Temp reduction suckback - Let's brainstorm

Maybe some folks have this all figured out and perhaps many others don't even know it's an issue. When you ferment at one temp and then reduce the temp later for any reason, you reduce the pressure in the fermenter pretty significantly. This can be trouble when cold crashing an ale or when ramping a lager down to 35 down from a diacytel rest.

If you use a bubbler airlock, you are certainly sucking the liquid in. If you use a medium sized blowoff into some liquid, you can definitely suck that back in too. My fermenting fridge isn't exactly a spotless sanctuary and I don't like the idea of that air nor oxygen getting into the fermenter.

My initial idea is to fill a plastic bag with CO2 and rubberband it over the blowoff tube (the end that is normally sitting in the starsan bucket). I'd do this right when I start dropping the temp. If I anticipate it properly, an alternative is to attach a deflated bag over the carboy neck as fermentation is still happening but winding down so that it fills with "free" co2 to be sucked back in during crashing.

Discuss.

Well two off the top of the head an easy one (space allowing) would be a couple coils of extra tube in a blow off, more for glass carboys that wont dent under the negative pressure. The math eludes me at the moment but I'm sure there could be a calculated amount that would allow for a draw, but not all the way back to the brew.

The other, which would work more for plastic/bettle bottles would be to attach a co2 cartridge injector near the top of the vessel and when the tempature drops to the point where negative pressure is apparent to open the valve and equalize it with a pure gas.

Fill your airlock w/vodka, and don't worry about the suckback. IMHO it's not enough of a concern, any oxygen that gets sucked in should rest above the blanket of CO2 that's on top of the beer right? And any aeration/oxidation that could possibly happen from the vodka splashing in to the beer would certainly be minimal.

Just my initial thoughts...

What about some kind of check valve?

wyzazz said:

IMHO it's not enough of a concern, any oxygen that gets sucked in should rest above the blanket of CO2 that's on top of the beer right?

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for a split second, yes. But, the laws of gas exchange would dictate that the CO2 and O2 would mix evenly.

Interesting discussion point Bobby. I'd obviously thought of the liquid that's getting sucked back (no biggee with vodka), but didn't consider the air exchange.

wyzazz said:

Fill your airlock w/vodka, and don't worry about the suckback. IMHO it's not enough of a concern, any oxygen that gets sucked in should rest above the blanket of CO2 that's on top of the beer right? And any aeration/oxidation that could possibly happen from the vodka splashing in to the beer would certainly be minimal.

Just my initial thoughts...

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I completely agree. Who cares? What is the issue Bobby? Are you worried about contamination or off flavors...it would seem that .5 ounces in 5 gallons is literally "a drop in the bucket."

I think the concern is the propensity for oxidation.

Some kind of on demand valve from your CO2 tank, not sure how but someone on HBT will figure it out or already does it.

Personally I am still waiting for the one button CIP solution (Kladue).

My vote is to bag the airlock all together prior to cold crash and cover it with sanitized foil. I don't really want vodka in my beer, and unless you're moving it around I wouldn't think the tiny bit of O2 would penetrate the cozy CO2 blanket mentioned above.

Slap a "jimmy" on your hose before putting it in the "box".

samc said:

Some kind of on demand valve from your CO2 tank, not sure how but someone on HBT will figure it out or already does it.

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We call that a regulator. You hit the nail on the head and didn't know it. You could just put a hose barb on your co2 line and put it in place of your air lock. Just set the regulator to a minimum and put the air lock back when it's chilled.

I just keep my air lock filled low enough that any negative pressure will only suck in a little air and not liquid.

I was just thinking about this the other day actually.

Sanitary filter in place of the airlock is my answer. That only stops the liquid but may increase the gas exchange.

I really think the small amount of O2 going in won't matter. If you're not transferring with CO2 it'll pick up way more O2 when you open the carboy to rack to keg, even more if you use a bottling bucket.

I think the only way to avoid it easily would be to use a fermenter that the walls won't collapse with vacuum and then completely shut it off to the atmosphere when you chill it. Could be quite easy if you used a normal rubber bung in a glass carboy, used a ball valve with a hose tail on each end, push one tail into the bung and then run a tube from the other to a normal blowoff. Then just turn the valve when it comes to cooling time.

It's not so much oxygen I'm worried about. It's the potential contamination inside my old nasty fermentation fridge. If you put vodka in the airlock, it's true there's no problem with liquid being sucked in, but there's also quite a bit of air going in with it after the level drops enough.
I'm trying to think of something simple but effective here and I'm not sure leaving my co2 bottle hooked up is fitting the bill. Thinking...

Bobby_M said:

I'm trying to think of something simple but effective here and I'm not sure leaving my co2 bottle hooked up is fitting the bill. Thinking...

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I usually don't cold crash, and never lager, so it's not much of an issue for me; but, how much is "sucked back" during an usual cold crash?

Could you just fill the airlock completely full? Or get a bigger airlock or something that you could fill more? Or will the physics of the suck back drain the liquid to get to air, no matter how much liquid it needs to suck?

I would not worry about it. From now on your beer will be cold, and nothing will grow fast. From a yeast biologist point of veiw...you got a lot of limiting factors for infection. Alcohol and yeast is your number 1 and 2. Vodka or star san is your best bet in an air lock.

Can you just take the negative pressure depending on your fermenter? Pop off your airlock and jam a rubber stopper in the neck of the carboy.
Modifying the ideal gas law (be prepared for liberal rounding):
P1/T1 = P2/T2, initially you are at atmospheric pressure and say 68F
1 atm/68F = x/34F, x = 0.5 atm. (convenient numbers )

So we are talking about going from atmospheric pressure (14 psi) to 0.5 atm (7 psi). Effectively a -7 psi pressure in your fermentation vessel upon cooling.

MATH FAIL: Didn't use absolute temperature. Correction posted here. <-1 psi change in pressure.

Does this seem like too much negative pressure on a glass carboy?

A Better Bottle would probably collapse a little bit. The change in volume necessary to equilibrate the pressure would depend on the headspace. Smaller headspace, less collapsing of the bottle.

I'd figure out a way to rig up a sterile filter like this to the air lock



I'm pretty sure I've seen them for sale at some web homebrew store whose name is currently escaping me.

If you want to try this but can't track down a place that sells singles, let me know

AZ_IPA said:

for a split second, yes. But, the laws of gas exchange would dictate that the CO2 and O2 would mix evenly.

Interesting discussion point Bobby. I'd obviously thought of the liquid that's getting sucked back (no biggee with vodka), but didn't consider the air exchange.

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Even if this did happen, I don't think there wouldn't be enough O2 to ruin your beer.

On another note, a sterile filter and check valve would probably satisfy those that are paranoid.

you are worrying over nothing.

If suckback really bothers you, use the "S" style airlocks for cold-conditioned beers.

Rushis said:

Can you just take the negative pressure depending on your fermenter? Pop off your airlock and jam a rubber stopper in the neck of the carboy.

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Speaking of which. I (used to) leave my carboys with sanitizer in them plugged with a rubber stopper. Where I had them stored had gotten pretty cold and I needed one. That stopper was wedged in so tight I about never got it out. It is permanently formed the the carboy.

jkarp said:

If suckback really bothers you, use the "S" style airlocks for cold-conditioned beers.

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It might keep the liquid out of the fermenter but seriously, if you saw the inside of my fermentation freezer, you'd understand why I want to keep that air out of my fermenter.

Bobby_M said:

It might keep the liquid out of the fermenter but seriously, if you saw the inside of my fermentation freezer, you'd understand why I want to keep that air out of my fermenter.

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I may be wrong but I thought the whole theory behind airlocks was anything that can hurt beer follows the laws of gravity. In other words, any airborne contaminant that makes it into the airlock will fall and be trapped by the liquid.

I definitely know what you mean about fermentation freezer funk - just did a bleach wash on mine a couple of weekends ago to beat back all the mold. Never had a contaminated lager yet (knocking on wood...)

Bobby_M said:

It might keep the liquid out of the fermenter but seriously, if you saw the inside of my fermentation freezer, you'd understand why I want to keep that air out of my fermenter.

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What about this idea? This type of setup is used to prevent moisture/liquids from entering your vacuum pump.

As the pressure drops the starsan will be pulled into the second bottle (here it is a 2L soda bottle). The starsan should keep the intake sanitary, but prevent any liquids from entering the fermenter.

This setup could be a temporary one inside of the fridge until the temps have dropped and stabilized. It could also be built with two flasks.

Just a thought. You could change the setup where the incoming air has to flow through the initial starsan, but if the temp rises any it will push the starsan out of the flask.

Rushis said:

Can you just take the negative pressure depending on your fermenter? Pop off your airlock and jam a rubber stopper in the neck of the carboy.
Modifying the ideal gas law (be prepared for liberal rounding):
P1/T1 = P2/T2, initially you are at atmospheric pressure and say 68F
1 atm/68F = x/34F, x = 0.5 atm. (convenient numbers )

So we are talking about going from atmospheric pressure (14 psi) to 0.5 atm (7 psi). Effectively a -7 psi pressure in your fermentation vessel upon cooling.

Does this seem like too much negative pressure on a glass carboy?

A Better Bottle would probably collapse a little bit. The change in volume necessary to equilibrate the pressure would depend on the headspace. Smaller headspace, less collapsing of the bottle.

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You need to use absolute temperatures for these calculations. Rankine is the absolute scale with degrees corresponding to Fahrenheit. To convert F to R, just add 460. 68 F = 528 R, 34 F = 494 R.

1 atm/528R = x/494R, x = 0.935 atm.

So we are talking about going from atmospheric pressure (14 psi) to 0.935 atm (13.1 psi). Effectively a -0.9 psi pressure in your fermentation vessel upon cooling.

Does this seem like too much negative pressure on a glass carboy?

Why not take off the airlock completely and use a carboy cap or a solid bung? That's what I do..............

rack your beer to a keg. purge the keg with CO2. and cold crash/lager in that. just keep the CO2 tank attached while the temp drops. as the CO2 contracts because of the cold more CO2 come out of the tank and equalize the pressure.


if you don't like to use a keg you can probably slap a carboy cap on there and do the same thing. have the airlock in the larger lead and connect your CO2 to the other lead. pressurize till you are just shy of pushing air out the airlock. the same thing should happen as above.

If you can calculate how much volume is changed from a given temperature drop, you can then plan for a CO2 reserve inline with your blowoff tube.

So for example, if you know that a drop from 50 degrees -> 35 degrees will cause a contraction of 1 liter, then you can put a 2 liter bottle inline like this diagram. When the fermentation is blowing off, the bottle will be filled with CO2. When the temp drops, this CO2 is what will be pulled back into the fermenter.

If you start with the 2 liter reserve bottle full of sanitizer, that sanitizer would then be displaced as the fermentation produces CO2. Then you know for sure that the reserve bottle has only CO2 in it when it starts to suck back in.

By adding a second bottle to catch the sanitizer as it blows out, you can then ensure that the only thing sucking back into the reserve bottle will be sanitizer. No gas exchange after fermentation stops.

tomwirsing said:

You need to use absolute temperatures for these calculations.

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Nice catch, this was a stupid mistake on my part. Thank you correcting the numbers Charles' Law would not make a lick of sense with a negative temperature. Should've stuck to SI units. Thanks for the info on Rankine, never looked at the absolute scale for degree F. Silly standard units

So with corrected numbers going from 285 K (54 F) to 274K (34 F) reduces the volume of your headspace by 4% ,assuming an open system as described hexmonkey and Brew-Happy. If your using a 6.5gal carboy for a 5.5gal batch you'ld be looking at a change in volume of 5 oz of gas.

Instead of trap setup, it might be workable to use a 1.25" OD tubing sunk into a starsan solution. With 1/4" wall, the ID would be 1", giving 3.14 sq. inches. Just 5" of tubing would give you 15.7 cubic inches or roughly 8 oz.

If you used 3 vertical feet of blowoff tubing (not counting the bending into the neck of the carboy), the tubing itself could 62 oz. There is no way you'ld generate enough negative pressure to lift a half gallon of water. Based on the volume change above, you could get by with 10 oz of liquid sitting a bucket and large diameter tube. The sanitizer solution would be drawn up the tube until the pressure equalized (less than 5" vertical up the tube).

Now I am intriqued. I'll have to try this the next time I crash cool, a 1.25" OD blow off tube and a pint of sanitizer.

Now let's see if my math fails.

That's probably the best solution for now. I've been using 1/2" blowoff tubing and that's just not big enough unless I coil it for a long run.

I often put warm PBW in a glass carboy and seal it with a solid stopper and as it cools it pulls a vacuum. I dunno how much vacuum but the stopper is a ***** to get out sometimes.

Just saw this thread. Building upon your orig idea:
A carboy cap with short tubing connected to both 'ports'. One piece of tubing has a small inline valve. The tubing without the valve has a balloon connected. Let the end of fermentation fill the balloon (or you could do it manually) and if the balloon runs out of gas just hook up your CO2 to the inline valve and carefully refill the carboy and balloon with gas. The balloon would also act as a relief valve (more like a burst disk) to ensure you don't over-pressurize the carboy (even if you do burst the balloon, just keep a slow stream of gas flowing until you attach and fill another balloon).

Also, surgical gloves can work well instead of balloons depending on what you're 'connecting' them to.

Bobby_M said:

It might keep the liquid out of the fermenter but seriously, if you saw the inside of my fermentation freezer, you'd understand why I want to keep that air out of my fermenter.

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Bobby - I didn't find this thread until someone gave me the link (see my new thread at https://www.homebrewtalk.com/f13/co...t-fluid-air-contaminating-181421/#post2099559). What was your final solution to this? Thanks.

I've never had enough suck back to completely drain my airlock down to the point where air would get in.

But Yooper's suggestion seems simple enough. Or you could just clean your fermentation freezer, ya know

EDIT: I do find it kind of hilarious that nobody suggested cleaning the freezer up to this point (that I saw). And we're supposed to be a cleanly bunch!!

Hexmonkey's diagram is what I planned to go with and I even have two PET containers with lids to do it. I just haven't rigged up the nipples yet. I realize it's probably a non issue but I like to find problems and obsess about them forever.

Bobby,
You could hook up a fixed liquid propane gas (LPG) regulator and CO2 tank to the fermenter. When serving cask ales on hand pump, the fixed LPG regulators are used to add CO2 as the beer is drawn out. I think the regulators are designed to apply .4 psi so it's not enough to push beer out of the hand pump and wouldn't be too much for a carboy. You'd only need it hooked up for 24 hours or so, long enough to crash the beer.
Nate

Spunding valve. Done.

(...and a pressurizable fermenter, duh)

You could just use a balloon.

+1 on using your normal airlock with vodka. Oxidation is non issue. People really need to quit over engineering everything...

I like the idea of racking to a purged keg and cold crashing in that. Then you can just hook it up and apply positive pressure with CO2 before you crack it.

weirdboy said:

I like the idea of racking to a purged keg and cold crashing in that. Then you can just hook it up and apply positive pressure with CO2 before you crack it.

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Are you saying to cold crash in the serving keg? Seems like that would defeat the purpose of the cold crash. I'm cold crashing a batch right now to get the suspended yeast and other debris to drop out so I can can then rack it to my serving keg nice and clean. (I'm under the assumption that "crack it" means serve it and not re-rack it to another keg.)