Spectrum Controls 1769sc-IF8u - Manual (Page 84)

74 Compact IO ™ Universal Input Module
thermoelements will typically have significant impurities of elements such
as palladium, iridium, iron, and silicon [38].
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 -2.6μV near 21°C to a maximum
of 2.3μV 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 3μV. At temperatures above 1100°C, an additional measurement
error of 3μV (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