The Digital Thermometer is a part of the Nuclear Control mod.
This item has the same use as a Thermometer, but the Digital Thermometer can read temperatures higher than the standard Thermometer.
The Digital Thermometer uses EU to function and so must be recharged periodically in a BatBox, MFE, or MFSU. Charging benches can be used also. Interestingly, it can also be repaired using a Talisman of Repair, however, this does not actually charge the item, and it will be reset to its previous charge when used.
Right clicking on your Nuclear Reactor while holding a Digital Thermometer will display the Hull heat, Water evaporation point and the Melting point of the reactor in the chat box.