Rockwell Automation 1762-IT4 Thermocouple/mV Input Module - Manual (Page 109)

Publication 1762-UM002A-EN-P - July 2002
Thermocouple Descriptions C-9
Type N Thermocouples This section describes Nickel-Chromium-Silicon Alloy Versus
Nickel-Silicon-Magnesium Alloy thermocouples, commonly referred to
as type N thermocouples. This type is the newest of the
letter-designated thermocouples. It offers higher thermoelectric
stability in air above 1000°C and better air-oxidation resistance than
types E, J, and K thermocouples. The positive thermoelement, NP, is
an alloy that typically contains about 84 percent nickel, 14 to 14.4
percent chromium, 1.3 to 1.6 percent silicon, plus small amounts
(usually not exceeding about 0.1 percent) of other elements such as
magnesium, iron, carbon, and cobalt. The negative thermoelement,
NN, is an alloy that typically contains about 95 percent nickel, 4.2 to
4.6 percent silicon, 0.5 to 1.5 percent magnesium, plus minor
impurities of iron, cobalt, manganese and carbon totaling about 0.1 to
0.3 percent. 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 4K) but that its Seebeck coefficient becomes very small below
20K. Its Seebeck coefficient at 20K is about 2.5
µ
V/K, roughly one-third
that of type E thermocouples which are the most suitable of the
letter-designated thermocouples types for measurements down to
20K. 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 3.25 mm 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