Spectrum Controls 1756sc-IF8u - Manual (Page 89)

Appendix B: Thermocouple Descriptions
77
much as 40uV (or 0.6°C equivalent) per minute when first brought up to
temperatures near 900°C.
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards
[7] specifies that the initial calibration tolerances for type J commercial
thermocouples be +/-2.2°C or +/-0.75% (whichever is greater) between
0°C and 750°C. Type J thermocouples can also be supplied to meet
special tolerances, which are equal to approximately one-half the standard
tolerances given above. Tolerances are not specified for type J
thermocouples below 0°C or above 750°C.
The suggested upper temperature limit of 760°C given in the above ASTM
standard [7] for protected type J thermocouples applies to AWG 8
(3.25mm) wire. For smaller diameter wires the suggested upper
temperature limit decreases to 590°C for AWG 14 (1.63mm), 480°C for
AWG 20 (0.81mm), 370°C for AWG 24 or 28 (0.51mm or 0.33mm), and
320°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
sheathed thermocouples having compacted mineral oxide insulation.
K Type
Thermocouples Nickel-Chromium Alloy Versus Nickel-Aluminum
Alloy Thermocouples
This type is more resistant to oxidation at elevated temperatures than
types E, J, or T thermocouples and, consequently, it finds wide application
at temperatures above 500°C. The positive thermoelement, KP, which is
the same as EP, is an alloy that typically contains about 89 to 90% nickel, 9
to about 9.5% chromium, both silicon and iron in amounts up to about
0.5%, plus smaller amounts of other constituents such as carbon,
manganese, cobalt, and niobium. The negative thermoelement, KN, is
typically composed of about 95 to 96% nickel, 1 to 1.5% silicon, 1 to 2.3%
aluminum, 1.6 to 3.2% manganese, up to about 0.5% cobalt and smaller
amounts of other constituents such as iron, copper, and lead. Also, type
KN thermoelements with modified compositions are available for use in
special applications. These include alloys in which the manganese and
aluminum contents are reduced or eliminated, while the silicon and cobalt
contents are increased.
The low temperature research [8] by members of the NBS Cryogenics
Division showed that the type K thermocouple may be used down to liquid
helium temperatures (about 4°K) but that its Seebeck coefficient becomes
quite small below 20°K. Its Seebeck coefficient at 20°K is only about
4uV/°K, being roughly one-half that of the type E thermocouple which is
the most suitable of the letter-designated thermocouples types for
measurements down to 20°K. Type KP and type KN thermoelements do
have a relatively low thermal conductivity and good resistance to corrosion
in moist atmospheres at low temperatures. The thermoelectric