The control variable is ultimately controlled by the ---- loop

The control variable is ultimately controlled by the controller loop because the controller is responsible for processing the input from sensors, comparing it to the desired setpoint, and then making adjustments to the control element to maintain the desired process condition. In a control system, the controller operates based on algorithms and control strategies to ensure that the output remains at the setpoint despite any disturbances or changes in the system.

This interaction is essential in various applications, such as temperature control, pressure regulation, or flow control, where the controller continuously monitors the process variable and makes the necessary adjustments to the control variable. By effectively managing these adjustments, the controller ensures that the overall system remains stable and operates within the desired parameters.