I finally hooked a PowerSwitch Tail II up to my Raspberry Pi and Fermwrap. The RPi has DS18B20 temperature probes directly measuring the temperature of the wort by going down through the airlock.
For this particular yeast strain, Northern Brewer recommends its optimal temperature range as 66-74*F (18.9-24.3*C).
What I did was subtract 66 from 74 to get 8. I rather arbitrarily chose that the fermwrap should be turned on when the temperature is lower than the bottom 30% of the range -- (8 * 0.3) + 66 = 68.4*F, and the Fermwrap should be turned off when the temperature is hotter than 60% of the range -- (8 * 0.6) + 66 = 70.8*F ---------- This is 20.2*C and 21.6*C respectively.
I left the house for several hours and came back to find the brew at 74.8*F -- 0.8*F over. The Fermwrap was turned off as it should have been (I am assuming it turned off exactly at 70.8*F). Yet the temperature increased way more than I had intended it.
I figured that by cutting the Fermwrap off at 70.8F, and given that the ambient temperature was dropping as it was getting later into the night, that the carboy's temperature would not have kept increasing.
What point should I cut the Ferwrap off? Is 60% of the scale too high?
Could the fact that I am surrounding the carboy and the fermwrap with a towel (to block sunlight) have anything to do with it?