Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 119)

Rockwell Automation Publication 1769-UM004B-EN-P - March 2010 119
Thermocouple Descriptions Appendix C
Type K Thermocouples This section describes nickel-chromium alloy versus nickel-aluminum alloy
thermocouples, called type K thermocouples. This type is more resistant to
oxidation at elevated temperatures than types E, J, or T thermocouples and,
consequently, it finds wide application at temperatures above 500 °C (932 °F).
The positive thermoelement, KP, which is the same as EP, is an alloy that typically
contains about 89 or 90% nickel, 9 or 9.5% chromium, both silicon and iron in
amounts up to about 0.5%, plus smaller amounts of other constituents such as
carbon, manganese, cobalt, and niobium. The negative thermoelement, KN, is
typically composed of about 95 or 96 percent nickel, 1…1.5% silicon, 1…2.3%
aluminum, 1.6…3.2% manganese, up to about 0.5% cobalt and smaller amounts
of other constituents such as iron, copper, and lead. Also, type KN
thermoelements with modified compositions are available for use in special
applications. These include alloys in which the manganese and aluminum
contents are reduced or eliminated, while the silicon and cobalt contents are
increased.
The low temperature research [8] by members of the NBS Cryogenics Division
showed that the type K thermocouple may be used down to liquid helium
temperatures (about 4 °K) but that its Seebeck coefficient becomes quite small
below 20 °K. Its Seebeck coefficient at 20 °K is only about 4 μV/K, being roughly
one-half that of the type E thermocouple which is the most suitable of the
letter-designated thermocouples types for measurements down to 20 °K. Type
KP and type KN thermoelements do have a relatively low thermal conductivity
and good resistance to corrosion in moist atmospheres at low temperatures. The
thermoelectric 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…1260 °C (-418…2300 °F) 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 (1382 °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. 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.