What is the main purpose of a loop diagram in instrumentation?

• A. To calibrate the loop
• B. To document maintenance history
• C. To map plant utilities
• D. To show signal flow from sensors through transmitters to controllers and actuators and indicators

Answer: (D)

Loop diagrams illustrate the path of the control signal through a process loop. They show how a measured variable is sensed, converted to a transmitter signal, processed by a controller, acted upon by an actuator, and finally displayed or indicated. This makes it clear which devices are in the loop and how they are connected, which is essential for troubleshooting, wiring changes, and commissioning. The description that matches what a loop diagram shows is the one that portrays signal flow from sensors through transmitters to controllers and actuators and indicators. The other ideas focus on calibration, maintenance records, or utility maps, which are not the primary purpose of a loop diagram.