Spectrum Controls 1756sc-IF8u - Manual (Page 96)

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ControlLogix ™ Universal Analog Input Module
0°C and 1450°C. Type R thermocouples can be supplied to meet special
tolerances of +/-0.6°C or +/-0.1% (whichever is greater).
The suggested upper temperature limit, 1480°C, given in the ASTM
standard [7] for protected type R thermocouples applies to AWG 24
(0.51mm) wire. This temperature limit applies to thermocouples used in
conventional closed-end protecting tubes and it is intended only as a rough
guide to the user. It does not apply to thermocouples having compacted
mineral oxide insulation.
S Type
Thermocouples Pla Pla Pla Pla Platin tin tin tin tinum-10% Rhodium um-10% Rhodium um-10% Rhodium um-10% Rhodium um-10% Rhodium Allo Allo Allo Allo Alloy y y y y V V V V Ver er er er ersus Pla sus Pla sus Pla sus Pla sus Platin tin tin tin tinum um um um um
T TT T T her her her her her mocouples mocouples mocouples mocouples mocouples
This type is often referred to by the nominal chemical composition of its
positive (SP) thermoelement: platinum-10% rhodium. The negative (SN)
thermoelement is commercially available platinum that has a nominal purity
of 99.99% [21]. An industrial consensus standard (ASTM E1159-87)
specifies that rhodium having a nominal purity of 99.98% shall be alloyed
with platinum of 99.99% purity to produce the positive thermoelement,
which typically contains 10.00 +/- 0.05% rhodium by weight. The
consensus standard [21] describes the purity of commercial type S
materials that are used in many industrial thermometry applications and
that meet the calibration tolerances described later in this section. It does
not cover, however, the higher-purity, reference-grade materials that
traditionally were used to construct thermocouples used as standard
instruments of the IPTS-68, as transfer standards and reference
thermometers in various laboratory applications, and to develop reference
functions and tables [27,28]. The higher purity alloy material typically
contains less than 500 atomic ppm of impurities and the platinum less than
100 atomic ppm of impurities [27]. Difference between such high purity
commercial material and the platinum thermoelectric reference standard,
Pt-67, are described in [27] and [28].
A reference function for the type S thermocouple, based on the ITS-90
and the SI volt, was determined recently from new data obtained in an
international collaborative effort involving eight national laboratories. The
results of this international collaboration were reported by Burns et al.
[28]. The new function was used to compute the reference table given in
this monograph.
Research [27] demonstrated that type S thermocouples can be used from
-50°C to the platinum melting-point temperature. They may be used
intermittently at temperatures up to the platinum melting point and
continuously up to about 1300°C with only small changes in their
calibrations. The ultimate useful life of the thermocouples when used at
such elevated temperatures is governed primarily by physical problems of
impurity diffusion and grain growth, which lead to mechanical failure. The
thermocouple is most reliable when used in a clean oxidizing atmosphere
(air) but may be used also in inert gaseous atmospheres or in a vacuum for