Rockwell Automation 1746-NT4 Series B,D17466.6.1 SLC 500 4-Channel Thermocouple/mV Input Module User Manual - Manual (Page 109)

1 Publication 1746-UM007C-EN-P - July 2004
Appendix
C
Thermocouple Restrictions
Following are some restrictions extracted from NBS Monograph 125
(IPTS-68) issued March 1974 on thermocouples J, K, T, E, R, and S.
J Type Thermocouple (Iron vs. Copper-Nickel <Constantan>)
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 gauge 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 reading 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
transformation, 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.”