Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 120)

120 Rockwell Automation Publication 1769-UM004B-EN-P - March 2010
Appendix C Thermocouple Descriptions
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 (1472 °F) and 1050 °C (1922 °F).
Both thermoelements of type K thermocouples are reasonably stable,
thermoelectrically, under neutron irradiation because 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 (±35.96 °F) or ±0.75% (whichever is greater) between
0 °C (32 °F) and 1250 °C (2282 °F), and ±2.2 °C (±35.96 °F) or ±2% (whichever
is greater) between -200 °C (-328 °F) and 0 °C (32 °F). In the 0…1250 °C
(32…2282 °F) 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 (32 °F). However, the same
materials may not satisfy the tolerances specified for the -200…0 °C (-328…32 °F)
range. If materials are required to meet the tolerances below 0 °C (32 °F), this
should be specified when they are purchased.
The suggested upper temperature limit of 1260 °C (2300 °F) given in the ASTM
standard [7] for protected type K thermocouples applies to 3.25 mm
2
(8 AWG)
wire. It decreases to 1090 °C (1994 °F) for 1.63 mm
2
(14 AWG), 980 °C
(1796 °F) for 0.81 mm
2
(20 AWG), 870 °C (1598 °F) for 0.51 or 0.33 mm
2
(24 or 28 AWG), and 760 °C (1400 °F) for 0.25 mm
2
(30 AWG).
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.