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Instrumentation Symbols and Identification
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General
The procedural needs of various users are different. The standard recognizes these needs,
when they are consistent with the objectives of the standard, by providing alternative symbolism
methods. A number of examples are provided for adding information or simplifying the symbolism,
as desired.
Process equipment symbols are not part of this standard, but are included only to illustrate
applications of instrumentation symbols.
Application to industries
The standard is suitable for use in the chemical, petroleum, power generation, air conditioning,
metal refining, and numerous other, process industries.
Certain fields, such as astronomy, navigation, and medicine, use very specialized instruments
that are different from the conventional industrial process instruments. No specific effort was made
to have the standard meet the requirements of those fields. However, it is expected that the
standard will be flexible enough to meet many of the needs of special fields.
Application to work activities
The standard is suitable for use whenever any reference to an instrument or to a control
system function is required for the purposes of symbolization and identification. Such references
may be required for the following uses, as well as others:
• Design sketches
• Teaching examples
• Technical papers, literature, and discussions
• Instrumentation system diagrams, loop diagrams, logic diagrams
• Functional descriptions
• Flow diagrams: Process, Mechanical, Engineering, Systems, Piping (Process) and
Instrumentation
• Construction drawings
• Specifications, purchase orders, manifests, and other lists
ANSI/ISA-5.1-1984 (R 1992)
• Identification (tagging) of instruments and control functions
• Installation, operating and maintenance instructions, drawings, and records
2.3.2 The standard is intended to provide sufficient information to enable anyone reviewing any
document depicting process measurement and control (who has a reasonable amount of process
knowledge) to understand the means of measurement and control of the process. The detailed
knowledge of a specialist in instrumentation is not a prerequisite to this understanding.
Application to classes of instrumentation and to instrument functions
The symbolism and identification methods provided in this standard are applicable to all classes
of process measurement and control instrumentation. They can be used not only to describe
discrete instruments and their functions, but also to describe the analogous functions of systems
which are variously termed "shared display," "shared control," "distributed control," and
"computer control."
Extent of functional identification
The standard provides for the identification and symbolization of the key functions of an
instrument. Additional details of the instrument are better described in a suitable specification,
data sheet, or other document intended for those requiring such details.
2.6 Extent of loop identification
The standard covers the identification of an instrument and all other instruments or control
functions associated with it in a loop. The user is free to apply additional identification — by
serial number, unit number, area number, plant number, or by other means.
Definitions
For the purpose of understanding this standard, the following definitions apply. For a more
complete treatment, see ISA-51.1 and the ISA-75 series of standards. Terms italicized in a
definition are also defined in this section.
Accessible: A term applied to a device or function that can be used or be seen by an operator
for the purpose of performing control actions, e.g., set point changes, auto-manual transfer, or
on-off actions.
Alarm: A device or function that signals the existence of an abnormal condition by means of an
audible or visible discrete change, or both, intended to attract attention.
It is not recommended that the term alarm switch or alarm be used to designate a device whose
operation is simply to close or open a circuit that may or may not be used for normal or abnormal
interlock, start-up, shutdown, actuation of a pilot light or an alarm device, or the like. The first
device is properly designated as a level switch, a flow switch, etc., because "switching" is what
the device does. The device may be designated as an alarm only if the device itself contains the
alarm function. [See also Table 1, note (13).]
Assignable: A term applied to a feature permitting the channeling (or directing) of a signal from
one device to another without the need for switching, patching, or changes in wiring.
Auto-manual station: Synonym for control station.
ANSI/ISA-5.1-1984 (R 1992) 11
Balloon: Synonym for bubble.
Behind the panel: A term applied to a location that is within an area that contains (1) the
instrument panel, (2) its associated rack-mounted hardware, or (3) is enclosed within the panel.
Behind the panel devices are not accessible for the operator's normal use, and are not
designated as local or front-of-panel-mounted. In a very broad sense, "behind the panel" is
equivalent to "not normally accessible to the operator."
Binary: A term applied to a signal or device that has only two discrete positions or states. When
used in its simplest form, as in "binary signal" (as opposed to "analog signal"), the term denotes
an "on-off" or "high-low" state, i.e., one which does not represent continuously varying quantities.
Board: Synonym for panel.
Bubble: The circular symbol used to denote and identify the purpose of an instrument or
function. It may contain a tag number. Synonym for balloon.
Computing device: 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.
Configurable: A term applied to a device or system whose functional characteristics can be
selected or rearranged through programming or other methods. The concept excludes rewiring
as a means of altering the configuration.
Controller: A device having an output that varies to regulate a controlled variable in a specified
manner. A controller may be a self-contained analog or digital instrument, or it may be the
equivalent of such an instrument in a shared-control system.
An automatic controller varies its output automatically in response to a direct or indirect input of a
measured process variable. A manual controller is a manual loading station, and its output is not
dependent on a measured process variable but can be varied only by manual adjustment.
A controller may be integral with other functional elements of a control loop.
Control station: A manual loading station that also provides switching between manual and
automatic control modes of a control loop. It is also known as an auto-manual station. In
addition, the operator interface of a distributed control system may be regarded as a control
station.
Control valve: A device, other than a common, hand-actuated ON-OFF valve or self-actuated
check valve, that directly manipulates the flow of one or more fluid process streams.
It is expected that use of the designation "hand control valve" will be limited to hand-actuated
valves that (1) are used for process throttling, or (2) require identification as an instrument.
Converter: A device that receives information in one form of an instrument signal and transmits
an output signal in another form.
An instrument which changes a sensor's output to a standard signal is properly designated as a
transmitter, not a converter. Typically, a temperature element (TE) may connect to a transmitter
(TT), not to a converter (TY).
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.
Digital: A term applied to a signal or device that uses binary digits to represent continuous
values or discrete states.
Distributed control system: A system which, while being functionally integrated, consists of
subsystems which may be physically separate and remotely located from one another.
ANSI/ISA-5.1-1984 (R 1992)
Final control element: The device that directly controls the value of the manipulated variable of
a control loop. Often the final control element is a control valve.
Function: The purpose of, or an action performed by, a device.
Identification: The sequence of letters or digits, or both, used to designate an individual
instrument or loop.