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

Page: 28 / 120

Since 1968 when the International Practical Temperature Scale of
1968 (IPTS68) 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:
• 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.
• 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, 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 83.8058 K (-189.3442 °C) to 961.78 ° C. The TTI-7-
R
is specified to measure temperature over the range
-200 °C to +962 °C. The actual range of temperatures which may
be measured depends on the type and range of the PRT.
Measuring Temperature
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