ISA Symbols Explained

Having a uniform set of documentation standard symbols is crucial for effective communication and understanding in various fields. In the field of process instrumentation and control, the International Society of Automation (ISA) has established a standardized set of symbols and identification codes that are used to represent various types of process instruments, control devices, and system components.

isa symbols

This article aims to provide a comprehensive explanation of ISA symbols and their significance.

ISA Symbols

ISA, or The International Society of Automation, is a non-profit professional association for individuals and organizations engaged in industrial automation from various sectors such as manufacturing, process industries, infrastructure, and energy. It aims to advance the understanding and application of automation and control technologies to improve productivity, safety, and sustainability.

In line with this goal, ISA has developed widely recognized automation standards in order to help promote interoperability, consistency, and best practices in automation and control system design and implementation worldwide. These include the Instrumentation Symbols and Identification standard, commonly known as the ISA symbols, which are defined in ANSI/ISA-5.1 document, created in conjunction with the American National Standards Institute (ANSI).

The ISA symbols serve as a universal language for communicating information about process instrumentation and control systems. They enable engineers, technicians, and operators to quickly understand the type of instrument, its function, and how it is connected within a system.

Basically, ISA Symbols are graphic building blocks that are used to construct diagrams in a concise, easily referenced manner. These include instrumentation diagrams, functional diagrams, binary logic diagrams, and electrical schematics for use in various industries such as the chemical, petroleum, power generation, metal refining, pulp and paper, and numerous other continuous, batch, discrete-part processing, and material-handling industries.

These diagrams can be used in various types of documents including conceptual drawings, such as process flow and utility flow diagrams; construction drawings such as engineering flow, mechanical flow, piping & instrumentation, and system flow diagrams; technical specifications; installation, operating, and maintenance manuals; teaching examples and technical papers; instrumentation, loop, logic, and functional diagrams; and purchase orders.

In essence, ISA symbols facilitate efficient design, maintenance, troubleshooting, and operation of industrial processes.

ISA Classification Of Instrumentation

ISA classifies instruments into four categories: primary, secondary, auxiliary, and accessory. These categories are essential for assigning loop and functional identities.

Primary instrumentation comprises devices and hardware that perform measuring, monitoring, controlling, or calculating functions. It includes transmitters, recorders, controllers, control valves, self-actuated safety and control devices, as well as application software functions that either require or allow user-assigned identifications.

Secondary instrumentation encompasses devices and hardware used for measuring, monitoring, or controlling purposes. This category includes level glasses, pressure gauges, thermometers, and pressure regulators.

Auxiliary instrumentation comprises devices and hardware that contribute to the effective operation of primary or secondary instrumentation. It includes purge meters, sample handling systems, and instrument air sets.

Accessory instrumentation consists of devices and hardware that are necessary for the effective operation of the measuring, monitoring, or control system, even though they do not perform measurement or control functions. Examples of accessory instrumentation are flowmeter tube runs, straightening vanes, and seal pots.

ISA Symbols And Their Meaning

ISA has established standardized symbols for pipe and instrumentation, monitoring and control loops, logic processes, and electrical circuits, shown in the following subsections.

It is important to note that ISA mandates to maintain size ratios when scaling the symbols up or down. In certain cases, symbols can be developed to represent devices and functions not covered by the standard or to simplify the depiction of commonly used instrumentation.

Instrumentation Device Symbols

Instrumentation symbols are used to represent different types of instruments. Discrete instruments are denoted by circles, shared instruments are depicted by a circle within a rectangle, computer functions are represented by hexagons, and PLC functions are symbolized by a diamond within a rectangle.

Instruments are also classified into four categories based on their location relative to the operator: primary location, secondary location, field mounted, and inaccessible.

Instrumentation Device Symbols

Function Symbols

Function symbols include signal processing function, interlock logic functions, and pilot light, among others.

Function Symbols

Primary Measurement Symbols

Primary measurement symbols include transmitters, flame detectors, sensing probes, and measurement devices for various parameters like flow, level, pressure, and temperature. The table below shows some examples of ISA primary measurement symbols.

Examples of ISA symbols for primary measurement devices

Secondary Measurement Symbols

Secondary measurement devices include sight glass, gage glass, pressure gage, and thermometer.

Symbols for secondary measurement devices

Auxiliary And Accessory Device Symbols

Auxiliary and accessory devices include insert probe, sample conditioner, straightening vanes, instrument purge, pressure seal, and thermowell.

Auxiliary and accessory device symbols

Line Symbols

Line symbols refer to the symbols used to represent the various types of connections in a processing plant. These are divided into two categories: instrument-to-equipment and instrument-to-instrument connections.

Line symbols for instrument-to-process and equipment connections
Examples of line symbols for instrument-to-instrument connections

Final Control Element Symbols

Final control elements are devices that directly influence or regulate the flow, pressure, level, or other process variables of a system. These elements are typically the last component in a control loop and are responsible for implementing control actions based on the signals received from the control system. These include valves, dampers, louvers, variable speed couplings, and electric motors, among others.

The table below shows some examples of ISA final control element symbols.

Examples of final control element symbols

Final Control Element Actuator Symbols

Final control element actuators are devices that are responsible for directly manipulating or controlling the flow, pressure, or position of a process fluid in response to signals from a control system. These actuators are typically installed at the end of the control loop and act as the final mechanism for adjusting the process variable.

The table below shows some examples of ISA final control element actuator symbols.

Final control element actuator symbols

Self-Actuated Final Control Element Symbols

Self-actuated final control elements are devices that can automatically regulate a process variable without the need for external control signals. These include regulators, safety valves, safety elements, and moisture traps.

The table below shows some examples of ISA self-actuated final control element symbols.

Examples of self-actuated final control element symbols

Control Valve Failure And De-Energized Position Indications

Control valve failure and de-energized position indications are mechanisms or signals used to indicate the status or position of a control valve in case of a failure or when the valve is not receiving power. These indications provide valuable information to operators or control systems to identify and respond to abnormal conditions or valve malfunctions.

Control valve failure and de-energized position indications

Functional Diagramming Symbols

Functional diagramming symbols are used to illustrate functional components including controllers, signal processors, and final control elements. The table below shows some examples of ISA functional diagramming symbols.

Functional diagramming symbols

Signal Processing Function Block Symbols

Signal processing function block symbols are used to depict specific signal processing operations or functions within a system. These symbols are used to illustrate the flow of signals and the operations performed on those signals.

The table below shows some examples of ISA signal processing function block symbols.

Examples of signal processing function block symbols

Binary Logic Symbols

Binary logic symbols are used to illustrate binary logic functions in a system. These symbols are used in logic diagrams, such as digital circuit diagrams or ladder logic diagrams, to depict the logical relationships and operations performed on binary signals.

The table below shows some examples of ISA binary logic symbols.

Examples of binary logic symbols

Electrical Schematic Symbols

Electrical schematic symbols are used to depict various electrical components, devices, and connections in electrical schematics, diagrams, or circuit drawings. These symbols represent electrical elements and facilitate clear communication in electrical engineering and design.

The table below shows some examples of ISA electrical schematic symbols.

Examples of electrical schematic symbols

ISA Symbol Identification Codes

In addition to graphical symbols, ISA has established identification codes that consist of a combination of letters and numbers. These provide further information about the device’s specifications, function, and location. By understanding these identification codes, professionals can accurately identify and specify the required instruments and devices for a given process.

Generally, two or three letters are employed. The first letter identifies the measured or initiating variable, while the second letter acts as a modifier. Any remaining letters provide identification of the specific function.

The table below defines the meanings of the individual letters when used to identify loop and device functions.

To illustrate, consider the tank depicted in the diagram below.

Example of a diagram using ISA symbols

The diagram above incorporates several instruments: a direct reading level indicator labeled LI17, a high-level detector denoted as LSH17, and a low-level detector referred to as LSL17. In this case, the first letter ‘L’ signifies the variable being measured, namely level. The letter ‘S’ denotes a switch, and ‘H’ and ‘L’ respectively indicate high and low.

Furthermore, the output from the level switch connects to an alarm denoted by the shared instrument symbol, specifically LAHL17. In this instance, ‘A’ signifies an alarm, while ‘H’ and ‘L’ respectively represent high and low. This configuration implies that the alarm will activate if the fluid level exceeds the set high level or falls below the set low level.

Scroll to Top
Complete... 50%
Please enter your name and email address below to receive a link to the ebook.

You’ll also receive regular tips to help you master Excel for engineering.



By Charlie Young, P.E.

Take your engineering to the next level with advanced Excel skills.