That makes some sense to my pea brain. And it works with my thought of adjusting ambient temps. Instead of just lowering my ambient temp hit and miss, I think I’ll drop it 1°f for every 10pts OG. Then raise it to the target temp after krausen drops, or roughly a week into fermentation. So if I want my 1.050 beer to ferment at 65°, I’d start it at 60 and after a week gradually walk it up to 65. I buy the earlier argument too that the internal temp rise is related to how high the ambient temp is. IE a 70° ambient would raise more than a 60°. But for low tech guys like me, I think dropping ambient 1° per 10pts wouldn’t’ hurt.
Jim, if I ferment a beer at 65, it will start at an ambient of 67ish, and then after fermentation begins, I will slowly drop the ambient down to keep the beer at 65. After it is pretty much done I will raise the beer to 68-with an ambient of 70ish. My goal is to keep the beer at an actual temperature, not allow it to vary over time. In your example the beer would start at 58ish and then if it took off climb into the 60s somewhere. But you would be fermenting it over several degrees as opposed to 65.
Now I’m confused. I thought internal temp would be higher than ambient.
It will during active fermentation, and only for a few days usually. As fermentation slows you will need to raise that ambient back up or the beer temp will drop. The yeast experts say the first 2-3 days is where temp control is most important.
The goal is to keep the beer at a desired temperature during peak fermentation.
Ok, maybe what I wrote was confusing. When I saw you say my beer would go down to 58° I was wondering how it would go below my ambient temp.
So kind of a related question…where do breweries measure their fermentation temp? I assume they use some sort of thermowell…but where do they place it? Since heat rises wouldn’t the upper half of the fermenter be warmer than the bottom half? I also know that they are all glycol jacketed fermenters. So wouldn’t the beer that is nearest the cooling jacket be dramatically cooler than say the beer in the middle of the fermenter? Or is the mixing action of fermentation enough to overcome any heat stratification? What about when fermentation is winding down? Is that enough questions? 
Interesting, but incorrect. He both misplaced a decimal (the graph should run from ~0-60°C), and totally neglected heat loss to the fluid around the fermenter.
Here’s a copy-paste from an NB topic about four years back:
[quote]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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Most tanks I’ve worked with have the thermowell set up on a TC about halfway up the height of the tank, and maybe 8-12" long to get it away from the jacket.
http://www.glaciertanks.com/Thermometer_Thermowell-Thermowell_9_x_1_2_NPT_Female_SS304.html