What is an indexer in instrumentation?

• A. A valve.
• B. A controller.
• C. A positioner.
• D. A transducer.

Answer: (C)

In instrumentation, controlling the exact position of an actuator is essential to regulate process variables. An indexer is the device that translates the control signal into a precise actuator position, which is the function of a positioner. It sits between the controller and the actuator and adjusts the actuator input (air pressure or electrical drive) so the valve reaches the commanded position, using feedback to hold and correct the position as needed. The valve itself is simply the final moving part, the controller provides the signal, and a transducer only converts energy forms. So the device that directly achieves the commanded position is the positioner.