Rockwell Automation 1790P-T4T0 CompactBlock LDX I/O Thermocouple Modules - Manual (Page 70)

Publication 1790-UM003A-EN-P
C-8 Thermocouple Descriptions
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.