ASL INTERCOM CTR5000, Руководство пользователя

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CTR5000 Operating Instructions
ASL operating instructions, model CTR5000 113
16. Appendix 4 - ITS-90
16.1 International Temperature Scale (ITS-90)
The purpose of this scale is to define procedures by which certain specified practical PRTs of
the required quality can be calibrated in such a way that the values of temperature
obtained from them can be precise and reproducible; at the same time matching the
corresponding thermodynamic values as closely as current technology permits. Since 1968
when the IPTS68 was adopted, there have been significant advances in the techniques
employed in establishing temperature standards and in the measurement of
thermodynamic temperature.
The ITS-90 scale has much improved continuity, precision and reproducibility compared
with IPTS68. The implementation of the ITS-90 scale according to its definition calls for
significant changes in equipment and procedure compared with IPTS68, 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. Particular features are –
ITS-90 specifies the use of the PRT up to the freezing point of silver, 961.78  C. 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
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.
 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, but requires a measurement at the freezing
point of aluminium, 660.323  C.
 Alternative definitions are permitted in certain sub-ranges, so that the calibration
of a PRT can be terminated at almost any fixed point. This provision allows
primary calibrations to be carried out with suitable PRTs over reduced ranges, and
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Note that each of these fixed points has an uncertainty associated with it.