Spectrum Controls 1769sc-IF8u - Manual (Page 91)

Appendix C: Thermocouple Descriptions 81
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 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