Rockwell Automation 1790P-T4T0 CompactBlock LDX I/O Thermocouple Modules - Manual (Page 69)

Publication 1790-UM003A-EN-P
Thermocouple Descriptions C-7
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. The positive thermoelement, KP, which is the
same as EP, is an alloy that typically contains about 89 to 90 percent
nickel, 9 to about 9.5 percent chromium, both silicon and iron in amounts
up to about 0.5 percent, plus smaller amounts of other constituents such
as carbon, manganese, cobalt, and niobium. The negative thermoelement,
KN, is typically composed of about 95 to 96 percent nickel, 1 to 1.5
percent silicon, 1 to 2.3 percent aluminum, 1.6 to 3.2 percent manganese,
up to about 0.5 percent 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 4K) but that its Seebeck coefficient
becomes quite small below 20K. Its Seebeck coefficient at 20K 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 20K. 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°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.