Temperature Sensors can be located to monitor either heater surface or part temperature. Small sensors are held in place with a protective cover of silicone rubber/fiberglass vulcanized to the heater body. Larger sensors have a protective cover (over mold) that is bonded to the heater body.
A thermocouple consists of two wires of dissimilar metals welded together into a junction. Heating the sensing junction generates a thermoelectric potential proportional to the temperature. This signal is used by electronic temperature controls. Thermocouples are simple but they need special extension wires. Most common are J and K types. They can withstand high temperature, they are resistant to shock and vibration.
But the thermocouple main advantage in foils temperature control is its small size and fast response. A fine wire thermocouple junction takes up little space and has low mass, making it suitable for point sensing and fast response. They can be built in heaters without increasing its thickness and without changes in its flexibility.