Rockwell Automation 1746-INT4 Thermocouple/MV Isolated/ User Manual - Manual (Page 100)

Publication 1746-UM614B-EN-P - September 2007
100 Thermocouple Descriptions
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% between -101…-59 °C (-149.8…-74.2 °F),
±0.8 °C (±33.4 °F) between -59…93 °C (-74…200 °F), and ±3/4%
between 93…371 °C (200…700 °F). 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% is
specified between -184…-59 °C (-300…-74 °F). The recommended
upper temperature limit for protected Type T thermocouples, 371 °C
(700 °F), applies to 1.6mm (14 AWG) wire. For smaller wires, it
decreases to 260 °C (500 °F) for 0.8mm (20 AWG) and 240 °C (464 °F)
for 0.5 or 0.3mm (24 or 28 AWG).”
E Type Thermocouple
(nickel-chromium versus
copper-nickel
<Constantan
(1)
>)
Type E thermocouples are recommended by the ASTM Manual [1970]
for use in the temperature range from -250…871 °C (-418…1600 °F) in
oxidizing or inert atmospheres. The negative thermoelement is subject
to deterioration above about 871 °C (1600 °F), but the thermocouple
may be used up to 1000 °C (1832 °F) for short periods.
The ASTM Manual [1970] indicates the following restrictions at high
temperatures. 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 not be used in atmospheres that
promote green-rot corrosion (those with low, but not negligible,
oxygen content).
The negative thermoelement, a copper-nickel alloy, is subject to
composition changes under thermal neutron irradiation since the
copper is converted to nickel and zinc.
(1)
It should be noted that the Constantan element of Type J thermoelements is not interchangeable with the Constantan element of Types T or N due to the different ration of
copper and nickel in each.