A sensing element responds to the value of what?

The correct answer is the measured variable because the sensing element of a control system is specifically designed to detect changes in a physical quantity such as temperature, pressure, level, or flow. This measured variable is the actual parameter that the system aims to monitor and control.

For instance, in a temperature control system, the sensing element could be a thermocouple or a thermistor that directly measures the temperature of the environment or process. As it responds to changes in this variable, it sends data to the controller, which then makes decisions based on the input from the sensing element to maintain the desired level.

In contrast, the controller, recorder, and control valve have different roles in the overall system. The controller processes the information received from the sensing element but does not respond to a value itself; instead, it acts based on that value. The recorder is responsible for documenting the data over time and does not interact directly with the process. Lastly, while the control valve is crucial for regulating the flow, it also operates based on commands from the controller, rather than responding directly to the measured value itself.