US 04 crazy warm fermentation

Ok, so a couple weeks back I brewed up a 1.065 OG batch, and pitched a pack of rehydrated US04.

Plastic bucket fermenter was placed in cool basement, per my lab thermometer laying next to bucket stayed pretty consistently in the 62-64F range (which feels accurate as our upstairs heat is set to 70F, and it's a lot cooler downstairs.)

Anyway, fermentation seemed to be pretty normal, but somewhere around day 2-3 I look at the fermometer strip on my bucket, and it had jumped from mid 60s day prior to mid 70s...like 74 to 76!

I've never had a fermentation jump the temp that much. I thought it was maybe 5* over ambient, not 10-12*! By next day it had dropped back to mid 60s, guessing fermentation had ended.

I did have a dark t shirt over the bucket, but can't believe that made that big a difference (this bucket is somewhat translucent and wanted to block light).

Is it likely that the fermometer is just wildly inaccurate?

Only way to tell if it's inaccurate is to compare it with a standard--i.e. one that you know is accurate. Another way is taste (solvent-like) & does the beer give you a headache!

I think with the shirt over it you were more than likely getting readings from both sides of the coin. Having the shirt over it will obviously assist in holding heat probably messing with the fermometer reading a bit. accounting for the 5 deg we expect, 5-7 deg off in that scenario I wouldn't feel would be too inaccurate when the ferment temps were ramped up from yeast activity. But you will need to test it.
Just my .02

Cheers
Jay

Ok, thanks, just curious if 10+ from ferm is even possible. I'm confident the ambient was 64 or less the entire time.

First I've used us04. Took it from 1.065 to 1.01 in about 5 days. I tasted sample at 10 days and it seemed tasty to me, so probably fine.