Fermometer v. Johnson Probe

My Johnson Controller is saying I am holding a constant temp of 68. However, my Fermometer is reading a constant 70.

Who do I believe?

The Johnson probe is taped half way up the wort (to the outside of the carboy). I have bubblewrap surrounding the probe to insulate it. The Fermometer is at the top of the wort (it marks the 5 gallon cut off).

Again, which should I go by? I want to be fermenting at 68 and I'm making a big beer (1.095 OG) so I'm worried about fussel alcohols.

Keep in mind that when the Johnson reads 68*F, the actual temperature can be anywhere from 68*F to 68.9*F as the LCD only reads to the nearest degree. Also the accuracy tolerance is +/-1*F so that the actual temperature could be from 67*F to 69.9*F if it's working properly. The Fermomters are rather crude instruments and I would guess that their accuracy is worse than the +/-1*F. IMO, the two are essentially in agreement, or close enough as they say. If it makes you feel any better, you could split the difference and call it 69*F. A one or two degree variance in the fermentation temperature won't make a detectable difference in the finished beer. Shoot for even lower temps if you are concerned about fusels. I like to ferment most of my ales on the cool side at about 62-64*F. I do let them warm up near the end of the fermentation to insure that they finish out completely.

turn your controller down a degree or two and call it good.

eastoak said:

turn your controller down a degree or two and call it good.

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+1