Rockwell Automation 1762-IT4 Thermocouple/mV Input Module - Manual (Page 103)

Publication 1762-UM002A-EN-P - July 2002
Thermocouple Descriptions C-3
1100°C, an additional measurement error of 3
µ
V (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 percent between 870°C and
1700°C. Type B thermocouples can also be supplied to meet special
tolerances of ±0.25 percent. 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
(0.51 mm) 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.
Type E Thermocouples This section describes Nickel-Chromium Alloy Versus Copper-Nickel
Alloy thermocouples, known as type E 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 20.3K) where their
Seebeck coefficient is about 8mV/°C. They may even be used down to
liquid helium temperatures (4.2°K) although their Seebeck coefficient
becomes quite low, only about 2mV/°C at 4K. 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 20K, the non-letter-designated
thermocouple, KP versus gold-0.07, 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.