Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 114)

114 Rockwell Automation Publication 1769-UM004B-EN-P - March 2010
Appendix C Thermocouple Descriptions
Studies by Ehringer [39], Walker et al. [25,26], and Glawe and Szaniszlo [24]
have demonstrated that thermocouples, in which both legs are platinum-rhodium
alloys, are suitable for reliable temperature measurements at high temperatures.
Such thermocouples have been shown to offer the following distinct advantages
over types R and S thermocouples at high temperatures: (1) improved stability,
(2) increased mechanical strength, and (3) higher operating temperatures.
The research by Burns and Gallagher [38] indicated that the 30-6 thermocouple
can be used intermittently (for several hours) up to 1790 °C and continuously
(for several hundred hours) at temperatures up to about 1700 °C with only small
changes in calibration. The maximum temperature limit for the thermocouple is
governed, primarily, by the melting point of the platinum-6% rhodium
thermoelement that is estimated to be about 1820 °C by Acken [40]. The
thermocouple is most reliable when used in a clean oxidizing atmosphere (air)
but also has been used successfully in neutral atmospheres or vacuum by Walker
et al [25,26], Hendricks and McElroy [41], and Glawe and Szaniszlo [24]. The
stability of the thermocouple at high temperatures has been shown by Walker et
al. [25,26] to depend, primarily, on the quality of the materials used for
protecting and insulating the thermocouple. High purity alumina with low
iron-content appears to be the most suitable material for the purpose.
Type B thermocouples should not be used in reducing atmospheres, nor those
containing deleterious vapors or other contaminants that are reactive with the
platinum group metals [42], unless suitably protected with nonmetallic
protecting tubes. They should never be used in metallic protecting tubes at high
temperatures.
The Seebeck coefficient of type B thermocouples decreases with decreasing
temperature below about 1600 °C (2912 °F) and becomes almost negligible at
room temperature. Consequently, in most applications the reference junction
temperature of the thermocouple does not need to be controlled or even known,
as long as it between 0…50 °C (32…122 °F). For example, the voltage developed
by the thermocouple, with the reference junction at 0 °C (32 °F), undergoes a
reversal in sign at about 42 °C (107.6 °F), and between 0…50 °C (32…122 °F)
varies from a minimum of -2.6 μV near 21 °C (69.8 °F) to a maximum of 2.3 μV at
50 °C (122 °F). Therefore, in use, if the reference junction of the thermocouple is
within the range 0…50 °C (32…122 °F), then a 0 °C (32 °F) reference junction
temperature can be assumed and the error introduced will not exceed 3 μV. At
temperatures above 1100 °C (2012 °F), an additional measurement error of 3 μV
(about 0.3 °C (32.5°F)) would be insignificant in most instances.
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards [7]
specifies that the initial calibration tolerances for type B commercial
thermocouples be ±0.5% between 870…1700 °C (1598…3092 °F). Type B
thermocouples can also be supplied to meet special tolerances of ±0.25%.
Tolerances are not specified for type B thermocouples below 870 °C (1598 °F).