Spectrum Controls 1746sc-NI8u - Manual (Page 113)

Appendix B: Thermocouple Descriptions 101
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%
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.
McLaren and Murdock [30-33] and Bentley and Jones [34] thoroughly
studied the performance of type S thermocouples in the range 0°C to
1100°C. They described how thermally reversible effects, such as
quenched-in point defects, mechanical stresses, and preferential oxidation
of rhodium in the type SP thermoelement, cause chemical and physical
inhomogeneities in the thermocouple and thereby limit its accuracy in this
range. They emphasized the important of annealing techniques.
The positive thermoelement is unstable in a thermal neutron flux because
the rhodium converts to palladium. The negative thermoelement is
relatively stable to neutron transmutation. Fast neutron bombardment,
however, will cause physical damage, which will change the
thermoelectric voltage unless it is annealed out.
At the gold freezing-point temperature, 1064.18°C, the thermoelectric
voltage of type S thermocouples increases by about 340uV (about 3%) per