I am going to add my details, I place the probe on the middle 'shelf' heat tape personally I glue it down with hot melt and tape over it with rubber electrical tape. I place my 'fail safe' t-stat probe on the top 'shelf' fixed the same way. The probes MUST be secure and I would highly recommend a back up or a t-stat with a low temp shut down relay (herpstats). Racks due to the movement of the tub can easily dislodge the probe that depending on the heat source can be a lethal event.