Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 116)

116 Rockwell Automation Publication 1769-UM004B-EN-P - March 2010
Appendix C Thermocouple Descriptions
They also should not be used in vacuum (at high temperatures) for extended
times because the chromium in the positive thermoelement, a nickel-chromium
alloy, vaporizes out of solution and alters the calibration. In addition, their use in
atmospheres that promote ‘green-rot’ corrosion of the positive thermoelement
should be avoided. Such corrosion results from the preferential oxidation of
chromium in atmospheres with low, but not negligible, oxygen content and can
lead to a large decrease in the thermoelectric voltage of the thermocouple with
time. The effect is most serious at temperatures between 800 °C (1472 °F) and
1050 °C (1922 °F).
The negative thermoelement, a copper-nickel alloy, is subject to composition
changes under thermal neutron irradiation because the copper is converted to
nickel and zinc.
Neither thermoelement of type E thermocouples is very sensitive to minor
changes in composition or impurity level because both are already heavily alloyed.
Similarly, they are also not extremely sensitive to minor differences in heat
treatment (provided that the treatment does not violate any of the restrictions
mentioned above). For most general applications, they may be used with the heat
treatment given by the wire manufacturers. However, when the highest accuracy
is sought, additional preparatory heat treatments may be desirable to enhance
their performance. Details on this and other phases of the use and behavior of
type KP thermoelements (EP is the same as KP) are given in publications by Pots
and McElroy [14], by Burley and Ackland [15], by Burley [16], by Wang and
Starr [17,18], by Bentley [19], and by Kollie et al. [20].
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards [7]
specifies that the initial calibration tolerances for type E commercial
thermocouples be ±1.7 °C (±35.06 °F) or ±0.5% (whichever is greater) between
0 °C (32 °F) and 900 °C (1652 °F), and ±1.7 °C (±35.06 °F) or ±1% (whichever
is greater) between -200 °C (-328 °F) and 0 °C (32 °F). Type E thermocouples can
also be supplied to meet special tolerances that are equal to ±1 °C (33.8 °F) or
±0.4% (whichever is greater) between 0 °C (32 °F) and 900 °C (1652 °F), and ±1
°C (33.8 °F) or ±0.5% (whichever is greater) between -200 °C (-328 °F) and 0 °C
(32 °F). Type E thermocouple materials are normally supplied to meet the
tolerances specified for temperatures above 0 °C (32 °F). The same materials,
however, may not satisfy the tolerances specified for the -200…0 °C (-328…32 °F)
range. If materials are required to meet the tolerances below 0 °C (32 °F), this
should be specified when they are purchased.