Spectrum Controls 1769sc-IF8u - Manual (Page 94)

84 Compact IO ™ Universal Input Module
positive thermoelement, which typically contains 10.00 ± 0.05 percent
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
short periods of time. However, type B thermocouples are generally more
suitable for such applications above 1200°C. Type S thermocouples should
not be used in reducing atmospheres, nor in those containing metallic vapor
(such as lead or zinc), nonmetallic vapors (such as arsenic, phosphorus, or
sulfur) or easily reduced oxides, unless they are suitably protected with
nonmetallic protecting tubes. Also, they should never be inserted directly
into a metallic protection tube for use at high temperatures. The stability of
type S thermocouples at high temperatures (>1200°C) depends primarily
upon the quality of the materials used for protection and insulation, and has
been studied by Walker et al. [25,26] and by Bentley [29]. High purity
alumina, with low iron content, appears to be the most suitable material for
insulating, protecting, and mechanically supporting the thermocouple wires.
Both thermoelements of type S thermocouples are sensitive to impurity
contamination. In fact, type R thermocouples were developed essentially
because of iron contamination effects in some British platinum-10 percent
rhodium wires. The effects of various impurities on the thermoelectric
voltages of platinum based thermocouple materials have been described by
Rhys and Taimsalu [35], by Cochrane [36] and by Aliotta [37]. Impurity