Instrument Terminology and Performance

This section was reprinted with format change only from the
work titled “Process Instrumentation Terminology (ANSI/
ISA-51.1-1979, Reaffirmed 26 May 1995)” with the permission
of The Instrumentation, Systems and Automation Society.
This permission is gratefully acknowledged. When using the
definitions in this document, please indicate, “This definition
is from ANSI/ISA-51.1–1979 (R1993), Process Instrumentation
Terminology. Copyright © 1993, ISA—The Instrumentation,
Systems and Automation Society.” For information, visit
www.isa.org.
The purpose of this standard is to establish uniform terminology
in the field of process instrumentation. The generalized
test procedures described in the section titled “Test Procedures”
are intended only to illustrate and clarify accuracyrelated
terms. It is not intended that they describe specific
and detailed test procedures.
This process instrumentation terminology standard is
intended to include many specialized terms used in the industrial
process industries to describe the use, performance,
operating influences, hardware, and product qualification of
the instrumentation and instrument systems used for measurement,
control, or both. Many terms and definitions relate
to performance tests and environmental influences (operating
conditions) as further explained in the “Introductory Notes”
section. Basically, this document is a guideline to promote
vendor/user understanding when referring to product speci-
fications, performance, and operating conditions. Process
industries include chemical, petroleum, power generation, air
conditioning, metallurgical, food, textile, paper, and numerous
other industries.
The terms of this standard are suitable for use by people
involved in all activities related to process instrumentation,
including research, design, manufacture, sales, installation,
test, use, and maintenance.
The standard consists of terms selected primarily from
Scientific Apparatus Makers Association (SAMA) Standard
PMC20.1 and American National Standards Institute (ANSI)
Standard C85.1. Additional terms have been selected from
other recognized standards. Selected terms and definitions
have not been modified unless there was a sufficiently valid
reason for doing so. New terms have been added and defined
where necessary.

This standard is primarily intended to cover the field
analog measurement and control concepts and makes no effort
to develop terminology in the field of digital measurement
and control.
INTRODUCTORY NOTES
Defined terms, where used as a part of other definitions,
set in italics to provide a ready cross-reference. In defining
certain performance terms, the context in which they are used
has been considered. It is fitting, therefore, that the philosophy
of performance evaluation on which these terms
based be explained.
Ideally, instruments should be designed for realistic operating
conditions (those they are likely to meet in service),
and they should be evaluated under the same conditions.
Unfortunately, it is not practical to evaluate performance
under all possible combinations of operating conditions.
test procedure must be used that is practical under laboratory
conditions and, at the same time, will make available, with
reasonable amount of effort, sufficient data on which
judgement of field performance can be made.
The method of evaluation envisioned is that of checking
significant performance characteristics such as accuracy rating,
dead band, and hysteresis under a set of reference operating
conditions, these having a narrow range of tolerances.
Reference performance is, therefore, to be evaluated and
stated in terms of reference operating conditions. Generally,
reference performance under reference operating conditions
represents the “best” performance that can be expected under
ideal conditions.
The effect of change in an individual operating condition,
such as ambient temperature, atmospheric pressure, relative
humidity, line voltage, and frequency, will be determined
individually throughout a range defined as “normal operating
conditions.” Logically, these can be expected to occur above
and below the values of reference operating conditions during
field operation.
While this approach does not duplicate all actual conditions,
where many operating variables may vary simultaneously
in random fashion, it does develop data from which

performance may be inferred from any given set of operating
conditions.
The effect of changes in an individual operating condition,
all other operating conditions being held within the
reference range, is herein called
operating influence.
There
may be an operating influence corresponding to a change in
each operating condition. In some cases, the effect may be
negligible; in others, it may have significant magnitude.
Tabulations of operating influences will usually denote
the performance quality level of a given design. Comparisons
of reference performance and operating influences for instruments
of a given design, or for different designs, will show
clearly their relative merits and probable performance under
actual operating conditions.

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