the percentage of head space in a keg, the pressure one force carbonates at, and the ability of the beer to take up the CO2.

I have always force carbed in the serving keg, after cold crashing and racking to the serving keg. Since starting to pressure ferment, I have been getting the pressure in the fermenting keg to the level I want it for force carbonation, and then cold crashing. If necessary, I top up the pressure during the cold crash, for example after I've lowered the pressure to take a sample.

However, I'm getting unexpected results since I started doing this. I have 50% head space in one of the pressure fermenters that's been in cold crash for nearly two weeks. I've been waiting for the beer to reach the carb level I want before transferring, but it's just not getting there. It's far under-carbed compared to where my serving keg would be if I were carbing in it. I'm shooting for 2.3 volumes, and know pretty much what that feels like on the palate for both force carbed and fermenter carbed beers, and I'd put this somewhere around 1.7-ish volumes - clearly lower, even if my carb-palate isn't as accurate as I think. I check the pressure in the fermenter daily with a gauge I know to be consistently accurate, which is the same one I've always used to check keg pressure. Could there be something to the relationship I mentioned in the top line? Could something else, say, the yeast cake or hop matter be interfering with the uptake of CO2?

As a comparison, I have another pressure fermenting keg that has 25% head space, and it has carbed to the expected level in the usual amount of time. This is what leads me to wonder about the relationship between CO2 uptake and percentage of head space.

@Lyrebird_Cycles @MHB @Pnutapper - any of you able to help? Am I missing something obvious?

 

[DJ_L3ThAL]

Hey mate, so I think what your actually experiencing in terms of lower carb than usual is due to the pressure ferment method is using the CO2 produced by the yeast to carbonate, however in order I get this level of carb the head space needs to be pressurised.

Because you have more head space in the aforementioned batch there will be more CO2 required to pressurise to the desired spunding valve pressure. So perhaps you need to set this value earlier and if you’ve had it the entire ferment perhaps there is not enough CO2 produced to pressurise the head space AND carbonate the beer. Make sense?

Edit: to simplify in case I’m rambling above, think of it as a mass balance. There is a finite amount of CO2 produced by the ferment, where as a cylinder of CO2 has the ability to supply a continuous amount for an extended period because it is at a far higher starting pressure.
I’m not sure on what maximum pressure you guys pressure ferment at and I’m also not sure why amount of CO2 mass is produced during a ferment but it would most definitely be a finitely amount and the more you increase the headspace the less CO2 you have to carbonate with. E.g. worst case condition is in a non-sealed container the batch won’t carbonate at all as it is all lost to the atmosphere... ok that was simplified I’ll stop now lol

 

[MHB]

In short No!
If you have a look at Carbonation Tables in Braukaiser you will see an equation that pretty much defines what the dissolved CO2 equilibrium content will be at a given temperature and pressure.
The factors in the equation that you have to pay attention to are: -
Temperature, remember that in fridge that hasn't got a fan in it there can be quite a lot of difference between the top and the bottom. where you measure the temperature might not be indicative of the temperature of the beer as a whole.
Pressure, in Bar (atmospheres, 101.3kPa) few home brew pressure gauges are really accurate enough to be better than indicative, for real control you would be looking at a very expensive narrow range gauge, or a not so expensive digital one (still about $100 or so), the one you are using is hopefully consistent if not as precise as we would all like.
Equilibrium, this is the point where the amount of CO2 going into the beer is the same as the amount coming out of solution and back into the head space, net CO2 content becomes stable for that given temperature and pressure - this takes time.

There are a lot of influences that affect the time it takes to reach the target equilibrium, if we assume you have a consistent temperature and pressure. The biggest one is the relationship between the surface area through which the CO2 has to pass and the volume of beer under the surface, that says that a less full keg would carbonate (reach equilibrium) faster than a fuller one as the surface area in most kegs remains the same.
One of the most common problems new keggers have is that the last part of a keg gets over carbonated.
It's often a combination of the bottom of the fridge being colder so as the keg empties the beer gets colder (takes up more CO2) and at the same time rising area to volume, so the same amount of CO2 and pressure will move toward equilibrium faster.

I haven't really taken all that hard a look at Pressure Fermentation (not interested at this point), but the same pressure/temperature/equilibrium conditions apply - I'm not sure what having a lot of yeast would do to CO2 solubility, at a guess not all that big an effect. I am fairly sure pressure and high dissolved CO2 will change the way yeast behaves and not in ways I want in the beer I'm brewing at this time.

You will need to make sure your temperature and pressure readings are consistent - from what you have said, I would be looking at my fermenter to make sure its holding pressure, it sounds like it's leaking (self spunding) at higher pressures, do a static pressure test (cold) and see if that's the problem.

Mark

 

[Mardoo]

OK, thanks. It didn't make sense to me that more headspace in a sealed vessel would make a difference, but at least now I know it doesn't make sense.

I test all my kegs by pressurising to 1 bar after cleaning and then checking the pressure again just before I use the keg again. I don't have a high-grade test gauge, but I did make a point of purchasing a decent one. At the very least I know that the pressure as measured in the keg on my Harris reg's gauge is consistent as measured on my test gauge. So yes, very unlikely to be as accurate as I might like, but it does give me an idea as to whether or not my fermenting kegs and serving kegs are leaking. I do know that this fermenting keg in particular tested as holding pressure prior to the ferment, but things can always change of course.

As to volumes of CO2 produced during fermentation DJ, I tried a closed, unvented test ferment that got up to 50psi before I decided that was far enough for my informational purposes. This particular ferment I let rise to 27psi by the end of the ferment at 22C, aiming for 2.3 volumes according to the Beersmith calculator. Clearly I am not shooting for perfection

 

[MHB]

One thing I cant help but wonder is why people are still talking in Volumes of CO2 and PSI. We went metric when I was still in primary school (just), after several years of school trying to learn imperial, I thought metric beat the imperial non-system hollow.
All the maths works better in metric, the Germans are now talking %CO2, I'm happy using g/L of CO2.
Even doing mass priming calculations is so much easier in metric that most imperial brewers do it in metric then convert the answer to silly units.
Mark

 

[manticle]

Probably because so many of the calculators are US based. I agree though - imperial does my head in.

That said - in my job, most of my measurements (and those who do what I do) are in mm but there are standards within the industry that insist on cm. Both metric but I still find cm silly and prone to error (people have a tendency to round off, less of an issue when the unit is smaller).

Way OT, sorry.

Mardoo - the only difference I can think might have an effect is the gas equibrilating over entire volumetric space. The less headspace, the more the gas will be in solution rather than between the two.

Someone more informed can either correct or confirm. Understanding how gas behaves in and out of solution is not currently my forte.

 

[MHB]

cm's are for dress makers, most tradies use mm and meters, just like g and kg, which makes a lot more sense than cm.
You are right about the more the head space the lower the pressure with the same amount of gas.
When you have a look at the amount of CO2 produced by a ferment, well you are going to have an excess unless your head space is many times greater than your liquid volume.
Just roughly
1g of fermented sugar produces 0.468g or (0.468kg/kg) of CO2
For a beer that drops 40 points (i.e. from 1.050 to 1.010), remember that that's apparent so in real terms (RDF=0.82ADF) its 40*0.82=32.8 points
Put that into Plato 0.0328=(4*oP)/1000 = 8.2oP
8.2% of the original mass, say you had 23L at 1.050 (12.5oP) the OM would have been 23*1.050=24.15kg
24.15*0.082= 1.9803kg of sugar
1.9803*0.468=0.8589kg (call it 859g)
The Formulae weight of CO2 is 44.01g/Mole so 19.5M
Each mole occupies 22.4L at STP so you are making about 437L of CO2

To get a rough idea of the pressure, if you packed all that CO2 into a Corny lets call it 19L, from P1V1=P2V2, 437L*1Bar=19L at x Bar
437/19=23Bar (2330kPa) (~338 psi for the luddites)

Just off the top of the head, hope I haven't flubbed any of the numbers - sure someone will point out any mistakes - but gives you a fair idea.
Marl

 

[breakbeer]

Sometimes I think I know a thing or two about brewing, then I read a post like the one above & realise I kinda don't

 

[Lyrebird_Cycles]

I'm shooting for 2.3 volumes, and know pretty much what that feels like on the palate for both force carbed and fermenter carbed beers, and I'd put this somewhere around 1.7-ish volumes - clearly lower, even if my carb-palate isn't as accurate as I think.

Just to cover off the basics, what temperature and pressure are you running?

Also, you make no mention of changing the pressure as you reduce the temperature. Is that just an oversight in your post?

Lastly, if you agitate the keg vigorously, does the gauge pressure dip?

 

[Mardoo]

True. Usually I do drop the temperature according to the BeerSmith calculations. This time I haven’t, so the beer has been at 4.1-4.6C and 27 psi and STILL not carbing. This is the first time I’ve ever done this, out of a see-if-it-carbs-faster interest. I’ve always dropped pressure according to temperature, as is recommended.

This is part of why I’ve felt so stymied by this. In my daily tastings to observe how the carb is developing, it’s been dropping, if anything. I’ve repressurised to 27psi after each tasting.

I’m chalking this up to making a mistake somewhere along the way that I haven’t noticed. This fermenting keg will be getting a very close examination after I clean it this week, that’s for sure.

 

[DJ_L3ThAL]

One other thought away from the theory side. I have seen some of the Keg King disconnects have the notch where the ball locks in sitting slight further away than all the other corny keg disconnects. So when you push it down it does not fully open the poppet. Granted for gas it shouldn't restrict it too much the problems I have had is on the liquid side where it restricts so much it causes foam when flowing. But just double double check that there is gas free flowing through the disconnect without having to continuously push down on the disconnect yourself.

 

[Grott]

DJ could be on to something as just like water you should be carbonating unless A) the gas is not getting in and/or B) your keg has a leak so no pressure. Check out both.

 

[TSMill]

Actual volume in heads pace is irrelevant if you are working to a set pressure/temperature. What will impact uptake is gas/liquid interface area, which may be related to liquid fill level in the keg (and hence, indirectly related to head space volume).

 

[Pnutapper]

@Pnutapper - any of you able to help? Am I missing something obvious?

I can't really add anything useful to what has already been said above mate. I tend to think that you may have a leak somewhere. At that temp and pressure, the water in your beer should be gobbling up the CO2 like crazy.

One thought though: Is it a new cylinder of gas? (i.e just been refilled?) Is it possible they may have filled a CO2 / N2 blend by accident?

 

[Mardoo]

Gotta be the keg. I’ve had two beers carbing the same way. One has carbed properly, the other not.

DJ’s point is of interest. I’m pretty sure I have all Cornelius disconnects going at the moment though.

 

[pcqypcqy]

I just did a 5g batch of dark mild in my 50 litre kegmenter, OG was only 1040. Had no issues getting it fully carbed prior to transfer only using the gas from fermentation.

If you're leaving the spunding valve on, check for leaks. I think mine leaks a little even when fully closed.

Also, on your method described in your first post, what I do is keep it at a sensible psi level for the first few days, and when i do my d rest I let the pressure increase right around to approx 20psi (warm). When I cold crash, this then settles around the 10 to 12psi mark which is my serving pressure in my keezer, so basically fully carbonated.

So maybe try that on your next one and see if that helps.

 

[Grott]

Good point with leaking from spunding valve. I found with a cheap disconnect the weight of the unit was putting angular pressure on the disconnect to post contact and was slowly leaking.

 

[pcqypcqy]

Actual volume in heads pace is irrelevant if you are working to a set pressure/temperature. What will impact uptake is gas/liquid interface area, which may be related to liquid fill level in the keg (and hence, indirectly related to head space volume).

And if the gas pressure is maintained to the headspace, this will also only affect the time it takes to carbonate. If the surface area is small, it will still get there in the end.

 

[pcqypcqy]

True. Usually I do drop the temperature according to the BeerSmith calculations. This time I haven’t, so the beer has been at 4.1-4.6C and 27 psi and STILL not carbing. This is the first time I’ve ever done this, out of a see-if-it-carbs-faster interest. I’ve always dropped pressure according to temperature, as is recommended.

This is part of why I’ve felt so stymied by this. In my daily tastings to observe how the carb is developing, it’s been dropping, if anything. I’ve repressurised to 27psi after each tasting.

I’m chalking this up to making a mistake somewhere along the way that I haven’t noticed. This fermenting keg will be getting a very close examination after I clean it this week, that’s for sure.

How are you measuring pressure?

If it's just with the spunding valve, I've found my spunding valve gauge can get a bit sticky, particularly if I've overfilled a keg on transfer or if I've had a big krausen explosion and had beer through the valve. Maybe disassemble and clean and try again.

If it's your gas bottle regulator, then I've got nothing.

 

[pirateagenda]

are the samples pouring heady and then tasting flat? I've had the same issue where i thought it was under carbed but was way over, subsequently frothing in the lines, but then the beer is flat because the gas has come out of solution.