How to prevent fermentation temperature control from sucking?

[bigboogieman]

I have an Anvil SS bucket fermenter, with the Anvil cooling system (pumping ice water through a coil), and a fermwrap, all hooked up to an Inkbird controller. This setup seems to do a good job of keeping fermentation temperatures within a couple of degrees F.

I just used this setup for the first time, and I noticed something I hadn't considered: when the heater kicks on, the contents of the fermenter expand, and air is expelled out of the airlock. So of course as it cools back down about 1-2 degrees F before the Inkbird kicks on again, the contents contract and room air starts getting sucked back in through the airlock.

When the yeast were active, this didn't seem to be a problem, because they were pushing out enough CO2 to overcome the suck-back. But what about when the yeast stop doing their thing? I like to leave the fermenter alone for a couple of weeks after the yeast are done. I disconnected the heating/cooling system once the yeast slowed down, for fear that I would end up oxygenating the beer. The ambient temperatures were getting down to about 63F. I would have rather kept the temperature stable the whole time, but I guess it probably doesn't matter that much after the yeast slow down?

Is this a problem I actually need to worry about? It will be worse in the summer as I would like to keep fermentation temperature around 68F (for US-05), and ambient gets up around 82F. How does everyone else without CO2 capabilities deal with the expansion/contraction cycles sucking in air?

 

[tracer bullet]

You might be interested in an ongoing discussion ehre about the same thing:

https://www.homebrewtalk.com/threads/cold-crashing.701885/#post-10217046
If you search the site for "suckback" you'll find threads pop up pretty frequently that might also have more answers.

 

[hotbeer]

With 2° F change you probably would be changing a volume of air by .5%. If you assume it to be all CO2 gas in the headspace, I'm not certain how much that would be.

But lets say you have a gallon of headspace, then maybe 1 tablespoons worth of change. If you use a blow off tube then you can easily deal with that much change and not let anything into the FV.

Changes in atmospheric pressure as highs and lows move through your area probably account for way more of a change of volume.

 

[Bobby_M]

A long blowoff tube would accommodate a couple degree shift. Certainly not an airlock.

 

[bigboogieman]

With 2° F change you probably would be changing a volume of air by .5%. If you assume it to be all CO2 gas in the headspace, I'm not certain how much that would be.

But lets say you have a gallon of headspace, then maybe 1 tablespoons worth of change. If you use a blow off tube then you can easily deal with that much change and not let anything into the FV.

Changes in atmospheric pressure as highs and lows move through your area probably account for way more of a change of volume.

I don't know why using a blow off tube didn't occur to me, so thanks! I had a lot of headspace because I did a 2.75 gal batch in a 7.5 gal fermenter. There sure seemed to be more than 1 tablespoon worth of change. But I'm going to try to get a blow off tube set up next time.

 

[hotbeer]

I had a lot of headspace because I did a 2.75 gal batch in a 7.5 gal fermenter.

Well that is 4.75 gallons worth of headspace. IIRC the calculator I used showed a little more than a tablespoon of volume increase from expansion for a gallon of air.

So for that much headspace that'd be a about 5 Tablespoons or a little over a ¼ cup of air to account for. So don't use a short blow off tube with a narrow ID. Fill the tube with a at least 5 TSP of water and you'll know how much you need to have before it sucks up the water in the bubbler jar.

5/8 ID might be ideal. I used 7/8" ID blow off tubes. But that was all they had when I went to the store except for 3/8" which is too small IMO.

However you mention turning off the heater and letting it go to ambient temps as low as 63°F. That's going to be about a full cups worth of air changing assuming the barometer is steady.

Volume2 = Temp2 Volume1/Temp1 note: temps are Kelvin 70°F = 294.26K 63°F = 290.37K

v2 = 290.37K * (4.75gallons / 294.26K
v2 = 4.68 gallons

for a change of .016 gallons or about 16.1 tablespoons or a little more than a cup of air going in. Of which, about 21% of that cup of air is oxygen. So assuming that 4.75 gallons of headspace was CO2, then 0.28% will be O2.

So not much at all. But how much is too much, I don't know. If you aren't making a supper hoppy NEIPA then I'm guessing it's not enough to notice.

Again, I'm no physics whiz by any means. I might have gotten something wrong in my thinking and calc's.

 

[bigboogieman]

Well that is 4.75 gallons worth of headspace. IIRC the calculator I used showed a little more than a tablespoon of volume increase from expansion for a gallon of air.

So is the headspace the only thing that really affects the expansion/contraction? The liquid expands/contracts so little that it's not really a factor?

 

[tracer bullet]

So is the headspace the only thing that really affects the expansion/contraction? The liquid expands/contracts so little that it's not really a factor?

Well, liquid expansion is a factor, but yes air is the bigger factor. Guess it depends how detailed you want to get, and you have to keep in mind that the exact temp you're talking about will change what's going on as well (it's not linear at all). But off the top, air will expand / contract something like 10 times the rate of the water at room temperature. That's not at all precise but if you want to get a mental picture of things, it might do. As things get colder that ratio changes, and IIRC it would change in a way that makes the liquid even less of a factor, ratio-wise that is.

Long story short... yeah it's mostly the headspace...

 

[hotbeer]

Temperature of the gas inside the FV the and atmospheric pressure outside of the FV are the things that will affect the movement of air/gases in or out of the FV the most, IMO. But like I said, physics isn't my strong point.

Liquids and solids do expand and contract with temperature, but IMO, we would really be getting nit picky about the amount of O2 you might be bringing into the FV because of that.

I feel that for most beers that aren't dry hopped or super hoppy, then amount of air from temperature changes isn't bringing that much destructive oxygen with it.