A device used to control the rate of flow of a process material through a pipe. It is actuated either electronically or pneumatically in which either a stem or diaphragm changes the position of a plug in a seat either restricting or opening the passage of flow. As the final control element in a control system, it is therefore responsible for changing the value of the manipulated variable to the output signal from the controller. Pneumatic control valves are either air-to-open or air-to-close. The application depends on safety consideration based on the impact of supplied air failure. An equal percentage valve characteristic is used to describe a type of control valve flow characteristic in which there are equal increments of valve plug movement for the change in flow rate. The change in flow rate with respect to movement is small when the valve plug is near its seat, and high when the valve plug is nearly wide open. A linear control valve characteristic is where the controlled flow is directly proportional to valve travel and is often used with distributive control systems or programmable logic controllers. A quick-opening valve characteristic provides a maximum change in flow rate at low movement of the stem and plug. It is used for on/off control.