durrettd said:
If your temp controller's sensor is taped to the fermenter, and if there is a 2-to-3 degree difference in your cooling (or heating) on and off temperatures, your beer will be constantly varying between the cut-on and cut-off temperatures.
You are correct. You solve that particular problem by using controllers with tighter tolerances than 2-3 degrees. An STC-1000 have a minimum differential of 0.3 C or 0.54 F. If you heat AND cool...then the total swing is 1.1F.
Second, if you keep the beer in a cold room (10F or so below desired beer temperature), then you only have to HEAT the beer to hold the temperature...and let it cool naturally. This approach drops the total swing back to ~1/2 F.
If your temp controller is sensing the air temperature, your beer will be fluctuating a fraction of the difference between the cut-on and cut-off temperatures.
In the short term, this is true...
assuming that the beer is not producing any heat of its own. My experience says that its about 1/3rd of the air temp swing. That said...it is HIGHLY dependent on local factors: size of the batch, fermenter material, size of the fridge, mass of the fridge, etc.
The other issue to consider is the "assumption" in bold above. Its not a valid assumption. An active fermentation produces a LOT of heat. A completed fermentation produces NONE. The beginning and the end produce somewhere in between. The amount of heat depends on how active the fermentation is. the more vigorous the fermentation, the more heat. Hefe yeast is notoriously active. I have seen hefe fermentation raise the temperature of the beer 12 F above the air temperature at the peak of fermentation. A more typical number is around 5 F, though.
If you hold a constant AIR temperature, then the extra heat of fermentation will simply raise the temperature of the beer by the amounts described above. So, you are holding your fridge at 60F, and the hefe raises another 12F for a BEER temperature of 72F. In fact if a Hefe yeast were allowed to get up above 68F or so, it might take off like a rocket and produce even more heat...maybe landing above 75F. Now you have a bubble-gum flavored beer.
Or....you anticipate this temperature rise...and set your fridge to 50F. 50F is too cold for the Hefe yeast to get started and ferment vigorously...so it ferments very slowly (if at all). Thus it produces only a couple degrees of temperature rise. Your hefe ends up fermenting at 53F (or so). Now you have zero Hefe flavors...and the fermentation may not complete.
These effects are most pronounced with Ales.
I've read that some yeasts are highly offended by temperature variations. That raises several questions: Do I believe everything I read? (Not for several decades.) How much variation does it take to affect yeast activity? How much does the temperature of the beer vary if the ambient air is varying 2-to-3 degrees? What am I missing here?
Some yeasts are ABSOLUTELY affected by temperatures, and temperature variations. Some not so much.
Kolsch yeast is notorious for dropping out of suspension if subjected to frequent, rapid, large, temp swings.
I cannot get Cal Ale to ferment below 65F...even that I have a hard time with.
hefe yeast I've described above.
Saison yeasts are notorious for quitting early without a significant rise in temperature over the ferment. But, you can't START high, or they will produce nasty high-order alcohols.
Most belgian strains produce very different flavors over a very small temperature range.
http://www.whitelabs.com/beer/BelgianYeastStrains.pdf
I'm playing around with WLP570 right now. And I promise the chart above is very accurate for that strain.
So, if you want to target a specific flavor from a belgian yeast, you have to control the BEER temperature withing a degree or two during the first 72 hours of fermentation---after that it doesn't matter as much. But, this is during the lag and ramp phases of fermentation where the heat produced by the yeast is increasing. Without good sensing of the beer temperature, you'd never be able to hold the needed temp range.
You don't have to believe what I'm writing....go do the experiments yourself. You will quickly find that sources like Jamil Z, White Labs, and Wyeast are very reliable...and won't need to continue to prove everything they say to yourself. I don't quote information from any source other than the ones I just listed. I've done a few tests of my own, and I've never managed to do anything except confirm the results reported by these authorities.
At this point, I take those sources information at face value. Any experiments I might try, in this regard, are limited to assessing a specific desired recipe characteristic rather than trying to completely characterize a yeast. If I want spicy, I target the spicy temp range. If its too much/not enough, I adjust a degree or two on the next iteration.