Rockwell Automation 1762-IT4 Thermocouple/mV Input Module - Manual (Page 102)

Publication 1762-UM002A-EN-P - July 2002
C-2 Thermocouple Descriptions
the purity of commercial type B materials that are used in many
industrial thermometry applications that meet the calibration
tolerances described later in this section. Both 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
-2.6
µ
V near 21°C to a maximum of 2.3
µ
V 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
µ
V. At temperatures above