I place my probe on the middle span of the flexwatt and in the middle slot of the rack. The probe MUST not move! I also hot melt mine and cover it with electrical tape. It must be firmly fixed and unmoving. I would highly recommend a fail safe to protect against the possibility of the probe being pulled free. It attach the fail safe on the top slot of the rack on the flexwatt. It is set 10º over the temp set on the primary.