Pneumatic control loop

A pneumatic control loop regulates a process variable using compressed air as the signal medium, the legacy standard on older marine plant before electronic and digital control. The transmitter converts the measured value to a proportional air pressure, classically 3 to 15 psi (0.2 to 1.0 bar), the pneumatic controller compares it to setpoint and outputs a corrective air signal, and a diaphragm actuator strokes the control valve. Pneumatics are intrinsically safe in flammable spaces and ride through electrical faults, so air control persists for boiler combustion and some cargo and fuel systems, often as backup to electronic loops.