Rockwell Automation 1771-IXE/D Thermocouple/Millivolt Input Module User Manual - Manual (Page 79)

Thermocouple Restrictions F–5
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 the Type E
commercial thermocouples be +/–1.7C between 0 and 316C and
+/–1/2 percent between 316 and 871C. Limits of error are not
specified for Type E thermocouples below 0C. Type E
thermocouples can also be supplied to meet special limits of error,
which are less than the standard limits of error given above:
+/–1.25C between 0 and 316C and +/–3/8 percent between 316 and
871C. The recommended upper temperature limit for protected
thermocouples, 871C, applies to AWG 8 (3.3mm) wire. For smaller
wires the recommended upper temperature decreases to 649C for
AWG 14 (1.6mm), 538C for AWG 20 (.8mm) and 427C for AWG 24
or 28 (0.5 or 0.3mm).
S (Platinum-10% Rhodium vs Platinum) & R (Platinum-13%
Rhodium vs Platinum) Type Thermocouples
“The ASTM manual STP 470 [1970] indicates the following
restrictions on the use of S {and R} type thermocouples at high
temperatures: They should not be used in reducing atmospheres, nor
in those containing metallic vapor (such as lead or zinc), nonmetallic
vapors (such as arsenic, phosphorous or sulfur) or easily reduced
oxides, unless suitably protected with nonmetallic protecting tubes.
They should never be inserted directly into a metallic primary tube.”
“The positive thermoelement, platinum–10% rhodium {13%
rhodium for R}, is unstable in a thermal neutron flux because the
rhodium converts to palladium. The negative thermoelement, pure
platinum, is relatively stable to neutron transmutation. However, fast
neutron bombardment will cause physical damage, which will
change the thermoelectric voltage unless it is annealed out.”
“The thermoelectric voltages of platinum based thermocouples are
sensitive to their heat treatments. In particular, quenching from high
temperatures should be avoided.”
“ASTM Standard E230–72 in the Annual Book of ASTM Standards
[1972] specifies that the standard limits of error for Type S {and R}
commercial thermocouples be +/–1.4C between 0 and 538C and
+/–1/4% between 538 and 1482C. Limits of error are not specified
for Type S {or R} thermocouples below 0C. The recommended
upper temperature limit for continuous use of protected
thermocouples, 1482C, applies to AWG 24 (0.5mm) wire.
* 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 ratio of copper and
nickel in each.