Rockwell Automation 1746-NT4 Series B,D17466.6.1 SLC 500 4-Channel Thermocouple/mV Input Module User Manual - Manual (Page 111)

Publication 1746-UM007C-EN-P - July 2004
Thermocouple Restrictions C-3
“They should not be used in sulfurous, reducing, or alternately
reducing and oxidizing atmospheres unless suitably protected with
protecting tubes. They should not be used in vacuum (at high
temperatures) for extended times because the Chromium in the
positive thermoelement vaporizes out of solution and alters the
calibration. They should also no be used in atmospheres that promote
“green-rot” corrosion (those with low, but not negligible, oxygen
content).”
“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 Type Thermocouple (Copper vs. Copper-Nickel <Constantan>)
“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 -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.”