Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 115)

Rockwell Automation Publication 1769-UM004B-EN-P - March 2010 115
Thermocouple Descriptions Appendix C
The suggested upper temperature limit of 1700 °C (3092 °F) given in the ASTM
standard [7] for protected type B thermocouples applies to 0.51 mm
2
(24 AWG)
wire. This temperature limit applies to thermocouples used in conventional
closed-end protecting tubes and it is intended only as a rough guide to the user. It
does not apply to thermocouples having compacted mineral oxide insulation.
Type E Thermocouples This section describes Nickel-Chromium Alloy Versus Copper-Nickel Alloy
thermocouples, known as type E thermocouples. This type, and the other
base-metal types, do not have specific chemical compositions given in standards;
rather, any materials whose emf-temperature relationship agrees with that of the
specified reference table within certain tolerances can be considered to be a type
E thermocouple. The positive thermoelement, EP, is the same material as KP. The
negative thermoelement, EN, is the same material as TN.
The low-temperature research [8] by members of the NBS Cryogenics Division
showed that type E thermocouples are very useful down to liquid hydrogen
temperatures (n.b.p. about 20.3 °K) where their Seebeck coefficient is about 8
mV/°C. They may even be used down to liquid helium temperatures (4.2 °K)
although their Seebeck coefficient becomes quite low, only about 2 mV/°C at 4
°K. Both thermoelements of type E thermocouples have a relatively low thermal
conductivity, good resistance to corrosion in moist atmospheres, and reasonably
good homogeneity. For these three reasons and their relatively high Seebeck
coefficients, type E thermocouples have been recommended [8] as the most
useful of the letter-designated thermocouple types for low-temperature
measurements.
For measurements below 20 °K, the non-letter-designated thermocouple,
KP versus gold-0.07, is recommended. The properties of this thermocouple have
been described by Sparks and Powell [12].
Type E thermocouples also have the largest Seebeck coefficient above 0 °C (32 °F)
for any of the letter-designated thermocouples. For that reason they are being
used more often whenever environmental conditions permit.
Type E thermocouples are recommended by the ASTM [5] for use in the
temperature range from -200…900 °C (-328…1652 °F) in oxidizing or inert
atmospheres. If used for extended times in air above 500 °C (932 °F), 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.