Rockwell Automation 1762-IT4 Thermocouple/mV Input Module - Manual (Page 114)

Publication 1762-UM002A-EN-P - July 2002
C-14 Thermocouple Descriptions
and physical inhomogeneities in the thermocouple and thereby limit
its accuracy in this range. They emphasized the important of
annealing techniques.
The positive thermoelement is unstable in a thermal neutron flux
because the rhodium converts to palladium. The negative
thermoelement is relatively stable to neutron transmutation. Fast
neutron bombardment, however, will cause physical damage, which
will change the thermoelectric voltage unless it is annealed out.
At the gold freezing-point temperature, 1064.18°C, the thermoelectric
voltage of type S thermocouples increases by about 340uV (about 3
percent) per weight percent increase in rhodium content; the Seebeck
coefficient increases by about 4 percent per weight percent increase at
the same temperature.
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards
[7] specifies that the initial calibration tolerances for type S commercial
thermocouples be ±1.5°C or ±0.25 percent (whichever is greater)
between 0°C and 1450°C. Type S thermocouples can be supplied to
meet special tolerances of ±0.6°C or ±0.1 percent (whichever is
greater).
The suggested upper temperature limit, 1480°C, given in the ASTM
standard [7] for protected type S 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 T Thermocouples This section describes Copper Versus Copper-Nickel Alloy
thermocouples, called type T thermocouples. This type is one of the
oldest and most popular thermocouples for determining temperatures
within the range from about 370°C down to the triple point of neon
(-248.5939°C). Its positive thermoelement, TP, is typically copper of
high electrical conductivity and low oxygen content that conforms to
ASTM Specification B3 for soft or annealed bare copper wire. Such
material is about 99.95 percent pure copper with an oxygen content
varying from 0.02 to 0.07 percent (depending upon sulfur content)
and with other impurities totaling about 0.01 percent. Above about
-200°C, the thermoelectric properties of type TP thermoelements,
which satisfy the above conditions, are exceptionally uniform and
exhibit little variation between lots. Below about -200°C the
thermoelectric properties are affected more strongly by the presence
of dilute transition metal solutes, particularly iron.
The negative thermoelement, TN or EN, is a copper-nickel alloy
known ambiguously as constantan. The word constantan refers to a