Do I need to move probe..

[JPerkins]

I put the probe of my thermostat on top of the flexwatt heat tape but under the tub and it seems to cycle properly but the inside of the tub only registers 78. Do I need to move the probe inside of the tub to reach desired temperature. The probe is causing a bubble around the probe could this be part of the problem?

Room temp 70-80 degrees
40 qt tub
11 inch flexwatt
and paper towel substrate