Allen-Bradley CompactBlock LDX 1790D-4T0, Руководство пользователя

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Publication 1790-UM003A-EN-P
Thermocouple Descriptions C-13
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
contamination usually causes negative changes [25,26,29] in the
thermoelectric voltage of the thermocouple with time, the extent of which
will depend upon the type and amount of chemical contaminant. Such
changes were shown to be due mainly to the platinum thermoelement
[25,26,29]. Volatilization of the rhodium from the positive thermoelement
for the vapor transport of rhodium from the positive thermoelement to the
pure platinum negative thermoelement also will cause negative drifts in
the thermoelectric voltage. Bentley [29] demonstrated that the vapor
transport of rhodium can be virtually eliminated at 1700°C by using a
single length of twin-bore tubing to insulate the thermoelements and that
contamination of the thermocouple by impurities transferred from the
alumina insulator can be reduced by heat treating the insulator prior to its
use.
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