Rockwell Automation 1790P-T4T0 CompactBlock LDX I/O Thermocouple Modules User Manual
Page 66
Publication 1790-UM003A-EN-P
C-4 Thermocouple Descriptions
Type E thermocouples should not be used at high temperatures in
sulfurous, reducing, or alternately reducing and oxidizing atmospheres
unless suitably protected with protecting tubes. 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 and 1050°C.
The negative thermoelement, a copper-nickel alloy, is subject to
composition changes under thermal neutron irradiation since 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 in order 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 or ±0.5 percent (whichever is greater) between
0°C and 900°C, and ±1.7°C or ±1 percent (whichever is greater) between
-200°C and 0°C. Type E thermocouples can also be supplied to meet
special tolerances which are equal to ±1°C or ±0.4 percent (whichever is
greater) between 0°C and 900°C, and ±1°C or ±0.5 percent (whichever is
greater) between -200°C and 0°C. Type E thermocouple materials are
normally supplied to meet the tolerances specified for temperatures above
0°C. The same materials, however, may not satisfy the tolerances specified
for the -200°C to 0°C range. If materials are required to meet the
tolerances below 0°C, this should be specified when they are purchased.