Analog Johnson Control question

Just got one of these and for its first trial I used it in a chest freezer with two simultaneously fermenting batches. The batches were pitched at the same time and pretty much fermented at the same rate. I taped the JC probe to the side of one of the plastic carboys.

To check against the JC, I also taped two other thermometers to the same carboy. I began fermentation with the JC set to 67, and it did a fine job actually keeping the beer around 67. After a few days I bumped up to 69 as fermentation slowed (using wlp002 so it's going quick), but I'm noticing that the JC is now maintaining the beer closer to 72-74.

I had hoped to edge up to that temp range, and I'm not worried about off flavors or anything - I'm just wondering if this is typical for these analog controllers. I did notice that the temp of fermentation was rarely lower than 67, and usually closer to 68, 69. I guess this is just an example of the imperfection of taping the probe to the side of the carboy?

Can anybody shed some light on what's going on, and what I should do to maintain better temps as fermentation slows?

kanzimonson said:

what I should do to maintain better temps as fermentation slows?

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My 2¢:

I don't bother trying to adjust the temp as fermentation slows. My goal is to maintain the temp where I want it during those first few violent days, and then I just let the wort cool as fermentation slows down.

Yea - actually - it should be doing the opposite. The wort should cool as fermentation slows.

How many days after fermentation started did you bump up the temp? You may have caused the yeast to start fermenting more vigorously, causing more heat.

As for Walker's suggestion on letting it go, I'm generally okay with that, but I've been using a lot of English yeasts lately so I want to make sure the yeast stay active long enough to clear up diacetyl.

Yes the heat output of the yeast is decreasing as fermentation slows, but combine the slight fermentation heat, plus the atmospheric leak of the freezer, plus my bump up on the temp control, and it's not a surprise that the temp rose a little bit. I bumped the dial up on day 3 of fermentation - it was clear that things were beginning to wind down, and from my visual and aural observation I don't think the extra two degrees kicked the yeast back into overdrive.

I just don't understand why it was perfectly regulating when I had it set at 67, but now it's at 69 the controller is keeping it at 74. It's almost like the controller is broken, but I've made sure it's still working by heating the probe with my hand, forcing the freezer to turn on.