Fast Sensor. Take a light gauge, Type J thermocouple, packed around with fiberglass, and stuff it in a 0.5" open-ended metal tube. Let the exposed thermocouple tip emerge from the fiberglass, flush with the open end of the tube. Fix the thermocouple on the machine frame under the disk so that the tip nearly touches as the disk rotates. This picks up a close approximation of the disk temperature and responds in a few seconds.
Temperature Controller. Run the thermocouple wires to a PID controller with a relay or triac output that controls the gas solenoid valve. This results in tight control and high yield.