Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 121)

Rockwell Automation Publication 1769-UM004B-EN-P - March 2010 121
Thermocouple Descriptions Appendix C
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 (1832 °F) and better
air-oxidation resistance than types E, J, and K thermocouples. The positive
thermoelement, NP, is an alloy that typically contains about 84% nickel,
14…14.4% chromium, 1.3…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…4.6% silicon, 0.5…1.5% magnesium, plus minor impurities of
iron, cobalt, manganese and carbon totaling about 0.1…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 2.5 μV/K, roughly one-third that of type E
thermocouples that 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 (2300 °F) 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
(2570 °F) for type NP and 1340 °C (2444 °F) 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.