This is a proportional controller; once the relation between the input and output are established the control system is left to produce an output which is proportional to the input. Disturbance inputs make it necessary for periodic calibration of the system to ensure proper functionality. With this arrangement a much smoother control of the furnace temperature is possible. As long as the outside conditions are constant so is the MEASURED VALUE, but if the load is disturbed no corrective action occurs without the operator changing the SET VALUE. Open loop control systems cannot compensate for disturbance of modeling errors. lt was noted that the system had no corrective measure when a disturbance input was applied thus creating an error. This showed one of the disadvantages of open loop control systems