In process control terminology, what is meant by a “process variable (PV)”? Choose the most accurate definition.

Introduction / Context:
In feedback control, clear terminology avoids design errors. The process variable, commonly abbreviated PV, is the measured quantity that reflects the current state of the process—such as temperature, level, pressure, or flow—which the controller watches and compares with the setpoint (SP). The controller then adjusts the manipulated variable (MV) to drive the PV toward SP.

Given Data / Assumptions:

• PV is the measurement coming from a sensor/transmitter.
• MV is the actuator command (e.g., valve position, heater power) that the controller changes.
• “Controlled variable” often colloquially refers to the same as PV when the loop is closed on it, but it is better to define PV precisely as the measured process parameter.

Concept / Approach:
Use the standard loop block model: SP (desired value) is compared to PV (measured value). The error e = SP - PV feeds a control law (e.g., PID), producing MV, which acts on the plant to influence PV. Therefore, PV is the process parameter that changes and is measured; it is not the manipulated variable itself.

Step-by-Step Solution:
Identify the role of PV as the sensor-measured quantity reflecting process state.Distinguish PV from MV (the controller output to the actuator).Select the option that captures PV as a changing, measured process parameter.

Verification / Alternative check:
Control textbooks and DCS/PLC HMIs label live measurements as PV; operators watch PV trends and alarms while adjusting SPs.

Why Other Options Are Wrong:
A: MV is not PV. B: “Controlled variable” can coincide with PV but the clearer, specific definition is the measured process parameter; option C states that unambiguously. D: Not all of A-C are simultaneously correct.

Common Pitfalls:
Mixing up PV and MV tags in code or graphics, leading to reversed control actions or confusing alarms.

Final Answer:
A process parameter that changes value and is measured for control.