Actual Fermentation Temperature?

[7brew]

I've got a thermowell probe and fermostat I use to control fermentation temp. I had always read that if I keep the probe outside the carboy, that the actual temp of the wort will be slightly higher because yeast activity causes heat.

So I have been sticking it in the center of the carboy for my last few brews, immersing it into the center of the wort.

When I checked the temp today, it was a perfect 68 as it should be. Then I sloshed the wort around to stir up the yeast, and the temp dropped to about 63. I'm assuming that the outside of the liquid was cooler than the inside, which caused the temp drop.

I can also imagine that as the heater warms up the fermentation chamber, the temp swings in the other way as well, warming the outside of the beer to a possible 72+ in order to affect the thermowell sitting in the center of the liquid.

These kind of temp swings are probably not good for yeast. Should I just go back to having the probe outside the beer, keeping the chamber a more stable temp, and just keep it a bit cooler than 68 to account for yeast heat?

 

[CGish]

7brew,

Here is my understanding. (I use a BrewPi with center thermowell for temp control.)

At the height of fermentation, there is a lot of movement in the carboy. This will keep the temperature of your beer more constant with the hottest part being right in the middle. Once the yeast slow down and stop, the temperatures will follow a gradient from the outside in while it stabilizes to ambient. However, once the yeast are done temperature becomes far less important. So having a thermowell in the center of the carboy is perfect. It insures the core temp is not too high and once the active phase is passed it does not matter.

Make sense?

Cody

 

[PGEduardo]

My guess is that you caught it at a transient (heater kicked off and outside was cooling more rapidly). I've always read that the difference in temp. from the center of wort to the exterior carboy was less than 3 degrees though. If this is true, I think either one of two things is happening:

1 - your temp probe in the thermowell is not coupled well and shaking it around caused the probe to come into contact, changing the reading.

2 - your ambient temperature is at too much of an extreme and after the heater kicks off the exterior of the fermenter is essentially "quenching" in really cold air.

If you had fairly tight control on the ambient temp my guess is that the temperature profile should be fairly flat. Were you in the midst of high krausen?

 

[RM-MN]

At any time except during temperature transitions when you are changing the temperature quickly, the temperature on the outside of the carboy will be extremely close to the wort temperature in the center. People who have dual probes have found a difference of 0.5 degree F. which in brewing is unimportant.

 

[PGEduardo]

Also, I forgot to ask how you took the outside reading - was the probe in contact with the carboy and insulated from ambient air?

 

[PoppinCaps]

Have you thought about wrapping the carboy with some insulation to decrease the quick swing in temperature when the fridge kicks on?

 

[BigFloyd]

At any time except during temperature transitions when you are changing the temperature quickly, the temperature on the outside of the carboy will be extremely close to the wort temperature in the center. People who have dual probes have found a difference of 0.5 degree F. which in brewing is unimportant.

This ^^^^ is why I simply tape the STC-1000 probe to the outside of the fermenter and tape bubble wrap over it.

 

[7brew]

Also, I forgot to ask how you took the outside reading - was the probe in contact with the carboy and insulated from ambient air?

I've got a second thermowell (the small one) suspended mid air, about center of the fridge (hanging). This way I can monitor the ambient temp and the beer temp. I just switch out the probes on the fly to see what the difference is.

I am just trying to lock down a system with the temp probes so it's one less variable affecting my beer and I can focus on other things to improve it.

It sounds like having the temp probe immersed in the beer is going to be the most accurate if there's only .5 degree difference between the center beer and outside beer. But I'm still curious how hot the outside has to get to start warming up the very center (or vice versa). Maybe not that hot if people are doing it this way and producing good beers.