Isotech TTI-7-R, Operator'S Handbook Manual

Page: 27 / 120

4.1 Measurement uncertainty and traceability
Measurement is usually made on the assumption that there is a true
value. Whenever a measurement is performed it is unlikely that the
measured value will equal the true value. The difference between the
two values is the measurement error which will lie within the specified
limits of uncertainty. Uncertainty is defined as an estimate
characterising the range of values within which the true value lies.
By taking a statistically significant number of measurement samples, a
distribution of results will emerge. Confidence in the distribution
increases as more measurements are made. Using statistical
methods, the distribution may be described in terms of mean, variance
and standard deviation. The uncertainty or precision limit of a particular
measurement is characterised by this distribution.
Traceability is defined as the property of a measurement that may be
related to appropriate reference standards through an unbroken chain
of comparisons. Through traceability it is possible to demonstrate the
accuracy of a measurement in terms of SI units.
4.2 International temperature scale
The purpose of the International Temperature Scale is to define
procedures by which certain specified practical thermometers
including PRTs and thermocouples of the required quality can be
calibrated. The values of temperature obtained from them can be
precise and reproducible, matching at the same time the
corresponding thermodynamic values as closely as current technology
permits.
4. Measuring Temperature
Page 4-1