Rockwell Automation 1790P-T4T0 CompactBlock LDX I/O Thermocouple Modules User Manual
Page 72
Publication 1790-UM003A-EN-P
C-10 Thermocouple Descriptions
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.
The performance of type N thermocouples fabricated in metal-sheathed,
compacted ceramic insulated form also has been the subject of
considerable study. Anderson and others [51], Bentley and Morgan [52],
and Wang and Bediones [53] have evaluated the high-temperature,
thermoelectric stability of thermocouples insulated with magnesium oxide
and sheathed in Inconel and in stainless steel. Their studies showed that
the thermoelectric instabilities of such assemblies increase rapidly with
temperature above 1000°C. It was found also that the smaller the diameter
of the sheath the greater the instability. Additionally, thermocouples
sheathed in Inconel showed substantially less instability above 1000°C
than those sheathed in stainless steel. Bentley and Morgan [52] stressed
the importance of using Inconel sheathing with a very low manganese
content to achieve the most stable performance. The use of special Ni-Cr
based alloys for sheathing to improve the chemical and physical
compatibility with the thermoelements also has been investigated by
Burley [54-56] and by Bentley [57-60].
Neither thermoelement of a type N thermocouple is extremely sensitive to
minor differences in heat treatment (provided that the treatment does not
violate any of the restrictions mentioned above). For most general
applications, they may be used with the heat treatment routinely given by
the wire manufacturer. Bentley [61,62], however, has reported reversible
changes in the Seebeck coefficient of type NP and NN thermoelements
when heated at temperatures between 200°C and 1000°C. These impose
limitations on the accuracy obtainable with type N thermocouples. The
magnitude of such changes was found to depend on the source of the
thermoelements. Consequently, when the highest accuracy and stability
are sought, selective testing of materials, as well as special preparatory
heat treatments beyond those given by the manufacturer, will usually be
necessary. Bentley’s articles [61,62] should be consulted for guidelines and
details.