Rockwell Automation 1790P-T4T0 CompactBlock LDX I/O Thermocouple Modules - Manual (Page 71)

Publication 1790-UM003A-EN-P
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
µV/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].