Rockwell Automation 1771-IXE/D Thermocouple/Millivolt Input Module User Manual - Manual (Page 76)

Thermocouple Restrictions F–2
Publication 1771-6.5.130 Ć May 1999
“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).
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 alters the
calibration. They should also not be used in atmospheres that
promote ”green–rot” corrosion (those with low, but not negligible,
oxygen content).”