Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 126)

126 Rockwell Automation Publication 1769-UM004B-EN-P - March 2010
Appendix C Thermocouple Descriptions
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 (1947.52 °F), the
thermoelectric voltage of type S thermocouples increases by about 340 μV
(about 3%) per weight percent increase in rhodium content; the Seebeck
coefficient increases by about 4% 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 (±34.7 °F) or ±0.25% (whichever is greater) between
0 °C (32 °F) and 1450 °C (2642 °F). Type S thermocouples can be supplied to
meet special tolerances of ±0.6 °C (±33.08 °F) or ±0.1% (whichever is greater).
The suggested upper temperature limit, 1480 °C (2696 °F), given in the ASTM
standard [7] for protected type S thermocouples applies to 0.51 mm
2
(24 AWG)
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 (698 °F) down to the triple point of neon (-248.5939 °C (-415.4690 °F).
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% pure copper with an
oxygen content varying from 0.02…0.07% (depending upon sulfur content) and
with other impurities totaling about 0.01%. Above about -200 °C (-328 °F), 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 (-328 °F) the thermoelectric properties are affected more
strongly by the presence of dilute transition metal solutes, particularly iron.