 Line Symbols Lines connect the symbols in a Piping and Instrumentation Diagram, or P&ID. Each line represents the physical connection between the instruments in a process control system. On the P&ID, different types of lines are used to indicate specifically what is passed along the connection from one instrument to the next. Let's examine the different types of lines used on a P&ID diagram and the types of connections they represent. Instrument Supply Pipe through which materials flow, steam, raw materials, and the final product. Connection to Process The instrument is connected directly to the process, a meter inserted into a pipe, material that passes through. The instrument measures the material and displays a flow reading. Pressured air ranging from 3 to 15 pounds per square inch is sent through a tube. The pressure quantity is a signal that indicates the value of a variable. Electronic signal Electronic signals indicate the value of a variable transmitted between two instruments. Hydraulic signal Hydraulic fluid, which represents a signal, is sent through a tube. Capillary tube This tube carries fluid that expands and contracts as the temperature to which it is exposed changes. The expansion and contraction varies pressure that is converted to a proportional electrical signal. Software Link These are signals sent between computers. Mechanical Connection A physical connection between two instruments. Congratulations! You have completed this activity, Lion Symbols.