MANIPULATED VARIABLE (MV): The Variable the Controller Actually Changes! 
In a control loop, the controller continuously monitors the process and decides:
“What should I change to bring the process toward the desired value?”
That variable being changed by the controller is called the Manipulated Variable (MV).
What is a Manipulated Variable?
A Manipulated Variable (MV) is a process variable that is adjusted by the controller to influence the Controlled Variable (CV) and maintain it near the desired setpoint.
In simple words:
CV = What we want to control
Example 1 — Flow Control
Suppose we want to maintain:
Flow = 100 m³/h
The controller measures the actual flow and adjusts the control valve position.
CV: Flow rate
MV: Control valve opening
If flow is too low:
Flow ↓ → Controller → Valve opens more → Flow ↑
Example 2 — Temperature Control
A reactor temperature must be maintained at 80°C.
The controller adjusts the steam control valve.
CV: Reactor temperature
MV: Steam valve opening / steam flow
Example 3 — Level Control
A tank level must remain at 60%.
The controller adjusts the outlet valve.
CV: Tank level
MV: Outlet valve position
Basic Control Loop
Setpoint → Controller → MV → Process → CV → Feedback → Controller
The controller uses the error between SP and CV to determine how much the MV should change.
Important Point
The MV is not necessarily a measured process variable.
For example:
Temperature controller → Steam valve
The temperature is measured.
The valve position is manipulated.
Therefore:
Temperature = MV
Valve position = MV
Quick Interview Question
What is the difference between CV and MV?
CV is the variable we want to maintain at the desired value, while MV is the variable manipulated by the controller to influence the CV.
Remember it this way:
CV = What you control
MV = What you manipulate
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