Spectrum Controls 1746sc-NI8u - Manual (Page 109)

Appendix B: Thermocouple Descriptions 97
supplied to meet the tolerances specified for temperatures above 0°C.
However, the same materials may not satisfy the tolerances specified for
the -200°C to 0°C range. If materials are required to meet the tolerances
below 0°C, this should be specified when they are purchased.
The suggested upper temperature limit of 370°C given in the ASTM
standard [7] for protected type T thermocouples applies to AWG 14
(1.63mm) wire. It decreases to 260°C for AWG 20 (0.81mm), 200°C for
AWG 24 or 28 (0.51mm or 0.33mm), and 150°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 thermocouples having compacted mineral
oxide insulation.
E Type
Thermocouples Nickel-Chromium Alloy Versus Copper-Nickel Alloy
Thermocouples
This type, and the other base-metal types, do not have specific chemical
compositions given in standards; rather, any materials whose emf-
temperature relationship agrees with that of the specified reference table
within certain tolerances can be considered to be a type E thermocouple.
The positive thermoelement, EP, is the same material as KP. The negative
thermoelement, EN, is the same material as TN.
The low-temperature research [8] by members of the NBS Cryogenics
Division showed that type E thermocouples are very useful down to liquid
hydrogen temperatures (n.b.p. about 20.3°K) where their Seebeck
coefficient is about 8uV/°C. They may even be used down to liquid
helium temperatures (4.2°K) although their Seebeck coefficient becomes
quite low, only about 2uV/°C at 4°K. Both thermoelements of type E
thermocouples have a relatively low thermal conductivity, good resistance
to corrosion in moist atmospheres, and reasonably good homogeneity. For
these three reasons and their relatively high Seebeck coefficients, type E
thermocouples have been recommended [8] as the most useful of the
letter-designated thermocouple types for low-temperature measurements.
For measurements below 20°K, the non-letter-designated thermocouple,
KP versus gold-0.07 at .% iron, is recommended. The properties of this
thermocouple have been described by Sparks and Powell [12].
Type E thermocouples also have the largest Seebeck coefficient above
0°C for any of the letter-designated thermocouples. For that reason they
are being used more often whenever environmental conditions permit.
Type E thermocouples are recommended by the ASTM [5] for use in the
temperature range from -200°C to 900°C in oxidizing or inert
atmospheres. If used for extended times in air above 500°C, heavy gage
wires are recommended because the oxidation rate is rapid at elevated
temperatures. About 50 years ago, Dahl [11] studies the thermoelectric