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Old 09-12-2012, 01:01 PM   #1
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Default Temp Control, Airlock ... Possilbe Oxidation Problem

Just wanted to share what I noticed so others can avoid a possible problem. I was looking at my airlock bubbling yesterday and noticed that it was bubbling in reverse! Air was being sucked in instead of CO2 blown out! I noticed that it went back to normal after the temp controller cycled off. Apparently, each time the temp controller cycles on, it cools the headspace enough to cause the airlock to pull outside air in. It was bubling in reverse about once every few seconds for maybe a minute or two. I would think that as this happens continuously, a lot of air gets into the fermenter over time. I can see this because I am using a fermentation chamber with a hole in the lid that just lets the neck of the fermenter stick out. I use a 6 gal Better Bottle with 5.25 gal of beer, so that's 3 quarts of headspace. I don't know if this would be a problem with buckets or glass carboys. It probably depends on the headspace size and the insulative properties of the fermenter. The wall thickness on the Better bottle is quite thin, so maybe that is a big factor. My temp controller turns on/off with a 1F differential. The controller probe is taped to the fermenter with layers of paper towel and duct tape for insulation. Other than this problem, my temp control has worked flawlessly for years. For those with chest freezers or refrigerators, you might not notice this problem if you can't see the airlock while it is running. I usually switch from blow off tube to 3 piece airlock after fermentation slows down. I don't know how much this affects the beer. It may explain a slight oxidative flavor I ocassionally get. From now on, I'm keeping my blow off tube on until I rack to secondary or keg. Just a heads up.

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Old 09-12-2012, 01:38 PM   #2
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So the top of the fermenter is out of the chamber? Wouldn't that be the cause of the suction? If the ambient air around the airlock would cool at the same rate as the head space( or close to the same rate), then it would dramatically decrease the pressure differential created between the two. Just a thought. We need someone with a glass door chamber to do some checking on this phenomenon for us! Great observation!

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Old 09-12-2012, 01:49 PM   #3
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Yea I would think that this could only happen if the entire fermenter wasn't inside the chamber. Theoretically, if it was totally enclosed, the air surrounding the fermenter would cool slightly faster or at the exact same rate as the air in the headspace, which means either a favorable pressure differential or no change in pressure.

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Old 09-12-2012, 02:10 PM   #4
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Either way, CO2 is heavier than O2, so the O2 sucked into your fermenter would stay on top of the CO2 layer and would never touch or oxidize your beer.

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Old 09-12-2012, 03:01 PM   #5
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Yea I would think that this could only happen if the entire fermenter wasn't inside the chamber. Theoretically, if it was totally enclosed, the air surrounding the fermenter would cool slightly faster or at the exact same rate as the air in the headspace, which means either a favorable pressure differential or no change in pressure.
The ambient air pressure inside the chamber is exactly the same as it is outside the chamber. The pressure is equal inside and outside the chamber since it does not have an airtight seal. The pressure differential exists between the ambient air and the air in the headspace. When the headspace cools down the gasses in the headspace contract. The lower pressure in the headspace pulls ambient air in through the airlock. It doesn't matter whether the airlock is inside or outside the chamber because the pressure is the same. We see this all the time even more dramatically when people have suckback when lagering.
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Old 09-12-2012, 03:06 PM   #6
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Either way, CO2 is heavier than O2, so the O2 sucked into your fermenter would stay on top of the CO2 layer and would never touch or oxidize your beer.
That is partially true and does help to prevent oxidation. However, there is diffusion and the laws of partial pressure which will allow some of the oxygen to get to the beer. The question is how much. I don't think this totally eliminates the concern especially over periods of days or weeks when fermentation is slowed and little or no C02 is being produced.
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Old 09-12-2012, 07:10 PM   #7
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The ambient air pressure inside the chamber is exactly the same as it is outside the chamber. The pressure is equal inside and outside the chamber since it does not have an airtight seal. The pressure differential exists between the ambient air and the air in the headspace. When the headspace cools down the gasses in the headspace contract. The lower pressure in the headspace pulls ambient air in through the airlock. It doesn't matter whether the airlock is inside or outside the chamber because the pressure is the same. We see this all the time even more dramatically when people have suckback when lagering.
If the headspace and the ambient air inside the fermentation chamber cool at the same rate though, how can there be a pressure differential? I would think the only way there can be a pressure differential is if the system is open to air outside the fermentation chamber (ie. through an airlock), where pressure would be higher.
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Old 09-13-2012, 02:12 PM   #8
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I guess I'm confused as to you chamber setup. Not sure how the warmer air above the air lock can not have a higher partial pressure than the cooling co2 in the headspace (some of which will be going back into solution) Pics would be great. Seems crazy that one degree differential setting would even cycle on for very long. Ive seen auction in an airlock but only during cold crashing where you're dramatically cooling the solution quickly. Still believe my confusion is based on your chamber set up. If its not air tight then what method of cooling are you using. Not doubting you at all. Would just like to wrap my head around it! Late fermentation o2 pick up is just plain awful! Unless you like cardboard I guess...

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Old 09-13-2012, 04:39 PM   #9
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Okay. Let's see if I can explain it better. It has nothing to do with the chamber setup except for the fact that I can see the airlock without disturbing anything. It would be the same as a refridgerator or freezer. In all cases, there is no difference in pressure inside or outside the chamber. If there were a difference, then air would rush in our out when you open the door or lid. The pressure can be the same even though the temperature is different.

Now on to the fermenter. Normally C02 gas is produced which causes gas to escape through the airlock by bubbling up through it. Assuming CO2 generation has stopped, the airlock will only bubble if you increase or decrease the temperature of the headspace. The beer will also expand and contract, due to temperature, but this is a much smaller effect so we can ignore that for illustrative purposes. When you heat up a gas, two things can happen; it can expand in volume or (if constrained) it can increase in pressure. The same is true in reverse. If a gas is cooled it can either contract in volume or decrease in pressure. If you periodically increase and decrease the temperature of the headspace, the gas will expand and contract pushing and pulling the liquid in the airlock in either direction. If this expansion and contraction is large enough, gas will be transported in both directions throught the airlock. This will happen regardless of the temperature surrounding the fermenter since it is always at the same pressure due to the fact that air is free to move into and out of the chamber (or refridgerator) without any restriction.

As I explained earlier, I don't know how much of a problem this is, but I just wanted to make people aware that it can occur and may possilby have an effect on the finished product (YMMV).

Hopefully that helps. Sorry for the long drawn out explaination.

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Old 09-13-2012, 07:31 PM   #10
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Ok yeah I get the ideal gas law and all, but I don't get how you can have a gas constricted by an air lock decrease in temperature but not pressure. If the temp of the gas in the headspace decreases it will decrease in volume AND pressure.

Also, I've never seen air sucked though the water of an air lock. If there would be a suction created, then wouldn't it suck the water back first (assuming that the water level covers the holes of the inner piece)?

One last thing is in your OP you stated that co2 was still being produced, and pushing out through the lock. If that's the case the I would think that the co2 generation would be vastly greater than any suction force created by a one degree differential. Maybe I'm just dense and not getting it. However, if you can document this phenomenon and prove it, then I would assume that data would drastically decrease 3 piece airlock production. They basically have one purpose and you would be proving it an ineffective design.

Anyway I hope your occasional oxidative flavors disappear forever and the world is filled even more good beer.

Cheers!

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