Rockwell Automation 1771-IXE/D Thermocouple/Millivolt Input Module User Manual - Manual (Page 77)

Thermocouple Restrictions F–3
Publication 1771-6.5.130 Ć May 1999
“ASTM Standard E230–72 in the Annual Book of ASTM Standards
[1972] specifies that the standard limits of error for Type K
commercial thermocouples be +/–2.2C between 0 and 277C and
+/–3/4 percent between 277 and 1260C. Limits of error are not
specified for the Type K thermocouples below 0C. Type K
thermocouples can also be supplied to meet special limits of error,
which are equal to one half the standard limits of error given above.
The recommended upper temperature limit for protected Type K
thermocouples, 1260C, applies for AWG 8 (3.3mm) wire. For
smaller wires it decreases to 1093C for AWG 14 (1.6mm), 982C for
AWG 20 (0.8mm), and 871C for AWG 24 or 28 (0.5 or 0.3mm).”
T (Copper vs Copper-Nickel <Constantan*>) Type Thermocouple
“The homogeneity of most Type TP and TN (or EN) thermoelements
is reasonably good. However, the Seebeck coefficient of Type T
thermocouples is moderately small at subzero temperatures (about
5.6uV/K at 20K), being roughly two–thirds that of Type E
thermocouples. This, together with the high thermal conductivity of
Type TP thermoelements, is the major reason why Type 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.”