I’ve seen many a debate on the forums about ambient versus internal temps during fermentation so I decided to buy a thermowell for my lagers. When I lagered without it, I kept the ambient temp at 52. This produced some fine lagers but conventional wisdom for those on the interior temperature side of the argument would say that the temp in the middle of the wort was higher than the 52 on the outside.
Well, I have my Johnson Controller set at 51F ±1degree and the probe in the thermowell. It’s either on 51 or 52 everytime I check. BUT, the ambient temp is 54F. I’m using the same thermometers that I used previously when I lagered without the thermowell, and they all were calibrated at 52F, including the Johnson controller.
I pitched a very healthy starter that had krausen coming out of the blow off tube in 2 days. It took off and is bubbling about as much as I’ve seen a lager bubble.
So, what am I missing in this little puzzle here? Shouldn’t the internal temp be higher than the ambient temp if the rule that fermentation raises wort temp is true???
Or maybe I’m just clueless. Can someone set me straight or tell me what I am missing?
Dave
that’s very strange. I feel like there must be something wrong with the measuring devices. Perhaps the thermo you used to use is reading high? I don’t have any experience with brewing lagers but I have had two fermenters side by side in the same fridge, one high gravity one low and seen the higher gravity read 8 degrees higher than the low at high krausen so I KNOW that fermentation creates heat.
Wort temperature is going to fluctuate less than air temperature because of the thermal mass. So, even if it’s on average higher than ambient, ambient will swing lower than the wort right after the cooling cycle has run, and higher than the wort later when things heat up.
Lagers fermentations also seem to be less vigorous than ales, so my completely unscientific guess is that the heat generated is less and the temperature difference is smaller.
I don’t believe that the temp in the middle of our small fermenters is significantly different from the temp on the outside surface of the fermenter. Fermentation causes currents in the liquid that mix it pretty well.
As for the air temp in the chamber, if it is at 54F and the controller is set to 51F +/- 1F, then the compressor should be running. Either the controller or air temp thermometer is inaccurate, or they are measuring different places and there is a difference in the air temp there.
I assume this is a chest freezer? If the carboy is in direct contact with the side of the freezer, I’ve noticed it will lose heat more quickly than if it is not.
It’s in my converted wine cooler. The compressor is on half the time and off half the time I check. And I check a lot as I love watching the yeaties do their thing.
Two lagers ago (~8 weeks), I did not have the thermowell and both thermometers and the controller were in synch.
I have to agree with Tom, that the heat is spread throughout the wort by the action of the yeast. I think the center stays cooler because it is in the middle of the thermal mass, thus being the last part to respond to the change in ambient temp.
I’ll have to try it with an ale next time.
Dave
Are you opening the freezer to measure the air temperature? That could make it go up pretty quickly.
Nope. Checking through the glass door.
As stated prev. in this string…the fermenter will change temp more slowly than the air. The air temp being 54 compared to 51 for the beer is not significant. Yes, at 54, the outside air is slowly heating the beer…but it would take a day or two at that temperature differential to see a difference in wort. So no worries.
Essentially by putting your temp sensor in the beer, you will find your fridge will cycle less often, but for longer times compared to keeping the air a constant temp. I have not found a significant difference in beer temp stabilty with the probe inside the beer or in the air. HOWEVER, in my setup, I only have a single fermentor in the fridge at a time. I imagine if I had three kegs of beer along with the fermentor, and I was sensing air… then I would find the fermentor fluctuating a lot more.
Either way, enjoy the fruits of your labor!
Where’s the probe? I re-read this and see that this is a converted wine fridge. The back where the element is will be much colder than the front by the door.
Fermentation is exothermic. The thermometers are not calibrated with each other. You need to compare the two thermometers in the same medium.
Or pull the temp probe out of the thermowell and hang it in the air for a few minutes until the temp stabilizes.
This is all I do to manage my fermentation temps. I also have a case-fan stirring the air gently around between two fermenters that are raised to the height of the hump by resting on a 1.5" by 6" board. My actively fermenting temp matches the surrounding air in the chamber. So for me using a thermowell is redundant. And I was not comfortable resting the fermenters directly in contact with the cold surface of the freezer.
Very pleased with the performance of the controller and chest freezer setup. It has exceeded all my wildest expectations. Easily the best brewing move I’ve ever made.
I really wouldn’t worry about this. The temperature of the beer is what matters, and you have that controlled.
I agree. Set your cooler temp a little lower than your desired wort temp and forget about it. Worry about important things, like what to brew next. 
One thing you can do is to take your probe out of the thermowell and let it sit according to how you normally had it in the past. Set it at the temperature you used and let it acclimate for a day. Then, put the probe back in the thermowell and read the temperature changes on the unit. This way, you will know what temperature you have been fermenting your lagers in the past (inside the carboy) and subsequently where to set the temperature in the future.
Thanks for the great feedback all! I’m trying to find a chest freezer on Craig’s list for cheap so I can do what Euge is doing. Until then, I like the mojo I have right now with the thermowell. Seems to be performing well as it doesn’t move from 51F whenever I check it.