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Quite a thermocline in the fermenter

So I’ve got this Kolsch fermenting in the basement, where it’s about 66-68 degrees ambient, and I want to keep it colder than that, so I have it sitting in a cooler with enough water to go about a third of the way up the carboy.

Since I’m an idiot, I got some of my fermometers wet and ruined them, and ordered a new batch, which I placed on the fermenters up higher where I’m not going to get them wet sitting the carboy in a water bath. However, the lower, wet fermometer strip on the Kolsch isn’t yet destroyed, and so I have two working fermometer strips on the carboy and, wow do they show different temps. (This is a five gallon better bottle.)

The lower one, which is in contact with the chilled water (I rotate in milk jugs full of frozen water) reads about 48-50 degrees. The top one, high and dry, reads 62-4. I think, “that can’t be right,” so I sanitize a long, skinny candy thermometer I’ve got that will slip right through the bung port in the top to see.

Result: up high, the temp reads 62 degrees. Down low, the beer reads 56. The chilled water itself reads 42.

So there it is, a major difference in the temp in a couple of temperature strata. I figure the lower strip reads lower than the internal temp because, duh, it’s in contact not only with the bottle but with the colder ice water outside. But I’m really surprised by the difference in the readings inside. I guess maybe I shouldn’t be, given the heat rising thing. But I guess I need to fill that water bath up a bit higher if I want to keep more of the carboy cold.

Not so much a question as an observation that might help others out.

the chilled water in the swamp cooler was 42 and some of the beer was 62? thats a 20 degree leap! was this during active fermentation? or after it settled down a little?

approximately how much chilled water would you say is in the cooler? is it less, equal, or more than what is in the fermenter?

i bet you would have a more consistent temperature if you had the chilled water all the way to the top of the cooler. or even better, enough water to reach the beer level in the fermenter.

yeah, I’m impressed by the difference too. This isn’t actively fermenting. It’s been in there a couple of weeks. The carboy was only about 1/3 in the water at that point. The top of the cooler is, of course, wide open, and the air was about 69 at that point.

I agree I need more water to keep that temp more consistent. I’d guess the ratio at the time of that reading was 5 gallons in the carboy to 3-4 gallons of water. I’ve put more water into the cooler now, and will top it off with even more once I see how the temp changes. I figure now it’s a bit of a science experiment.


When I use a swamp cooler, I fill the water to the same level as the beer. While the beer is actively fermenting the yeast will keep it well-stirred as they move up and down, but once fermentation is over the beer’s just going to sit there and match temp with its surroundings, water or air.

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