Author Topic: Fermentation Temperature versus Ambient Temperature  (Read 5773 times)

Offline a10t2

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Re: Fermentation Temperature versus Ambient Temperature
« Reply #15 on: December 12, 2009, 08:57:38 AM »
A couple of things still lurking in the gray cells that bother me.... Yes, Palmer addresses the exothermic action of fermenting beer, and says it can run as high as +10. Then in the same chapter he says pick a spot with an ambient temperature of 65-72 degrees to place your fermenter, which is exactly what I suspect the majority of folks do.

I don't want to speak for John Palmer, but I'm guessing that's because if he said "pick a spot with an ambient temperature of 60-64°F" most beginning brewers would say, "Damn! I guess I can't make beer!"

Plus, the lag time for the temp to reach the probe may cause the outer areas of the fermenter to be too cold before the controller shuts off.

The center of the fermenter and the outside will be the same temperature, at least within our measurement tolerances. Convection AND conduction will see to that.

This came up in a thread over at NB recently, which was the first time I'd actually bothered to do a simple estimate:

I went ahead and did the math. Fermentation of glucose (I couldn't find the free energy for any disaccharides) releases 235 kJ/mol. Assuming a 20 L batch and 10% drop in sugar (about average, I think, for a 1.060 beer) that's about 2600 kJ total. Without any heat being removed, that would raise the temperature of the 20 L of water by 31°C.

You can try to estimate how much of that heat will get removed by the air, but there are a lot of assumptions involved. Assuming the heat gets released at a constant rate over 72 hours, that's a steady state release of 14 W. Just eyeballing, a bucket has a surface area of about 0.55 m^2. I'm just going to use Newton's law of cooling for simplicity, with a coefficient of 10 W/m^2-K; that could be off by as much as half. Anyway, it works out to a differential of 2.5°C, about 4.5°F.
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Offline dhacker

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Re: Fermentation Temperature versus Ambient Temperature
« Reply #16 on: December 12, 2009, 09:35:11 AM »
I use a Thermowell now and I have it set at a 1 degree differential - probably not thee best for my compressor. But honestly it doesn't seem to come on any more than before.
Some digital controllers do have adjustable differentials. I set mine to 1degF and set the probe into a thermowell. Works well. I don't have to check it every few hours.

I'm using a Johnson A419 and I've just been reluctant to dial it in to a 1 degree differential. But it seems I've needlessly worried about the compressor cycle.
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Offline bonjour

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Re: Fermentation Temperature versus Ambient Temperature
« Reply #17 on: December 12, 2009, 09:50:10 AM »
Remember that brewing beer can be as difficult or as simple as you want.

It's not rocket science,  but it can be if you want it to be.

If you think about it there are two major "activities" that occur in brewing beer,  Preparing the wort for fermentation, and the step that actually makes the beer fermentation.

Most brewers start out paying attention to the first one because it's hands-on, and ignore the other because it will be done in a week or two and is very hands-off.

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Fred Bonjour
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Everything under 1.100 is a 'session' beer ;)