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WIKA operating instructions, model CTR3000
14191761.01 05/2019 EN/DE
7. Technical information about temperature
7. Technical information about temperature
7.1 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.
7.2 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.
Since 1968 when the International Practical Temperature Scale of 1968 (IPTS-68) was adopted, there have been significant
advances in the techniques employed in establishing temperature standards and in the measurement of thermodynamic
temperature. The International Temperature Scale of 1990 (ITS-90) gives practical effect to these improvements. Particular
features are:
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ITS-90 specifies the use of the PRT up to the freezing point of silver, 961.78 °C [1,763.2 °F / 1,234.93 K]. The platinum 10 %
rhodium/platinum thermocouple is no longer specified for use in the scale, though it and other noble metal thermocouples
will continue to be used as secondary standards.
■
New, more precise, fixed points have been introduced and mathematical procedures for calculating resistance temperature
equivalents have been revised so as to reduce the ‘non-uniqueness’ of the scale: that is, to reduce the differences which
occur between different, identically calibrated PRTs. In particular, the calibration of a PRT can no longer be extrapolated
beyond the freezing point of zinc, 419.527 °C [787.149 °F / 692.677 K], but requires a measurement at the freezing point of
aluminium, 660.323 °C [1,220.581°F / 993.473 K].
■
Alternative definitions are permitted in certain sub-ranges; the calibration of a PRT can be terminated at almost any fixed
point. This allows primary calibrations to be carried out with suitable PRTs over reduced ranges, and will be of special
importance to metrology standards departments which need to make precise measurements at ambient temperatures.
■
The part of the ITS-90 scale which may be measured by PRTs extends from -189.3442 °C [-308.8196 °F / 83.8058 K] to
+961.78 °C [+1,763.2 °F / 1,234.93 K]. The CTR3000 is specified to measure temperature over the range -200 ... +962 °C
[-328 ... +1,764 °F / 73.15 ... 1,235.15 K] . The actual range of temperatures which may be measured depends on the type
and range of the PRT.
The ITS-90 scale has much improved continuity, precision and reproducibility compared with IPTS-68. The implementation
of the ITS-90 scale according to its definition calls for changes in equipment and procedure compared with IPTS-68, but
lower uncertainties of calibration are achievable in all parts of the range. However, the instruments and equipment needed to
implement the ITS-90 scale in calibration laboratories will be substantially the same.
