Rockwell Automation 1790P-T4T0 CompactBlock LDX I/O Thermocouple Modules - Manual (Page 76)

Publication 1790-UM003A-EN-P
C-14 Thermocouple Descriptions
McLaren and Murdock [30-33] and Bentley and Jones [34] thoroughly
studied the performance of type S thermocouples in the range 0°C to
1100°C. They described how thermally reversible effects, such as
quenched-in point defects, mechanical stresses, and preferential oxidation
of rhodium in the type SP thermoelement, cause chemical 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.