Rockwell Automation 1762-IT4 Thermocouple/mV Input Module - Manual (Page 107)

Publication 1762-UM002A-EN-P - July 2002
Thermocouple Descriptions C-7
The suggested upper temperature limit of 760°C given in the above
ASTM standard [7] for protected type J thermocouples applies to AWG
8 (3.25 mm) wire. For smaller diameter wires the suggested upper
temperature limit decreases to 590°C for AWG 14 (1.63 mm), 480°C for
AWG 20 (0.81 mm), 370°C for AWG 24 or 28 (0.51 mm or 0.33 mm),
and 320°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 sheathed thermocouples having compacted mineral oxide
insulation.
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
µ
V/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.