Temperature control n00b question

[lgoolsby]

I just recently became the owner of an old freezer hooked up to a Johnson control and I have a n00b question that I am sure will spark off some kind of flame war.

I noticed today that I have my controller set at 68F but according to the thermometer sticker on the side of my carbouy (and we all know they are completely reliable) the beer is at least 72F. I know that beer gets warmer during fermentation so my question is should I adjust the Johnson down lower (~64F given the differential), or should I leave it be? The recipe calls for fermentation at 68F, so does that mean 68 on the controller or the for the beer?

 

[BigB]

Here is what I like to do... I tape the temp probe to the side of the fermentor and cover it with a sponge... That way I am measuring the vessel rather than ambient... However, I have a Ranco with the flexible probe lead. You may have the controller set at 68, but the vessel is going to be higher... Therefore, set your controller temp lower if you are measuring ambient.

 

[DeafSmith]

I have found the Fermometer temp. stickers to be pretty accurate if the carboy and ambient temps are about the same. If the ambient temp. is quite a bit different, then this web site:

http://tkachenterprises.com/Products.html

says:

QUOTE
People have often inquired about the accuracy of the fermometer and what it is measuring when placed on the outside of the fermenter in contact with ambient air. To answer those questions, I conducted an experiment where I put warm water in a glass carboy and placed the entire setup outside in thirty degree weather and compared the fermometer to internal temperatures measured by an immersion fermometer as they came into equilibrium.
What I learned is that for every ten degree difference in temperature between the atmosphere and the fluid in the fermenter, the temperature was off by approximately one degree as follow.
0-10 degrees difference between ambient and internal temperature ~ 1 degree difference between fermometer and immersion thermometer
10-20 degrees difference between ambient and internal temperature ~ 2 degree difference between fermometer and immersion thermometer
20-30 degrees difference between ambient and internal temperature ~ 3 degree difference between fermometer and immersion thermometer
So in most normal fermentation situations, the fermometer is only off by about 0.0 - 0.5 degrees from the internal fermenting temperature.
END QUOTE


I try to simulate having the probe immersed in the beer by securing a gel pack covered by some insulation (towel, folded up T-shirt, etc.) to the carboy or keg with a bungee cord. I slip the sensor probe under the gel pack so it is in between the gel pack and carboy (or keg) with the insulation covering the gel pack as completely as possible.

 

[jfowler1]

There are a couple schools of though on this, and I have personally participated in some pretty passionate debate.

It basically boils down to the brewer's opinion about how ambient temps reflect liquid temps during fermentation. I have been told that measuring ambient is perfectly accurate, but in my experience (and as the OP found) the temperature on a fermenting carboy can be quite different than the temperature set on a controller reading ambient air.

Personally, I have had success with temperature control by keeping my carboy in an ambient temp ~12-15 degrees below where I want fermentation temps to be, and using a johnson control + fermwrap + carboy cap thermowell to heat the liquid to fermentation temp as needed. Fermentation generates heat on its own (i've heard as much as 10 degrees above ambient), and the fermwrap is really there to just assist the yeast to warmer temps as fermentation slows.

Good arguments have been made against my set up because it is not very energy efficient to have competing cold and hot energy sources, but at the end of the day, it achieves a measured wort temp of my choice with a 1 degree +/- differential. IMO, the name of the game is controlling wort temps, not conserving energy.

If you have time, this thread really spells out my system (with pics), as well as some good opposing arguments.

https://www.homebrewtalk.com/f11/help-fermwrap-fridge-using-2-single-stage-rancos-240433/

As I said, this system has worked great for me, but to each his own.

Joe