Spectrum Controls 1756sc-IF8u - Manual (Page 99)

Appendix B: Thermocouple Descriptions
87
oxidizing atmosphere (air) but also has been used successfully in neutral
atmospheres or vacuum by Walker et al [25,26], Hendricks and McElroy
[41], and Glawe and Szaniszlo [24]. The stability of the thermocouple at
high temperatures has been shown by Walker et al. [25,26] to depend,
primarily, on the quality of the materials used for protecting and insulating
the thermocouple. High purity alumina with low iron-content appears to
be the most suitable material for the purpose.
Type B thermocouples should not be used in reducing atmospheres, nor
those containing deleterious vapors or other contaminants that are reactive
with the platinum group metals [42], unless suitably protected with
nonmetallic protecting tubes. They should never be used in metallic
protecting tubes at high temperatures.
The Seebeck coefficient of type B thermocouples decreases with
decreasing temperature below about 1600°C and becomes almost
negligible at room temperature. Consequently, in most applications the
reference junction temperature of the thermocouple does not need to be
controlled or even known, as long as it between 0°C and 50°C. For
example, the voltage developed by the thermocouple, with the reference
junction at 0°C, undergoes a reversal in sign at about 42°C, and between
0°C and 50°C varies from a minimum of -2.6uV near 21°C to a maximum
of 2.3uV at 50°C. Therefore, in use, if the reference junction of the
thermocouple is within the range 0°C to 50°C, then a 0°C reference
junction temperature can be assumed and the error introduced will not
exceed 3uV. At temperatures above 1100°C, an additional measurement
error of 3uV (about 0.3°C) would be insignificant in most instances.
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards
[7] specifies that the initial calibration tolerances for type B commercial
thermocouples be +/-0.5% between 870°C and 1700°C. Type B
thermocouples can also be supplied to meet special tolerances of +/-
0.25%. Tolerances are not specified for type B thermocouples below
870°C.
The suggested upper temperature limit of 1700°C given in the ASTM
standard [7] for protected type B thermocouples applies to AWG 24
(0.51mm) wire. This temperature limit applies to thermocouples used in
conventional closed-end protecting tubes and it is intended only as a rough
guide to the user. It does not apply to thermocouples having compacted
mineral oxide insulation.
N Type
Thermocouples Nickel-Chromium-Silicon Alloy Versus Nickel-Silicon-
Magnesium Alloy Thermocouples
This type is the newest of the letter-designated thermocouples. It offers
higher thermoelectric stability in air above 1000°C and better air-oxidation
resistance than types E, J, and K thermocouples. The positive
thermoelement, NP, is an alloy that typically contains about 84% nickel, 14