We all know fermentation temp is important, and in a perfect world it would be best to have a stable temp with no swings, but realistically would a 2 degree swing between 64 and 66F cause problems? thank you
nope
Cause problems? No. However, if you’re trying to maintain consistency in your brewing, than you will want to recreate that two degree differential with each batch. That said, if the fermentation temp bumped up or down two degrees for a short time period — a couple hours — then went back to the previous temperature, no problem; I don’t believe you would ever notice.
My fridge controller is set to +/-2*F. Well within expectations.
A swing of 5 F wouldn’t cause problems as long as it didn’t go too high
cool, if i set my fridge to 65 for example with a 1 degree differential, when the temps get to 66 the fridge kicks on but because of the thermal mass of the beer the ambient temp gets way cooler and even when the fridge kicks off at 65 it continues to drop to 64-63 then rises back up to 65, i use a thermowell so for the most part my fridge does not cycle on very often cause the beer insulates the probe, but i was just curious about this my last 2 batches where not the best so i am trying to pay attention to all the variables even if it means getting a little OCD
You’ve described a fairly normal fermentation. I think your problem may be elsewhere.
FWIW the procedure I use, recommended by the manufacturers of most temperature control devices, is to place the probe in the air in the chamber rather than in a thermowell or otherwise monitoring liquid temperature. Because of the thermal mass of the beer you mention, as the air swings 2 degrees Fahrenheit either side of the set temperature, the liquid temperature will remain rock steady. This is easier on both the beer and the compressor in your fridge. You just need to separately monitor the liquid temperature and adjust your set temperature for the differential between ambient and beer, which will change with the level of fermentation activity (fermentation being an exothermic process.) Or just pick an ambient temperature that will ensure your beer is within an acceptable temperature range throughout fermentation, which is what I have learned to relax and do rather than being overly OCD.
I attach the probe to the fermenter becasue that’s the temp I want to know.
I do the same. I bought one of those thermo wells and silicone bung and it didn’t perform well and it would not stay in place. It’s now in the local landfill.
i had the same problem with the silicone bung one, now i use a carboy cap with a thermowell, alot easier then the bung i find, i also have a roll of parafilm and i tend to seal up the carboy cap with it, i just go overboard sometimes but i sleep well
I know we’ve been here before, Denny. I tried the probe in a thermowell one time, at your recommendation, and the result was big swings in the beer temperature, and either cooling or heating running almost nonstop. Because of the much greater thermal mass of the beer compared to the air, by the time the controller registers that set temperature has been achieved, ambient has gone far beyond it in whichever direction; then the beer coasts in that direction, and the over correction cycle starts in the other direction… My experience aligns with the theory of the manufacturers, and the experience of many other brewers. There is clearly, however, a set of brewers who get good results from probing the beer instead of the air, like Denny. So whatever works for you. But if one method doesn’t, try the other.
Silicone stoppers are the work of the devil and should be banned by international treaty. ;) They will not stick in anything. Being slippery, of course, is exactly what silicone is designed to do. Rubber is the answer.
Forgot we’d discussed this…but I’m pretty sure I didn’t recommend a thermowell since I have no experience with them. I put the probe against the side of the fermenter, put a paper towel over it and duct tape it down. But I agree…whatever works!
No, you’re right Denny, you just recommended I try monitoring beer instead of air. At the time I happened to be using fermenter with a thermowell, so that was how I accomplished it. An irrelevant detail as I see it. What might not be irrelevant is that said fermenter was Speidel, made of plastic with a high insulating value. Had I tried the method with my previous glass carboys or my current 10 gallon corny, just sticking the probe to the side, my results might have been different. Obviously many factors come into play. But I’m not fixing my system, because it isn’t broke. And I’ve also adopted the approach of exercising less precise control, just setting acceptable ambient conditions and proceeding to RDWHAHB while the beer sees to itself. I think you influenced me in that too. 
I installed a 10” thermowell thru the lid of my fermenter. Works great.