Rockwell Automation 1762-IT4 Thermocouple/mV Input Module - Manual (Page 108)

Publication 1762-UM002A-EN-P - July 2002
C-8 Thermocouple Descriptions
When oxidation occurs it normally leads to a gradual increase in the
thermoelectric voltage with time. The magnitude of the change in the
thermoelectric voltage and the physical life of the thermocouple will
depend upon such factors as the temperature, the time at temperature,
the diameter of the thermoelements and the conditions of use.
The ASTM Manual [5] indicates that type K thermocouples should not
be used at high temperatures in sulfurous, reducing, or alternately
oxidizing and reducing atmospheres unless suitably protected with
protecting tubes. They also should not be used in vacuum (at high
temperatures) for extended times because the chromium in the
positive thermoelement, a nickel-chromium alloy, vaporizes out of
solution and alters the calibration. In addition, avoid their use in
atmospheres that promote “green-rot” corrosion [9] of the positive
thermoelement. Such corrosion results from the preferential oxidation
of chromium in atmospheres with low, but not negligible, oxygen
content and can lead to a large decrease in the thermoelectric voltage
of the thermocouple with time. The effect is most serious at
temperatures between 800°C and 1050°C.
Both thermoelements of type K thermocouples are reasonably stable,
thermoelectrically, under neutron irradiation since the resulting
changes in their chemical compositions due to transmutation are
small. The KN thermoelements are somewhat less stable than the KP
thermoelements in that they experience a small increase in the iron
content accompanied by a slight decrease in the manganese and
cobalt contents.
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards
[7] specifies that the initial calibration tolerances for type K
commercial thermocouples be ±2.2°C or ±0.75 percent (whichever is
greater) between 0°C and 1250°C, and ±2.2°C or ±2 percent
(whichever is greater) between -200°C and 0°C. In the 0°C to 1250°C
range, type K thermocouples can be supplied to meet special
tolerances that are equal to approximately one-half the standard
tolerances given above. Type K thermocouple materials are normally
supplied to meet the tolerances specified for temperatures above 0°C.
However, the same materials may not satisfy the tolerances specified
for the -200°C to 0°C range. If materials are required to meet the
tolerances below 0°C, this should be specified when they are
purchased.
The suggested upper temperature limit of 1260°C given in the ASTM
standard [7] for protected type K thermocouples applies to AWG 8
(3.25 mm) wire. It decreases to 1090°C for AWG 14 (1.63 mm), 980°C
for AWG 20 (0.81 mm), 870 for AWG 24 or 28 (0.51 mm or 0.33 mm),
and 760°C for AWG 30 (0.25 mm). These temperature limits apply to
thermocouples used in conventional closed-end protecting tubes and
they are intended only as a rough guide to the user. They do not
apply to thermocouples having compacted mineral oxide insulation.