Spectrum Controls 1769sc-IF8u - Manual (Page 89)

Appendix C: Thermocouple Descriptions 79
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 4mV/
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.
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.