Rockwell Automation 1771-IXE/D Thermocouple/Millivolt Input Module User Manual - Manual (Page 78)

Thermocouple Restrictions F–4
Publication 1771-6.5.130 Ć May 1999
“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).
E (Nickel-Chromium vs Copper-Nickel <Constantan*>) Type
Thermocouple
“Type E thermocouples are recommended by the ASTM Manual
[1970] for use in the temperature range from –250 to 871C in
oxidizing or inert atmospheres. The negative thermoelement is
subject to deterioration above about 871C, but the thermocouple may
be used up to 1000C 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.”