[cornershot]

 I have a Johnson digital controller on my fermentation fridge. I tape the probe to the side of the carboy or bucket and insulate it with a handful of fiberglass insulation in a zipper bag. The controller always reads the exact same temperature as the stick-on fermometer.

[tschmidlin]

I keep hearing that 10 degrees, and sometimes it is 5 degrees, but it always anecdotal.

If you do the stoichiometry for how much energy is released during fermentation, transferred over the surface area of a 5 gal bucket or carboy in air, it works out to a 3-6°F average differential over 72 hours of fermentation, depending on fermenter material, humidity, airflow, etc. 5-10°F at high krausen seems totally reasonable to me.

The other thing to keep in mind is that the warmer it is the faster it ferments, so the more it warms up.  So if ambient is 65 the beer could be 68, but if ambient is 75 the beer could be 85.  It will depend on all of the things above, including the yeast strain.

[Herminator]

All really great feedback.  Learned a lot with this batch.  I turned off the heater and the chamber has dropped down to a 65 ambient temp.  Obviously missed the crucial time, but live and learn.  Thanks again for all the feedback and suggestions.  Time to switch up some things. 

[hopfenundmalz]

Based on what I have read, that could reach 10 degrees higher....which would put me at much higher than suggested temps.

I keep hearing that 10 degrees, and sometimes it is 5 degrees, but it always anecdotal.  I suppose at some point I will set up my own experiment, but with all the agitation going on in the fermentor from thermal gradients and CO2 production, during active fermentation I can't see how there is going to be significant differences in temperatures anywhere in the beer.  The difference in temperature between a thermowell and something on the outside of the fermentor will strongly depend upon the type of device and how it is mounted.  A hard cylindrical temperature probe (like from a Ranco controller) makes horrible physical contact (almost no surface area contact) with the cylindrical wall of the fermentor, so it is important to cover them well with an insulator (bubble wrap) and lots of tape because the probe will measure the temperature of the air around it better than it will measure the temperature of the fermentor.  A stick-on thermometer makes great contact with the fermentor via its adhesive, but you're relying on something that has one side exposed to the air and the other side that has a thermal insulator (glass/plastic) between it and a large thermal mass (beer).

As Sean says, if you really want to know, you need to calibrate them, but make sure you calibrate them like you will use them because you might find the stick-on ones have one offset when on a glass carboy, and another when on a plastic carboy or bucket.  The good thing about all of this is that if you set everything up the same, you'll get consistent results.  You need to keep in mind that if someone says they get great results fermenting at 64F, not knowing how they are measuring temperature and what their offsets are, that might not be the same 64F to you. (Yeah, I know, I hate those "it is different from system to system" answers too...)

At the 2011 NHC, Terence Sullivan of Sierra Nevada said that they had a multiple probe sensor made and put in an 800 barrel fermenter. They found an 8 degree difference in temperature. Big fermenter, but there you go.

[Herminator]

Found an interesting estimate graph at the following link.  They estimate fermentation temp rises based on OG/SG.  Kind of interesting. 

http://homebrew.stackexchange.com/questions/5563/can-we-estimate-the-temperature-rise-in-the-primary-due-to-fermentation/5564#5564

[klickitat jim]

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.

[repo]

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.

[klickitat jim]

Now I'm confused. I thought internal temp would be higher than ambient.

[repo]

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.

[klickitat jim]

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.

[phunhog]

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?

[a10t2]

Found an interesting estimate graph at the following link.  They estimate fermentation temp rises based on OG/SG.  Kind of interesting.

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:

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.

[a10t2]

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?

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