Spectrum Controls 1756sc-IF8u - Manual (Page 100)

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ControlLogix ™ Universal Analog Input Module
to 14.4% chromium, 1.3 to 1.6% silicon, plus small amounts (usually not
exceeding about 0.1%) of other elements such as magnesium, iron,
carbon, and cobalt. The negative thermoelement, NN, is an alloy that
typically contains about 95% nickel, 4.2 to 4.6% silicon, 0.5 to 1.5%
magnesium, plus minor impurities of iron, cobalt, manganese and carbon
totaling about 0.1 to 0.3%. The type NP and NN alloys were known
originally [16] as nicrosil and nisil, respectively.
The research reported in NBS Monograph 161 showed that the type N
thermocouple may be used down to liquid helium temperatures (about
4°K) but that its Seebeck coefficient becomes very small below 20°K. Its
Seebeck coefficient at 20°K is about 2.5uV/°K, roughly one-third that of
type E thermocouples which are the most suitable of the letter-designated
thermocouples types for measurements down to 20°K. Nevertheless,
types NP and NN thermoelements do have a relatively low thermal
conductivity and good resistance to corrosion in moist atmospheres at low
temperatures.
Type N thermocouples are best suited for use in oxidizing or inert
atmospheres. Their suggested upper temperature limit, when used in
conventional closed-end protecting tubes, is set at 1260°C by the ASTM
[7] for 3.25mm diameter thermoelements. Their maximum upper
temperature limit is defined by the melting temperature of the
thermoelements, which are nominally 1410°C for type NP and 1340°C for
type NN [5]. The thermoelectric stability and physical life of type N
thermocouples when used in air at elevated temperatures will depend upon
factors such as the temperature, the time at temperature, the diameter of
the thermoelements, and the conditions of use. Their thermoelectric
stability and oxidation resistance in air have been investigated and
compared with those of type K thermocouples by Burley [16], by Burley
and others [13,44-47], by Wang and Starr [17,43,48,49], by McLaren and
Murdock [33], by Bentley [19], and by Hess [50].
Type N thermocouples, in general, are subject to the same environmental
restrictions as types E and K. They are not recommended for use 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 and silicon in the positive thermoelement, a nickel-
chromium-silicon alloy, vaporize out of solution and alter the calibration. In
addition, their use in atmospheres with low, but not negligible, oxygen
content is not recommended, since it can lead to changes in calibration due
to the preferential oxidation of chromium in the positive thermoelement.
Nevertheless, Wang and Starr [49] studied the performances of type N
thermocouples in reducing atmospheres, as well as in stagnant air, at
temperatures in the 870°C to 1180°C range and found them to be
markedly more stable thermoelectrically than type K thermocouples under
similar conditions.