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CALIBRATION CONCEPTS
Zafer Turgay DA
Quality Manager / Chief of wind speed calibration laboratory
September 2011
Turkish State Meteorological Service
Ankara/Turkey
Definition of Calibration
A set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system or values represented by a material measure or a reference material, and the corresponding values realized by standards.
Four main reasons for having an instrument calibrated:
1. To establish and demonstrate traceability.
2. To ensure readings from the instrument are consistent with other measurements.
3. To determine the accuracy of the instrument readings.
4. To establish the reliability of the instrument i.e. that it can be trusted.
Terminology
RESULT OF MEASUREMENT : value attributed to a measure, obtained by measurement.
When a result is given, it should be made clear whether it refers
to:
- the indication
- the uncorrected result
- the corrected result and whether several values are averaged.6/10
UNCORRECTED RESULT :result of a measurement before correction for systematic error.
CORRECTED RESULT : result of a measurement after correction for systematic error.
ACCURACY OF MEASUREMENT : closeness of the agreement between the result of a measurement and a true value of a measure.
1. "Accuracy" is a qualitative concept.
2. The term precision should not be used for "accuracy"
REPEATABILITY : closeness of the agreement between the result of successive measurements of the same measure carried out under the same conditions of measurement.
These conditions are called repeatability conditions include:
- the same measurement procedure,
- the same observer,
- the same measuring instrument, used under the same conditions,
- the same location,
- repetition over a short period of time.
REPRODUCIBILITY : closeness of the agreement between the results of measurements of the same measure carried out under changed conditions of measurement.
A valid statement of reproducibility requires specification of the conditions changed.
The changed conditions may include:
- principle of measurement,
- method of measurement,
- observer,
- measuring instrument,
- reference standard,
- location,
- conditions of use,
- time.
EXPERIMENTAL STANDARD DEVIATION : for a series of n measurements of the same measure, the quantity is characterizing the dispersion of the results and given by the formula:
being the results of the measurement and being the arithmetic mean of the n results considered.
UNCERTAINTY OF MEASUREMENT : parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measure.
The parameter may be, for example, a standard deviation (or a given multiple of it), or the half with of an interval having a stated level of confidence.
Uncertainty of measurement comprises, in general, many components. Some of these components may be evaluated from the statistical distribution of the results of series of measurements and can be characterized by experimental standard deviations. The other components, which can also be characterized by standard deviations, are evaluated from assumed probability distributions based on experience or other information.
It is understood that the result of the measurement is the best estimate of
the value of the measure, and that all components of uncertainty, including those arising from systematic effects, such as components associated with corrections and reference standards, contribute to the dispersion.
ERROR : result of measurement minus a true value of the measure.
DEVIATION : value minus its reference value.
RANDOM ERROR : result of a measurement minus the mean that would result from an infinite number of measurements of the same measure carried out under repeatability conditions. Random error is equal to error minus systematic error. Because only a finite number of measurements can be made, it is possible to determine only an estimate of random error.
SYSTEMATIC ERROR : mean that would result from an infinite number of measurements of the same measure carried out under repeatability conditions minus a true value of the measure.
Systematic error is equal to error minus random error.
CORRECTION : value added algebraically to the uncorrected result of a measurement to compensate for systematic error. The correction is equal to the negative of the estimated systematic error. Since the systematic error cannot be known perfectly, the compensation cannot be complete.
Traceability
Property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties.
The concept is often expressed by the adjective traceable. The unbroken chain of comparisons is called a traceability chain.
Uncertainty
The uncertainty of measurement associated with the input estimates is evaluated according to either a 'Type A' or a 'Type B' method of evaluation. The Type A evaluation of standard uncertainty is the method of evaluating the uncertainty by the statistical analysis of a series of observations. In this case the standard uncertainty is the experimental standard deviation of the mean that follows from an averaging procedure or an appropriate regression analysis. The Type B evaluation of standard uncertainty is the method of evaluating the uncertainty by means other than the statistical analysis of a series of observations. In this case the evaluation of the standard uncertainty is based on some other scientific knowledge.
Expanded Uncertainty of Measurement
Within EAL it has been decided that calibration laboratories accredited by members of the EAL shall state an expanded uncertainty of measurement U, obtained by multiplying the (*LpJ L ` ~ )
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