Spunding Before Cold Crash

[SamInNJ]

I’m spunding in a keg for the first time. i attached the valve and it hit 25psi after finishing fermentation at 68 degrees (so far so good)

I then chilled the keg to 38 degrees, and i would expect the pressure to drop as more co2 is absorbed, but the spunding valve gauge has barely budged.

Do i have an overcarbed beer now? i was trying to gauge for the appropriate carb level at 68 so that it would be ready to go at serving but something seems off

 

[VikeMan]

I then chilled the keg to 38 degrees, and i would expect the pressure to drop as more co2 is absorbed, but the spunding valve gauge has barely budged.

The pressure should ultimately drop to about 9 PSI or so. Not so much because more CO2 will be absorbed (a little will, but not that much), but rather because the CO2 is much less energetic at the lower temperature.

You say the gauge has barely budged. Has the beer actually chilled significantly already? 5 gallons of beer is is a lot of thermal mass. Also, why is your spunding valve still attached?

Do i have an overcarbed beer now? i was trying to gauge for the appropriate carb level at 68 so that it would be ready to go at serving but something seems off

At 68F, that 25 PSI indicates ~2.3 volumes of CO2 in the beer. When it chills, the total amount of CO2 in the beer + headspace doesn't change. A very small amount of CO2 will move from the headspace to the beer, but not enough to move the needle on your volumes of CO2 (in the beer) when rounding to a tenth.

So, if your beer was at 25 PSI at 68F, it's not going to be overcarbonated at 38F.

 

[SamInNJ]

I left the spunding valve on because i didnt think it would hurt and was curious to see the psi drop on the meter, but didnt. My temp probe is taped to the side of the keg and its been about 24 hours so i would hope the liquid is cooled sufficiently

 

[VikeMan]

My temp probe is taped to the side of the keg and its been about 24 hours so i would hope the liquid is cooled sufficiently

That's not going to accurately measure beer temperature, especially during a crash, when the difference between beer temp and ambient temp is so large.

I left the spunding valve on because i didnt think it would hurt and was curious to see the psi drop on the meter, but didnt.

If the gauge doesn't move after significant chilling, there's something wrong with it. The keg is (for practical purposes) a closed system, and there's nowhere for more pressure (offsetting what should be a pressure drop) to be coming from, unless fermentation wasn't complete.

 

[Dland]

Try releasing the pressure from valve before taking cold reading.

 

[McMullan]

I've made similar observations. I don't normally cold crash but have been for lagers recently. I assumed the yeast were still active and/or there was little chance of much more CO2 going into solution. I kept a close eye on it first time, as I was worried about a vacuum forming in the keg, but it never happened. Gauge went from about 15psi to 12psi and stayed there for weeks. I'm using blowtie spunding valves. As far as I can tell the cheapo gauges are reliable and pretty accurate. I've had 2-3 going at the same time and they all behave the same way. I think our expectations might be biased by our understanding of force carbonating and how quickly CO2 moves from the headspace into solution. Is spunding/natural carbonation a different model? Obviously, the 'CO2 is CO2' folks will say 'no'.

 

[DavidWood2115]

I have observed similar behavior when cold crashing my spunded beer. I think there are two things going on here on two different time scales.

As @VikeMan says, the small amount of CO2 in the headspace won't significantly change the volumes of CO2 in the finished beer. However, the small surface area and lower temperature means it could take quite a while to reach equilibrium. So it could take days to reach 9 psi.

In the short run (overnight), the change in pressure of the headspace will follow the Ideal gas law PV=nRT. If the initial temperature and pressure were 25PSI and 68F and the new temperature is 39F, you might be tempted to conclude that the new pressure P = 25psi * 39F/68F = 14psi. However, this would be wrong, since both the pressure and temperature in this equation are absolute, not relative. So the correct pressure at 39F is P = (25psi + 14.7psi) * 277K/293K - 14.7 = 22.8psi (68F is 20C = 293K and 39F is 4C = 277K). So your pressure should drop slightly, but not much.

Since I usually want to taste my finished beer as soon as it cools, I typically release the headspace pressure when I put it in the keezer and hook it up to the gas.

 

[Bobby_M]

I then chilled the keg to 38 degrees, and i would expect the pressure to drop as more co2 is absorbed, but the spunding valve gauge has barely budged.

I think the trip up here is that the pressure drop of the headspace is actually not caused by it dissolving in to the beer. When the 25psi was created in the headspace, the same 25psi was already in solution in the beer. Since the CO2 molecules are being produced in the liquid, they would instantly dissolve unless the headspace pressure was lower than the partial pressure of CO2 already dissolved. In other words, as beer is fermenting in a closed, temperature stable system, it should be pretty close to equilibrium at any given time. This is very different from the carbonation process where external CO2 is introduced into the headspace first.

The reason why pressure drops in the head space as the whole system cools down is based on the ideal gas laws. As temperature drops, so does pressure. The system is closed so the amount of CO2 doesn't change. The tank is rigid so the volume doesn't change. When you're cooling a tank that is maybe half full of beer and half empty head space, the gas in the headspace will definitely cool down faster than the beer so that co2 pressure will drop first and cause CO2 to come out of solution to seek equilibrium as it always does. As the beer temperature finally catches up (down?) to the final temperature of the tank, the headspace, etc. It will settle back in at the pressure found on the charts.

68°F @ 25psi is 2.33 volumes of CO2.
2.33 volumes at 38°F is 9.5psi

If this doesn't happen, I can only think of a few reasons.

1. The beer wasn't done fermenting.
2. The average temp of the beer at 25psi wasn't actually 68F.
3. The average temp of the beer isn't 38F now.
4. The pressure gauge is off.

 

[DavidWood2115]

I think the trip up here is that the pressure drop of the headspace is actually not caused by it dissolving in to the beer. When the 25psi was created in the headspace, the same 25psi was already in solution in the beer. Since the CO2 molecules are being produced in the liquid, they would instantly dissolve unless the headspace pressure was lower than the partial pressure of CO2 already dissolved. In other words, as beer is fermenting in a closed, temperature stable system, it should be pretty close to equilibrium at any given time. This is very different from the carbonation process where external CO2 is introduced into the headspace first.

Bobby, I don't think this is right. The partial pressure of 2.33 volumes at 68F is 25psi at equilibrium. But if you (somehow instantaneously, for a thought experiment) drop the temperature of the entire system to 38F, the partial pressure of 2.33 volumes of CO2 in the beer drops to 9.5psi while the CO2 pressure in the headspace only drops to 22.8psi according to the ideal gas law (see my post #7). So some of the CO2 in the headspace will be absorbed into the beer to reach equilibrium (just like with force carbonation). Assuming a reasonably full keg, this will have little impact on the volumes of CO2, but will reduce the headspace pressure to ~9.5psi. And it could take a long time to reach equilibrium due to the low temperature and small surface area.

 

[Bobby_M]

Bobby, I don't think this is right. The partial pressure of 2.33 volumes at 68F is 25psi at equilibrium. But if you (somehow instantaneously, for a thought experiment) drop the temperature of the entire system to 38F, the partial pressure of 2.33 volumes of CO2 in the beer drops to 9.5psi while the CO2 pressure in the headspace only drops to 22.8psi according to the ideal gas law (see my post #7). So some of the CO2 in the headspace will be absorbed into the beer to reach equilibrium (just like with force carbonation). Assuming a reasonably full keg, this will have little impact on the volumes of CO2, but will reduce the headspace pressure to ~9.5psi. And it could take a long time to reach equilibrium due to the low temperature and small surface area.

Yeah that's a good point about absolute temps.

 

[theredviper]

Try releasing the pressure from valve before taking cold reading.

I second this. I always see a drop 24h after cold crashing. Pull the valve and clear it out and put it back on.