Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 125)

Rockwell Automation Publication 1769-UM004B-EN-P - March 2010 125
Thermocouple Descriptions Appendix C
Research [27] demonstrated that type S thermocouples can be used from -50 °C
(-58 °F) 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 (2372 °F) 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 (2192 °F). 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 (>2192 °F) 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 (3092 °F) 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…1100 °C (32…2012 °F).
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.