Allen-Bradley MicroLogix 1200 User Manual page 113

Thermocouple Descriptions C-13
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.
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
AB Parts quenched-in point defects, mechanical stresses, and preferential
oxidation of rhodium in the type SP thermoelement, cause chemical
Publication 1762-UM002A-EN-P - July 2002