Chapter_10_ Instrumentation

Auto/manual switch

A switch that allows a process technician to select either automatic or manual control from the front of a controller.

Bumpless transfer

The act of changing the controller from manual to automatic (or vice versa) without a significant change in controller output.

Cascade control loop

A control loop characterized by the output of one controller becoming the setpoint of another.

Control valve

The most common final control element in the processing industry that manipulates a process flow in response to a control signal.

Derivative action

A controller output response that is proportional to the rate at which the controlled variable deviates from the setpoint.

Fail-last (fail-in-place)

The condition of the valve seat upon loss of instrument air or power failure.

Local controller

A controller physically mounted in the processing area near the other instruments in the control loop.

Remote controller

A controller that is not located in the processing area with the transmitter and control valve.

Split-range controller

A controller where the output signal is divided between two final control elements.

Integral action (reset)

A controller output response that is proportional to the length of time the controlled variable has been away from the setpoint.

Proportional band (proportional gain)

The amount of deviation of the controlled variable from the setpoint required to move the output of the controller through its entire range.

Ratio controller

A controller designed to ratio (or proportion) flow rate between two separate flows entering a mixing point.

Variable speed motor

An electric motor controlled to operate within a selectable RPM range to adjust the flow rates in a process.

Final control element

The last active device in an instrument control loop that directly controls the manipulated variable.

Louver or damper

Devices used to control airflow, usually by positioning open or closed to regulate draft or temperature settings.