Although I don't have a helix, my ranco does this too.
Where do you have the probes located?
It should be like this:
Tstat probe: Tape on top of heat tape of the middle tub your speaking of.
Thermometer probe: Hot glue to the inside of the tub where the hot spot will be .
I then adjust the tstat until the thermometer probe is reading my desired temp. If it is off by more then 20 degrees there could be issues, but I have to set my ranco to 96-97 to achieve 90 degrees.
Good luck!
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