A process automation or automation system (PAS) is used to automatically control a process such as chemical, oil refineries, paper and pulp factories. The PAS often uses a network to interconnect sensors, controllers, operator terminals and actuators. A PAS is often based on open standards in contrast to a DCS (distributed control system), which is traditionally proprietary. However in recent times the PAS is considered to be more associated with SCADA systems. PAS is the lowest level of automation, while MES (manufacturing execution system) is considered to be directly positioned above a PAS. Process automation involves using sensors, actuators, computer technology and software engineering to help power plants and factories in industries as diverse as paper, mining and cement operate more efficiently and safely.