andy77 wrote:
You're not stirring things up, good data point.
This would be my question: if you're controlling the temp via a probe taped to the side, what's the actual temp swing of the wort. If the measured temp swings from (in you example) from 61, back down to 60 very quickly because you're measuring the glass temp, then everything equalizes, then the fridge turns back on again and cools back down again very quickly, the wose case scenario is overcycling the fridge. But if the actual wort temp, over the long term stays at 60-61 then that's a good ferment.
Remember that the goal isn't to measure the fermentation temps exactly, but to maintain a desired temp through a whole fermentation.
At any rate, I tape to the side of the carboy, and have great fermentations.
From what I can tell, the beer temp isn't reducing for the externally mounted probe. The "outside" probe hits a 1 degree drop in temperature pretty quickly, probably in about 15 minutes. To drop the thermowell probe temperature 1 degree, it takes more like 45 minutes to an hour. In 15 minutes (the time the freezer would run with the external probe location), I don't think the temperature of the beer is dropping too much. I also don't think the glass gets cold enough to drop the beer any further, so I don't think that everything is equalizing at the correct temperature. I actually think that the internal temperature is increasing, since it's not being cooled enough to maintain the correct temp.
To me, it seems like the outside probe is measuring the temperature of the glass, not the beer. When the freezer is not running, it's generally fine, since the glass is roughly the same temp as the beer. When it's being cooled though, measuring the temp of the glass and not the beer seems like it could be inaccurate. Over time, I believe it builds upon itself. I think that is compounded with an especially aggressive yeast (e.g. belgian strains).
To be fair, I am in California, and it gets warm in the room where the freezer is located. The freezer is fighting external temps also, so if it isn't running long enough to cool the fermenting beer, the fermentation can get out of control pretty quickly. I think that's exactly what was happening to me. I was getting fusels. I could taste them, and I got dinged on several judging sheets for them. Those beers fermented at the low end of the temperature range, according to the external probe, but they still ended up tasting hot. Combining that with the temperature difference I showed in the picture, I have a reason to believe the temperature measured with the external probe was inaccurate enough to cause issues.
As with everything, my experience is just that, my personal experience. I just wanted to throw out an example where the external probe mounting may not be ideal. I've learned that I get better control and better beers with a thermowell. For $14.00 (http://www.brewershardware.com/16-Stainless-Steel-Thermowell.html) and the price of a carboy hood, I know that my temperature is where I want it. It's definitely worth the price to me.
