Spectrum Controls 1756sc-IF8u - Manual (Page 90)

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ControlLogix ™ Universal Analog Input Module
homogeneity of type KN thermoelements, however, was found [8] to be
not quite as good as that of type EN thermoelements.
Type K thermocouples are recommended by the ASTM [5] for use at
temperatures within the range -250°C to 1260°C in oxidizing or inert
atmospheres. Both the KP and the KN thermoelements are subject to
deterioration by oxidation when used in air above about 750°C, but even
so, type K thermocouples may be used at temperatures up to about
1350°C for short periods with only small changes in calibration. 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, their use in atmospheres that promote “green-rot” corrosion
[9] of the positive thermoelement should be avoided. 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% (whichever is greater) between
0°C and 1250°C, and +/-2.2°C or +/-2% (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