Rockwell Automation 1790P-T4T0 CompactBlock LDX I/O Thermocouple Modules - Manual (Page 64)

Publication 1790-UM003A-EN-P
C-2 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 Pt-6 percent rhodium thermoelement which 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 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°C and 50°C. For
example, the voltage developed by the thermocouple, with the reference
junction at 0°C, undergoes a reversal in sign at about 42°C, and between
0°C and 50°C varies from a minimum of -2.6
µV near 21°C to a maximum
of 2.3
µV at 50°C. Therefore, in use, if the reference junction of the
thermocouple is within the range 0°C to 50°C, then a 0°C reference
junction temperature can be assumed and the error introduced will not
exceed 3
µV. At temperatures above 1100°C, an additional measurement
error of 3
µV (about 0.3°C) 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 percent between 870°C and 1700°C. Type B
thermocouples can also be supplied to meet special tolerances of ±0.25
percent. Tolerances are not specified for type B thermocouples below
870°C.