April 2011 - Control Global

a s k t h e e x p e r t S
Measurement of Drift, Stability, Offset, Bias.
“Ask the Experts” is moderated by Béla Lipták, process control consultant and editor of the Instrument Engineer’s Handbook
(IEH). He is now recruiting new contributors for the 5th edition. If you are qualified to contribute to this volume, or if you are
qualified to answer questions in this column or want to ask a question, write to liptakbela@aol.com.
QIn determining some of the static and dynamic characteristics
of sensors and other instruments, I find the
definitions of the following terms somewhat confusing.
I find it difficult to differentiate between them. For example.
the ISA dictionary gives the definitions for drift and
stability as follows:
Drift:
ANSI/ISA-S51.1-1979 (R1993). [ISARP67.04.01-2000.ISA-
RP67.04.02-2000]. — An undesired change in output over a
period of time, where the change is unrelated to the input,
environment or load.
[ANSI/ISA-75.05.01-2000] 3. — An undesired change in
the output/input relationship over time.
[ISA-37.1-1975 (1992)] — An undesired change in output
over time that is not a function of the measurand. Drift is
usually expressed as the change in output over a specified
time with fixed input and operation conditions. It is usually
used in the context of analog transducers, analyzers, etc.
Repeatability
Random error
(Precision)
Systematic error
(Bias)
Stability:
[ISA-37.1-1975 (R1982)] — The ability of a transducer to
retain its performance characteristics for a relatively long period
of time. Unless otherwise stated, stability is the ability of
a transducer to reproduce output readings obtained during its
original calibration, at room conditions, for a specified period
of time. It is then typically expressed as being “within X percent
of full scale output for a period of Y months.
[SA-RP55.1-1975(R1983)]— In data processing, a measure of
the ability of a device to maintain constant volumes for one or
more parameters that describe its operation. Freedom from undesirable
deviation. A measure of the controllability of a process.
In our work, these two terms are used quite interchangeably.
When we are calibrating an instrument, and it has an
offset, we say it has drifted. Similarly most test equipment,
such as dry bath calibrators, has stability (both long-term and
short-term) in its specifications. As an aside, stability is also
used to define a process controller that brings the process to
new setpoint after a disturbance. But, at that time, we know
that we are dealing with a process.
I would like to know how we differentiate between these
two characteristics? How do we measure them separately
when evaluating a sensor or another instrument? Similarly,
in order to correctly determine and publish offset and bias
of the various instruments, how should we better understand
what they are?
Yusaf Muti
yousaf_ zai_khan81@ yahoo.com
Illegitimate
error
definition of terms
Total error
(Inaccuracy)
Figure 1. Terms describing the error (inaccuracy) of a sensor
AThe first 150 pages of Volume 1 of the Instrument Engineers’
Handbook deals with such general topics. In
this age of advertisements that show attractive people
and places instead of guaranteed and clearly defined performance
data, it is hard to know how good a sensor is. Some
manufacturers do not even test all their sensors, and their
specifications do not even state how the published numbers
have been arrived at.
For this reason, it would be useful if ISA recommended
the testing methods and also recommended that the performance
found be printed on all instrument specifications, no
matter where the device was produced around the world.
For example, ISA could recommend that all drift specifi-
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