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Especially in the classical chemical industry are typically "unique". Compared with more reproducible processes there is little experience or models about the real behavior. These industries especially those who run continuous productions involving potentially hazardous materials and process conditions are faced with a historical dilemma: availability of the production and plant safety without false trips of the process control caused by SIS-loops [safety instrumented systems]. With the introduction of smart sensors along with new physical principles for process automation the chemical industry has seen a continuous improvement during the last 2 decades in terms of reliability and availability. In order to achieve proven-in use status fast and due to economical pressure the same type of field instruments are used for process control loops, for monitoring and process optimizations purposes as well as for SIS-loops. The presentation will discuss methods and approaches how these "contradictional" requirements can be solved in a consents not in a compromise, based on the norm IEC 61508/61511 and the GMA/VDE 2650 or NE107 [NAMUR-recommendation]. A special focus is made on "check in place" methods to verify proper function under process conditions even with conditions where instruments are applied by chance or intentionally in applications that are "out of spec".