Spectrum Controls 1746sc-NI8u - Manual (Page 117)

Appendix B: Thermocouple Descriptions 105
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.
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards
[7] specifies that the initial calibration tolerances for type N commercial
thermocouples be +/-2.2°C or +/-0.75% (whichever is greater) between
0°C and 1250°C. Type N thermocouples can also be supplied to meet
special tolerances that are equal to approximately one-half the standard
tolerances given above. Tolerances are not specified for type N
thermocouples below 0°C.
The suggested upper temperature limit of 1260°C given in the ASTM
standard [7] for protected type N thermocouples applies to AWG 8
(3.25mm) wire. It decreases to 1090°C for AWG 14 (1.63mm), 980°C for
AWG 20 (0.81mm), 870°C for AWG 24 or 28 (0.51mm or 0.33mm), and
760°C for AWG 30 (0.25mm). These temperature limits apply to
thermocouples used in conventional closed-end protecting tubes and they
are intended only as a rough guide to the user. They do not apply to
hermocouples having compacted mineral oxide insulation.