Rockwell Automation 1771-IXE/D Thermocouple/Millivolt Input Module User Manual - Manual (Page 75)

Appendix
F
Publication 1771-6.5.130 Ć May 1999
Thermocouple Restrictions
(Extracted from NBS
Monograph 125 (IPTS-68))
Following are some restrictions extracted from NBS Monograph 125
(IPTS–68) issued March 1974 on thermocouples J, K, T, E, R and S:
J (Iron vs Copper-Nickel <Constantan*>) Type Thermocouple
The J thermocouple “is the least suitable for accurate thermometry
because there are significant nonlinear deviations in the
thermoelectric output from different manufacturers. ... The total and
specific types of impurities that occur in commercial iron change
with time, location of primary ores, and methods of smelting.”
“Type J thermocouples are recommended by the ASTM [1970] for
use in the temperature range from 0 to 760C in vacuum, oxidizing,
reducing or inert atmospheres. If used for extended times above
500C, heavy gage wires are recommended because the oxidation rate
is rapid at elevated temperatures.”
“They should not be used in sulfurous atmospheres above 500C.
Because of potential rusting and embrittlement, they are not
recommended for subzero temperatures. They should not be cycled
above 760C even for a short time if accurate readings below 760C
are desired at a later time.”
“The negative thermoelement, a copper–nickel alloy, is subject to
substantial composition changes under thermal neutron irradiation,
since copper is converted to nickel and zinc.”
“Commercial iron undergoes a magnetic transformation near 769C
and <an alpha – gamma> crystal transformation near 910C. Both of
these transformations, especially the latter, seriously affect the
thermoelectric properties of iron, and therefore, the Type J
thermocouples. ... If Type J thermocouples are taken to high
temperatures, especially above 900C, they will lose accuracy of their
calibration when they are recycled to lower temperatures.”
General