A closed-loop method of controlling a process in which information about the controlled variable is fed back to the input and compared against a desired value. The difference between the two signals is called the error or deviation. The feedback can be accomplished by a human operator as in manual control, or by the use of instruments as in automatic control. In a negative feedback control, the applied counteracting disturbance is motivated by the difference between the desired value and the actual value of the controlled variable. For example, in the manual control of the heating of a liquid in a vessel with steam, an operator periodically measures the temperature of the liquid; if this temperature is below the desired value, the steam flow is increased by opening the valve slightly. In the automatic control of the vessel, a temperature-sensitive device is used to produce a signal, which may be electrical or pneumatic, and is proportional to the measured temperature. This signal is fed to a controller, which compares it with some desired value or set point. Where there is a difference in signals, the controller changes the opening of the steam control valve to adjust the temperature.