Rockwell Automation 1771-IXHR , D17716.5.80 HIGH RESOL.THERMOCOUPLE - Manual (Page 58)

Thermocouple Restrictions
Appendix C
CĆ5
T thermocouples are less suitable for use in the subzero range than Type E
thermocouples.”
“Type T thermocouples are recommended by the ASTM [1970] for use in the
temperature range from –184 to 371C in vacuum or in oxidizing, reducing or
inert atmospheres. The recommended upper temperature limit for continuous
service of protected Type T thermocouples is set at 371C for AWG 14 (1.6mm)
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 Roeser
and Dahl [1938] observed negligible changes in the thermoelectric voltage of
Nos. 12, 18, and 22 AWG Type TP thermoelements after heating for 30 hours in
air at 500C. At this temperature the Type TN thermoelements have good
resistance to oxidation and exhibit only small changes in thermal emf with long
exposure in air, as shown by the studies of Dahl [1941].” ... “Operation of Type
T thermocouples in hydrogen atmospheres at temperatures above about 370C is
not recommended since severe embrittlement of the Type TP thermoelements
may occur.”
“Type T thermoelements 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 thermoelement is converted
to nickel and zinc.”
“Because of the high thermal conductivity of Type TP thermoelements, special
care should be exercised in the use of the thermocouples to insure that both the
measuring and reference junctions assume the desired temperatures.”
ASTM Standard E230–72 in the Annual Book of ASTM Standards [1972]
specifies that the standard limits of error for Type T commercial thermocouples
be +/–2 percent between –101 and –59C, +/–.8C between –59 and 93C and
+/–3/4 percent between 93 and 371C. Type T thermocouples can also be
supplied to meet special limits of error, which are equal to one half the standard
limits of error given above (plus a limit of error of +/–1 percent is specified
between –184 and –59C). The recommended upper temperature limit for
protected Type T thermocouples, 371C, applies to AWG 14 (1.6mm) wire. For
smaller wires it decreases to 260C for AWG 20 (0.8mm) and 240C for AWG 24
or 28 (0.5 or 0.3mm).