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Negative Pressure with the 6.5 gallon big mouth bubbler

I’m trying to figure out if I’m doing something wrong.

I have a fermentation going for a mead with 5lbs blackberries, 10 lbs honey, 5 tbsp rosehips, 2 cinnamon sticks and 1.5 tsps of ground cloves. Mixed the must yesterday, measured at 1.090, about 5 points higher than what I wanted but no room to add extra water. Don’t think 5 points will make too much of a difference. Pitched Nottingham Ale Yeast w/ GoFerm.

Today I’ve noticed negative pressure in the airlock a few times. I opened it today just to punch down the fruit cap (adding nutrient tomorrow) and gave the lid a firm seal, and for a while, the liquid in the S airlock was all in the wrong side, as if it was being sucked in. I pressed as hard as the lid would go, but it kept stabilizing back to negative until about an hour ago.

Right now it’s showing positive and bubbling, but it’s only been positive the past hour or so. Was there a reason it might have been negative to begin with?

You will get suck back somewhat if you pitched warmer than ambient temperature. As the air in headspace cools the air gets denser and suck back has to occur. Its either boyles or charles law sorry forget which. The suck back is nothing to worry about especially since the yeast wants oxygen at this stage.

After I pitch yeast in my 6.5 gallon widemouth, I put the lid on with stopper(s) in place and shake the wort for 5 minutes to aerate it. I assume you did the same or similar. If not, you could get suck back as well just due to absorption of oxygen and nitrogen. If you are using canned berries, they are under negative pressure so they want to suck up some oxygen and nitrogen. Oxygen is only soluble at 8 ppm at room temp and pressure so it would not be a lot I suspect but I have not calculated the volume suck back do to this.

In summary, don’t worry about it


additional info

That makes sense now, and I feel better. I pitched at about 80-90 degrees F, since the blackberries were pasteurized (at 160 and no higher) and additional water to fill only lowered the temp a small amount.

I was more worried there might be an issue with the seal on the lid of the carboy or something, but everything you’ve explained makes sense with what I’ve done.

I have noticed today after feeding it some Fermaid K and punching the fruit cap, the pressure is pushing the berries against the lid, and it was enough to pop the lid a bit and break the seal. I’ve resorted to duct-taping the lid down.

I had a bit less headspace than I wanted when I mixed the must, but it was like an inch, inch and a half. Enough that I couldn’t add the extra water I wanted to bring it from 1.090 to 1.085 (when I first made this, it was a 1 gallon batch that started at 1.085 and I’m trying to replicated it but at 5 gallons).

I’d rather try and wait for primary to finish in a week or so before I transfer to the siphonless bucket.

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i start my fruit wines in a 2 gallpon bucket then rack to 1 gallon jugs seems to work

I was actually forced this morning to syphon a gallon off. Fruit was literally pushing against the lid.

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