When selecting ON-OFF control method, the output control is executed by ON/OFF or full output. The output control is OFF when the process value reaches the set value, and the output control is ON when the process value is lower than the set value. This method will fluctuate temperature unstably. Contrasting to ON-FF method, the PID control is to control the output by continuously proportional calculation to compensate for the change in the system. When the process value is close to the set value, PID control will adjust the output by percentage to reach a stable temperature situation.