Rockwell Automation 1746-INT4 Thermocouple/MV Isolated/ User Manual - Manual (Page 98)

Publication 1746-UM614B-EN-P - September 2007
98 Thermocouple Descriptions
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 ±2.2 °C (±35.9 °F) between 0…277 °C (32…531 °F)
and ±3/4% between 277…760 °C (531…1400 °F). Limits of error are
not specified for Type J thermocouples below 0 °C (32 °F) or above
760 °C (1400 °F). 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, 760 °C (1400 °F), applies to 3.3mm (8 AWG) wire. For
smaller wires, the recommended upper temperature decreases to
593 °C (1099 °F) for 1.6mm (14 AWG), and 371 °C (700 °F) for 0.5 or
0.3mm (24 or 28 AWG).”
K Type Thermocouple
(nickel-chromium versus
nickel-aluminum) 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 500 °C (932 °F).
Type K thermocouples may be used at liquid hydrogen temperatures.
However, their Seebeck coefficient (about 4 uV/K at 20 k) is only
about one-half of that of 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…1260 °C
(-418…2300 °F) in oxidizing or inert atmospheres. Both the KP and
the KN thermoelements are subject to oxidation when used in air
above about 850 °C (1562 °F), but even so, Type K thermocouples
may be used at temperatures up to about 1350 °C (2462 °F) 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).