Allen-Bradley MicroLogix 1200 User Manual page 110

C-10 Thermocouple Descriptions
Publication 1762-UM002A-EN-P - July 2002
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.
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 percent (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