In a separator level control loop, which component computes the error relative to the setpoint?

• A. Level sensor measures level; controller computes error; output adjusts control valve to regulate inflow/outflow
• B. Level sensor
• C. Control valve
• D. Pump

Answer: (B)

In a separator level control loop, the error is created by the controller. It compares the setpoint (the desired level) with the measured level from the level sensor and computes the deviation, typically as error = setpoint minus measured level. The controller then uses that error to determine the appropriate control signal, which is sent to the valve (the actuator) to adjust inflow or outflow and bring the level toward the setpoint. The level sensor simply provides the current level; the valve and pump execute the resulting action but do not perform the arithmetic to determine the error.