Rockwell Automation 1762-IT4 Thermocouple/mV Input Module - Manual (Page 116)

Publication 1762-UM002A-EN-P - July 2002
C-16 Thermocouple Descriptions
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.