Allen-Bradley MicroLogix 1200 User Manual page 104

C-4 Thermocouple Descriptions
Publication 1762-UM002A-EN-P - July 2002
Type E thermocouples are recommended by the ASTM [5] for use in
the temperature range from -200°C to 900°C in oxidizing or inert
atmospheres. If used for extended times in air above 500°C, heavy
gauge wires are recommended because the oxidation rate is rapid at
elevated temperatures. About 50 years ago, Dahl [11] studied the
thermoelectric stability of EP and EN type alloys when heated in air at
elevated temperatures. His work should be consulted for details. More
recent stability data on these alloys in air were reported by Burley et
al. [13]. 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,