Interpreting P&IDs can often be very challenging especially for beginners. In this piece, I shall be elaborating on some commonly misunderstood terms used in P&IDs to enable the beginner better understand how to interpret the P&ID drawings of their respective plants.
Computing Device
This is a device or function that performs one or more calculations or logic operations, or both, and transmits one or more resultant output signals. A computing device is sometimes called a computing relay.
Converter
A device that receives information in one form of an instrument signal and transmits an output signal in another form is called a converter. An instrument which changes a sensor's output to a standard signal is properly designated as a transmitter, not a converter. Typically, a flow element (FE) may connect to a Flow transmitter (FT), not to a converter (FY). A converter is also referred to as a transducer; however, "transducer" is a completely general term, and its use specifically for signal conversion is not recommended. An I to P (current to pneumatic) converter is a converter we often come across in P&ID drawings.
Local
This is the location of an instrument that is neither in nor on a panel or console, nor is it mounted in a control room. Local instruments are commonly in the vicinity of a primary element or a final control element. The word "field" is often used synonymously with local.
Local Panel
This is a panel that is not a central or main panel. Local panels are commonly in the vicinity of plant subsystems or sub-areas. The term "local panel instrument" should not be confused with "local instrument." From my explanation on the word local above, a local instrument implies an instrument in the field.
Monitor
A monitor is a general term for an instrument or instrument system used to measure or sense the status or magnitude of one or more variables for the purpose of deriving useful information. The term monitor is very often unspecific when used in P&ID drawings — sometimes meaning analyzer, indicator, or alarm. Monitor can also be used as a verb
Panel
A panel is a structure that has a group of instruments mounted on it, houses the operator-process interface, and is chosen to have a unique designation. The panel may consist of one or more sections, cubicles, consoles, or desks. Panel is the Synonym for board on P&IDs
Panel-mounted
This is the term applied to an instrument that is mounted on a panel or console and is accessible for an operator's normal use. A function that is normally accessible to an operator in a shared-display system is the equivalent of a discrete panel-mounted device.
