Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 122)

122 Rockwell Automation Publication 1769-UM004B-EN-P - March 2010
Appendix C Thermocouple Descriptions
In addition, their use in atmospheres with low, but not negligible, oxygen content
is not recommended, because 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…1180 °C (1598…2156 °F) 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 (1832 °F). 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 (1832 °F) 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
(392 °F) and 1000 °C (1832 °F). 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 (±35.96 °F) or ±0.75% (whichever is greater) between
0 °C (32 °F) and 1250 °C (2282 °F). 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 (32 °F).