Common Terms Used to describe P&ID Drawings

Interpreting or describing P&ID may be a very challenging task for beginners. In this article, we will show some common terms used in P&ID that make beginners or freshers understand how to describe the P&ID diagrams of their respective industrial projects.

Computing Device:

  • This device is commonly known as an arithmetic and logic unit.
  • This device performs all required arithmetic calculations and logic operations and displays the computed output valve to output devices.
  • This computing device is known as a computing relay.


  •  It is an electronic device that receives a measured valve from the sensor as its input.
  • This device receives a sensor’s output signal in one form and translates the same information into proportional another form of signal designated as a transmitter.
  • Basically, the physical quantity of the signal is converted to a proportional electrical quantity. Such as temperature in degrees to mill volts or in ohms in case of T/C or RTDs.
  • A converter is also known as a transducer which is a completely general term.
  • Generally, a Flow Element (FE) is connected to a Flow Transmitter (FT), but not to a converter (FY).
  • Though a device called a transducer sometimes may not be recommended for signal conversion.
  • Current (I) to Pneumatic (P) converter is the most recommended converter that we often come across it in most P&ID drawings.


  • A sensor is the input device called detector or primary element it is a part of the instrument loop.
  • This sensor initially senses or detects the process value of a process variable, and assumes a corresponding, predetermined, and intelligible state or output.
  • The sensor is an integral element within the functional loop.

Transducer and Transmitter:

  • The term transducer is a general device that converts or modifies the information received by the sensor.
  • The transducer produces a resultant output signal by sensing a process variable through the medium of a sensor in the form of a physical quantity and this signal is converted into proportional electrical quantity.
  • Depending upon the application type, the transducer can be considered as a primary element, transmitter, relay, converter, or another device.
  • This device has an output whose steady state value varies only as a predetermined function of the process variable.
  • The sensor may be an integral element of the transmitter.
  • A transmitter is a device that is required when the measured instrument signal needs to be sent to some distance or to a central control room.


  • The term monitor in instrumentation is used to sense, measure, and control the status or magnitude of more than one process variable to derive some useful information.
  • The monitor can also be used as a verb and is unspecific when used in P&ID drawings sometimes meaning analyzer, indicator, or alarm.


  • A panel is a structure or a rack that consists of a group of instruments mounted in it.
  • This panel houses the operator process interface and has a unique designation.
  • The panel includes one or more sections, cubicles, consoles, or desks.
  • The panel is a synonym for the board on Piping & Instrumentation Drawing.


  • The term local defines the instrument type and installed location for particular process control in industry.
  • The instrument is neither in a panel nor neither console nor it is mounted in the control room.
  • A local instrument is commonly in the sector or in area of a primary element or a final control element.
  • The word FIELD is often used synonymously with the term local.

Local Panel:

  • The term local panel is not a central panel or main panel.
  • Local panels are commonly seen in the sector or in the area of plant subsystems or sub areas of plant.
  • Note that the term Local Panel Instrument must not arise confusion with another term Local Instrument.
  • The word local instrument implies an instrument in the field.

Panel Mounted:

  • The term panel mounted is applicable to an instrument that is mounted in a panel or in a console.
  • It makes it easy to access for normal use for an operator.
  • Generally, a function that is accessible by an operator in a shared display system is the equivalent of a discrete panel-mounted device.

Shared Display:

  • The term shared display is the operator interface device usually a video screen called a monitor.
  • This displays the process and control information from various sources of that control system at the command of the operator.

Shared Controller:

  • The term shared controller is a type of electronic controller that includes pre-programmed algorithms.
  • These algorithms are usually accessible, configurable, and assignable.
  • This controller allows various process variables to be controlled by a single device.

Set Point:

  • For every instrument, the term set point is an input variable for setting a desired fixed value of the controlled variable.
  • The set point is set manually, automatically, or through the program.
  • Both set point and controlled variable values are expressed in the same units.
  • The term set point is used in closed loop system and is most important for controlling a process within the design or operating parameter of the process.

Pilot Light:

  • A pilot light indicates the existence of number of normal conditions a system or device.
  • It is different from an alarm light, used to indicate an abnormal condition
  • In other words the monitor light is noted as pilot light.

Rabert T

As an electrical engineer with 5 years of experience, I focus on transformer and circuit breaker reliability in 110/33-11kV and 33/11kV substations. I am a professional electrical engineer with experience in transformer service and maintenance. I understand electrical principles and have expertise troubleshooting, repairing, and maintaining transformers, circuit breakers, and testing them. Tweet me @Rabert_infohe

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