A controller takes action to compensate for the disturbances or load changes in the system. If the controller is properly tuned, the process response to this action by returning to the set point. These is an interval of time between initiation of an input change and start of the resulting response for each instrument in the system. Because of dead time in the response of the instruments, some lag will occur and process will not respond instantaneously to the control action. As a result the process will oscillate around the set point slightly as the controller tries to bring the process back to set point. Process response to a disturbance and resulting controller action can be shown on process reaction curve. A process reaction curve shows changes in the value of the control variable over a period of time.Analyzing a process reaction curve can provide a valuable information about process response and controller action. If the controller is properly tuned, the process reaction curve will be ¼ decay ratio curve. That is each oscillation will be ¼ size of the previous oscillation as the process return to the set point