Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 128)

128 Rockwell Automation Publication 1769-UM004B-EN-P - March 2010
Appendix C Thermocouple Descriptions
At this temperature the type TN thermoelements have good resistance to
oxidation and exhibit only small voltage changes heated in air for long periods of
time, as shown by the studies of Dahl [11]. Higher operating temperatures, up to
at least 800 °C (1472 °F), are possible in inert atmospheres where the
deterioration of the type TP thermoelement is no longer an anomaly. The use of
type T thermocouples in hydrogen atmospheres at temperatures above about
370 °C (698 °F) is not recommended because type TP thermoelements may
become brittle.
Type T thermocouples are not well suited for use in nuclear environments
because both thermoelements are subject to significant changes in composition
under thermal neutron irradiation. The copper in the thermoelements is
converted to nickel and zinc.
Because of the high thermal conductivity of type TP thermoelements, special
care should be exercised when using the thermocouples to be sure that the
measuring and reference junctions assume the desired temperatures.
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards [7]
specifies that the initial calibration tolerances for type T commercial
thermocouples be ±1 °C (±33.8 °F) or ±0.75% (whichever is greater) between
0 °C (32 °F) and 350 °C (662 °F), and ±1 °C (±33.8 °F) or ±1.5% (whichever is
greater) between -200 °C (-328 °F) and 0 °C (32 °F). Type T thermocouples can
also be supplied to meet special tolerances which are equal to approximately
one-half the standard tolerances given above. Type T thermocouple materials are
normally supplied to meet the tolerances specified for temperatures above 0 °C
(32 °F). However, the same materials may not satisfy the tolerances specified for
the -200…0 °C (-328…32 °F) range. If materials are required to meet the
tolerances below 0 °C (32 °F), this should be specified when they are purchased.
The suggested upper temperature limit of 370 °C (698 °F) given in the ASTM
standard [7] for protected type T thermocouples applies to 1.63 mm
2
(14 AWG)
wire. It decreases to 260 °C (500 °F) for 0.81 mm
2
(20 AWG), 200 °C (392 °F)
for 0.51 or 0.33 mm
2
(24 or 28 AWG), and 150 °C (302 °F) for 0.25 mm
2
(30 AWG). 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.