Spectrum Controls 1769sc-IF8u - Manual (Page 95)

Appendix C: Thermocouple Descriptions 85
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
percent) per weight percent increase in rhodium content; the Seebeck
coefficient increases by about 4 percent per weight percent increase at
the same temperature.
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards
[7] specifies that the initial calibration tolerances for type S commercial
thermocouples be ±1.5°C or ±0.25 percent (whichever is greater)
between 0°C and 1450°C. Type S thermocouples can be supplied to meet
special tolerances of ±0.6°C or ±0.1 percent (whichever is greater).
The suggested upper temperature limit, 1480°C, given in the ASTM
standard [7] for protected type S thermocouples applies to AWG 24 (0.51
mm) wire. This temperature limit applies to thermocouples used in
conventional closed-end protecting tubes and it is intended only as a rough
guide to the user. It does not apply to thermocouples having compacted
mineral oxide insulation.