PID is an Auto (closed loop) control uses a process sensor to determine the difference between the process value and the set point. Then the controller applies power to a control output load in order to reduce that difference. Some processes need to maintain a temperature or process value closer to the set point than on-off control can provide. Proportional control provides closer control by adjusting the output when the temperature or process value is within a proportional band. When the value is in the band, the controller adjusts the output based on how close the process value is to the set point; the closer to set point the lower the output. This is similar to backing off on the gas pedal of a car as you approach a stop sign. It keeps the temperature or process value from swinging as widely as it would with simple on-off control. With so many brands and methods of tuning, call Seagate today to help in your control needs!