Rockwell Automation 1771-IXHR , D17716.5.80 HIGH RESOL.THERMOCOUPLE - Manual (Page 56)

Thermocouple Restrictions
Appendix C
CĆ3
“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.”
“ASTM Standard E230–72 in the Annual Book of ASTM Standards [1972]
specifies that the standard limits of error for Type J commercial thermocouples
be +/–2.2C between 0 and 277C and +/–3/4 percent between 277 and 760C.
Limits of error are not specified for Type J thermocouples below 0C or above
760C. Type J thermocouples can also be supplied to meet special limits of error,
which are equal to one half the limits given above. The recommended upper
temperature limit for protected thermocouples, 760C, applies to AWG 8
(3.3mm) wire. For smaller wires the recommended upper temperature decrease
to 593C for AWG 14 (1.6mm), and 371C for AWG 24 or 28 (0.5 or 0.3mm).
* It should be noted that the Constantan element of Type J thermoelements is
NOT interchangeable with the Constantan element of Types T or N due to the
different ratio of copper and nickel in each.
K (Nickel-Chromium vs Nickel-Aluminum) Type Thermocouple
“This type is more resistant to oxidation at elevated temperatures than the Types
E, J or T thermocouples and consequently it finds wide application at
temperatures above 500C.”
“Type K thermocouples may be used at” liquid hydrogen “temperatures.
However, their Seebeck coefficient (about 4uV/K at 20K) is only about
one–half of that of Type E thermocouples. Furthermore, the thermoelectric
homogeneity of KN thermoelements is generally not quite as good as that of EN
thermoelements. Both the KP and the KN thermoelements do have a relatively
low thermal conductivity and good resistance to corrosion in moist atmospheres
at low temperatures.”
“Type K thermocouples are recommended by the ASTM [1970] for continuous
use at temperatures within the range –250 to 1260C in oxidizing or inert
atmospheres. Both the KP and the KN thermoelements are subject to oxidation
when used in air above about 850C, but even so, Type K thermocouples may be
used at temperatures up to about 1350C for short periods with only small
changes in calibration.”
“They should not be used in sulfurous, reducing, or alternately reducing and
oxidizing atmospheres unless suitably protected with protecting tubes. They
should not be used in vacuum (at high temperatures) for extended times because
the Chromium in the positive thermoelement vaporizes out of solution and