How can you differentiate P &ID diagrams from Loop Diagrams as used In Instrumentation and Control Systems?
Usually (but not always) P&IDs will show the On Base functions and connections for Process Instrumentation. Loop Diagrams will show the complete "loop" all the way back to Control System with all the interposing connections.
P&ID shows elements relevant to the process flow...
sensors/transmitters, valves, tanks, pipes, etc.
Loop diagram shows strictly electrical loops...
transmitters, field panels, I/O connectors, etc.
P&ID is relevant to understanding the process, loop diagrams are useful for installing and troubleshooting electrical loops.