Keg [Apparently] Failing to Carbonate

[Rhizome]

Hey folks,
So, after doing both force carb and natural carb on my kegs, I've decided I like natural carb better (there are endless threads on this, I know, but just my preference). To better monitor when it's complete, I bought a bleeder valve with a pressure gauge on it that attaches to the gas in stem of the keg. This allows me to watch the pressure increase as it carbonates. In the past, I've seen that once it hits 30 psi at room temperature and levels out, the beer ends up perfectly carbonated when chilled and delivered at 10 psi. This has always made sense to me because 30 psi is the equilibrium pressure for about 2.5 volumes of CO2 at 74 deg, and 10 psi is the equilibrium pressure for that same 2.5 volumes at my approximate serving temperature of 40 F.

Now to the question--my most recent keg has been sitting at about 15 psi for 3 weeks now (basically equal to the head pressure I put on it to seal the lid). This makes me think it didn't carbonate. On the one hand, my first thought is a leak in the keg, but it also seemed unlikely that the pressure would sit at virtually the same reading for 3 weeks with the leak being exactly the same rate as the carbonation that's being added from the conditioning. Thoughts?

I plan to chill the keg and try it and if it's indeed flat, I'll put 10-12 psi on it and do the shake-shake force carb method. So long as I don't over-pressure it, the keg should reach the same 2.5 volumes of CO2, yes?

Thanks in advance.

 

[boogyman]

My guess is that there might not have been enough priming sugar in there to really get it going. You could try dumping some more in, but at this point, it might be easier to force carb.

If you're worried about overpressurizing by force carbing, use the "set it and forget it method". If you're patient enough to wait for natural carbing, another week with letting it sit at 10-12psi should be a piece of cake.

 

[Rhizome]

Thanks for the reply. It's the same amount of priming sugar I've used successfully in the past, so it's a bit of a mystery to me. I have a feeling I'll be force carbing this time around!

 

[Rhizome]

So, I've double checked my sugar (1/3 cup for a 5 gallon keg batch, which has worked for me in the past and I believe is standard), I realized my apparent attenuation is quite high. This is an American Pale that had an OG of 1.050 and FG 1.006 for an attenuation of 88%. I used Wyeast 1056 which should give 73-77%. I've heard of yeast getting "tired" and not being able to fully attenuate in high gravity beers. Could the same thing happen here even in a lower gravity beer whereby the yeast didn't have anything left to carbonate my keg?

 

[JKoravos]

Your keg wouldn't sit at 15 psig if there wasn't any CO2 being generated (unless the headspace was mostly air). If you hit the keg with 15 psig of CO2 it would be completely absorbed into the beer over the course of a few days, so you would see your gauge slowly dropping over that period. Either your beer started generating CO2 then died out, or the beer wasn't generating any CO2 and the headspace of your keg was essentially all air (this seems unlikely).

Maybe heat the keg up to ~85 and see if you get any pressure building up.

You may want to gently agitate the keg, too.

 

[david_42]

The apparent attenuation won't make any difference, since the priming sugar added after the initial is the same and 5.7% ABV is within the abilities of all ale yeasts.

Possibly your room temperature is a bit lower as we move into Fall? That would increase the absorption of CO2 and reduce the pressure. Chill it and see what you get.

 

[IrregularPulse]

Your keg wouldn't sit at 15 psig if there wasn't any CO2 being generated (unless the headspace was mostly air). If you hit the keg with 15 psig of CO2 it would be completely absorbed into the beer over the course of a few days, so you would see your gauge slowly dropping over that period.

I don't think this is true. The keg is still going to be pressurized at 15psi. This is why you can burst a keg to seal it and then store it for several months and...guess what? When you pull the relieve there is still the same pressure bursting out. With your explanation if I hit it with 15psi, the beer would absorb in in a couple days so surely in 4months, I wouldn't get any hiss from my PRV when pulled,let along the full force release. this isn't theory, This is what I saw with my over oaked PA while letting it sit to mellow.

 

[david_42]

When you pull the relieve there is still the same pressure bursting out.

Not true. The CO2 you add to seal the keg will be absorbed and the pressure will drop.

On the other hand, I have often seen big beers continue to ferment after being kegged. It only takes a point or two to keep the pressure on. My 2003 Old Bog Water fermented an extra 3 points over the first year.

 

[elmetal]

I don't think this is true. The keg is still going to be pressurized at 15psi. This is why you can burst a keg to seal it and then store it for several months and...guess what? When you pull the relieve there is still the same pressure bursting out. With your explanation if I hit it with 15psi, the beer would absorb in in a couple days so surely in 4months, I wouldn't get any hiss from my PRV when pulled,let along the full force release. this isn't theory, This is what I saw with my over oaked PA while letting it sit to mellow.

the reason you get a hiss after having bled the headspace is because the headspace, is not unpressurized but the beer has co2 in solution, co2 comes out of solution and pressurizes the headspace to match the pressure of the beer.

that's why you get a hiss.

that's also how you lower the carbonation on an overcarbonated beer

 

[Rhizome]

david_42,
Your comment that the head pressure applied to seal the keg will drop as it dissolves into solution makes sense. The CO2 is seeking equilibrium at that temperature and pressure. Anyone have experience with the time scale on which this happens? Is it hours, days, weeks? In other words, if I apply gas, remove it and see the pressure drop from 10 to 3 psi in 4-6 hours and stabilize, is that likely because it's dissolving into solution?

 

[shortyjacobs]

I'm betting yes, rhizome. Since we carbonate our kegs with the "set it and forget it" method in 1 week with 2.4 volumes CO2 at 12 PSI, and since you have such a small headspace in a full keg, (what, like 1/2 gallon at best?), it's quite conceivable that you could use up most of the CO2 in that small headspace in a few hours...

 

[JKoravos]

david_42,
Your comment that the head pressure applied to seal the keg will drop as it dissolves into solution makes sense. The CO2 is seeking equilibrium at that temperature and pressure. Anyone have experience with the time scale on which this happens? Is it hours, days, weeks? In other words, if I apply gas, remove it and see the pressure drop from 10 to 3 psi in 4-6 hours and stabilize, is that likely because it's dissolving into solution?

It's possible, yes. That doesn't mean you don't have a small leak or some other issue. There are a lot of factors that determine how fast the pressure will drop, but a 7psi drop over 4-6 hours due to dissolution is possible.

EDIT: shorty beat me to it.

I've been wanting to do some CO2 dissolution rate studies since this seems to be an area of confusion for homebrewers. There's probably data out there in trade journals, but it would be interesting to get some direct numbers for dissolution rates in a corny as opposed to modeling based on existing data.

 

[Rhizome]

Thanks for the feedback, gents. I had already driven myself mad submerging every connection on the keg and looking for bubbles. After failing to find a leak anywhere, I thought more of it and posted the above. After a day of watching it drop and topping it off to 10 psi again, it finally stabilized and held at 10 and is now creeping up. Seems that it took a day to dissove enough gas that 10 psi was at equilibrium. Next time I may just leave the regulator on it for a day then disconnect and let the conditioning go.