A type of feedback control used to control a process in which a process operator makes the judgement to adjust a process variable. The operator receives a signal, usually from a gauge monitoring the temperature, pressure, flow, or level, and manually adjusts a valve. Opening or closing the valve to alter the flow of cooling water, steam, gases, etc., results in changes to the process conditions. The process operator continually monitors the gauges making more adjustments until the required process conditions are met.