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Measurement Errors
13
states (12.24 and 12.25) on the meter.
An
example where “bobble” is of significant
concern is the case where some action is taken when a value changes above or be-
low a certain quantity-and the digital display bobbles above and below the criti-
cal threshold.
Environmental Static Errors
All sensors and instruments operate in an environment, which sometimes affects
the output states. Factors such as temperature (perhaps the most common error-
producing agent), electromagnetic fields, and radiation
must
be
considered
in
elec-
tronic measurement systems.
Characteristic Static Errors
These static errors are still left after reading errors and environmental errors are ac-
counted for. When the environment is well within the allowable limits and is
un-
changing, and when there is no reading error, there will be a residual error
remaining that is a function of the measurement instrument or process itself. Er-
rors found under this category include zero offset error, gain error, processing er-
ror, linearity error, hysteresis error, repeatability error, resolution error, and
so
forth. Also included in the characteristic error is any design or manufacturing de-
ficiencies that lead to error. Not all of the ”ticks” on the ruler are truly 1.0000 mm
apart at all points along the ruler. While it is hoped that the errors are random,
so
that the overall error is small, there is always the possibility of a distinct bias or er-
ror trend in any measurement device.
For digital systems one must add to the resolution error a
quantization
error
that emerges from the fact that the output data can only take on certain discrete val-
ues. For example, an 8-bit analog-to-digital converter allows 256 different states,
so
a
0
to
10
volt range is broken into 256 discrete values in 39.06 mV steps. A poten-
tial that is between two of these steps is assigned to one or the other according to
the rounding protocol used in the measurement process.
An
example is the weight
sensor that outputs 8.540 volts, on a 10-volt scale, to represent a certain weight. The
actual &bit digitized value may represent 8.502,8.541, or 8.580 volts because of the
f
0.039 volt quantization error.
Dynamic Errors
Dynamic errors arise when the measurand is changing or in motion during the
measurement process. Examples of dynamic errors include the inertia of mechan-
ical indicating devices (such as analog meters) when measuring rapidly changing
parameters. There are a number of limitations in electronic instrumentation that
fall into this category, especially cases where a frequency, phase, or slew rate limi-
tation is present.
Instrument Insertion Error
The fundamental rule of making measurements is that
the measurement
process
should not significantly alter the phenomenon being measured.
Otherwise, the measur-
and is actually the altered situation, not the original situation that is of true inter-
est. Examples
of
this error are
found
in
many
places.
For example,
a
voltmeter
with