Rockwell Automation 1790P-T4T0 CompactBlock LDX I/O Thermocouple Modules - Manual (Page 78)

Publication 1790-UM003A-EN-P
C-16 Thermocouple Descriptions
Type T thermocouples are recommended by the ASTM [5] for use in the
temperature range from -200°C to 370°C in vacuum or in oxidizing,
reducing or inert atmospheres. The suggested upper temperature limit for
continuous service of protected type T thermocouples is set at 370°C for
AWG 14 (1.63 mm) thermoelements since type TP thermoelements
oxidize rapidly above this temperature. However, the thermoelectric
properties of type TP thermoelements are apparently not grossly affected
by oxidation since negligible changes in the thermoelectric voltage were
observed at NBS [10] for AWG 12, 18, and 22 type TP thermoelements
during 30 hours of heating in air at 500°C. 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,
are possible in inert atmospheres where the deterioration of the type TP
thermoelement is no longer a problem. The use of type T thermocouples
in hydrogen atmospheres at temperatures above about 370°C is not
recommended since type TP thermoelements may become brittle.
Type T thermocouples are not well suited for use in nuclear environments
since 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 ensure
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 or ±0.75 percent (whichever is greater) between
0°C and 350°C, and ±1°C or ±1.5 percent (whichever is greater) between
-200°C and 0°C. 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. However, the same
materials may not satisfy the tolerances specified for the -200°C to 0°C
range. If materials are required to meet the tolerances below 0°C, this
should be specified when they are purchased.
The suggested upper temperature limit of 370°C given in the ASTM
standard [7] for protected type T thermocouples applies to AWG 14 (1.63
mm) wire. It decreases to 260°C for AWG 20 (0.81 mm), 200°C for AWG
24 or 28 (0.51 mm or 0.33 mm), and 150°C for AWG 30 (0.25 mm). 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.