Rockwell Automation 1794-IT8,D17946.5.7 Thermocouple/Millivolt Input Module User Manual
Page 69
Thermocouple Restrictions
B–5
Publication 1794-6.5.7
R (Platinum–13% Rhodium vs Platinum) and
S (Platinum–10% 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.
T (Copper vs Copper–Nickel
“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.”