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Whether you are a seasoned veteran or just getting started in this industry
it can be challenging trying to understand and remember the meanings of the
various terms used to describe accuracy. This is no small thing of
course. High accuracy remains one the most sought after features in
flowmeters today. Understanding the terms that relate to the way
manufacturers specify their own products' accuracy is essential in any
product evaluation.
Here then are some of the most commonly used terms to
describe accuracy and their meanings:
 | Precision
Precision is the ability to produce the same value within given
accuracy bounds when successive readings of a specific quantity are
measured. Precision represents the maximum departure of all readings
from the mean value of the readings. Thus, a measurement cannot be more
accurate than the inherent precision of the combined primary and secondary
precision.
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| Error
Error is the deviation of a measurement, observation, or
calculation from the truth. The deviation can be small and inherent in the
structure and functioning of the system and be within the bounds or limits
specified. Lack of care and mistakes during fabrication, installation, and
use can often cause large errors well outside expected performance bounds. Since the true value is seldom known, some prefer to use the
term uncertainty.
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| Uncertainty
Uncertainty describes the possible error or range of error which
may exist. People often classify errors and uncertainties into
spurious, systematic, and random types.
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| Spurious Errors
Spurious errors are commonly caused by accident, resulting in
false data. Misreading and intermittent mechanical malfunction can cause
discharge readings well outside of expected random statistical distribution
about the mean. A hurried operator might incorrectly estimate discharge. Spurious errors can be minimized by good supervision, maintenance,
inspection, and training. Experienced, well-trained operators are more
likely to recognize readings that are significantly out of the expected
range of deviation. Repeating
measurements does not provide any information on spurious error unless
repetitions occur before and after the introduction of the error. On a
statistical basis, spurious errors confound evaluation of accuracy
performance.
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| Systematic Errors
Systematic errors are errors that persist and cannot be considered
entirely random. Systematic errors are caused by deviations from standard
device dimensions. Systematic errors cannot be detected by repeated
measurements and usually cause persistent error on one side of the true
value. For example, error in determining the inside diameter of a pipe causes systematic error
in calculating flow. The error for this case can be corrected when
discovered by adjusting to accurate dimensional measurements. Broken,
and/or defective flowmeter components can cause systematic errors. This kind of systematic
error is corrected by maintenance or replacement of parts or the entire
instrument.
Calibration equations can have systematic errors, depending on the
quality of their derivation and selection of form. Equation errors are
introduced by selection of equation forms that usually only approximate
calibration data. These errors can be reduced by finding better equations or
by using more than one equation to cover specific ranges of measurement. In
some cases, tables and plotted curves are the only way to present
calibration data.
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| Random Errors
Random errors are caused by such things as the estimating required
between the smallest division on a head measurement device and water surface
waves at a head measuring device. Repeating readings decreases average random
error by a factor of the square root of the number of readings.
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| Total Error
Total error of a measurement is the result of systematic and
random errors caused by component parts and factors related to the entire
system. Sometimes, error limits of all component factors are well known. In
this case, total limits of simpler systems can be determined by computation
(Bos et al., 1991). In more complicated cases, different investigators may
not agree on how to combine the limits. In this case, only a thorough
calibration of the entire system as a unit will resolve the difference. In
any case, it is better to do error analysis with data where entire system
parts are operating simultaneously and compare discharge measurement against
an adequate discharge comparison standard.
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| Comparison Standards
Comparison standards for water measurement are systems or devices
capable of measuring discharge to within limits at least equal to the
desired limits for the device being calibrated. Outside of the functioning
capability of the primary and secondary elements, the quality of the
comparison standard governs the quality of calibration.
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| Discrepancy
Discrepancy is simply the difference of two measurements of the
same quantity. Even if measured in two different ways, discrepancy does not
indicate error with any confidence unless the accuracy capability of one of
the measurement techniques is fully known and can be considered a working
standard or better. |
Sources:
United States Bureau of
Reclamation
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