Rockwell Automation 1746-INT4 Thermocouple/MV Isolated/ User Manual - Manual (Page 101)

Publication 1746-UM614B-EN-P - September 2007
Thermocouple Descriptions 101
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 ±1.7 °C (±35 °F) between 0…316 °C (32…600 °F)
and ±1/2% between 316…871 °C (600…1600 °F). Limits of error are
not specified for Type E thermocouples below 0 °C (32 °F). Type E
thermocouples can also be supplied to meet special limits of error,
which are less than the standard limits of error given above ±1.25 °C
(±34.3 °F) between 0…316 °C (32…600 °F) and ±3/8% between
316…871 °C (600…1600 °F), applies to 3.3mm (8 AWG) wire. For
smaller wires, the recommended upper temperature decreases to
649 °C (1200 °F) for 1.6mm (14 AWG), 538 °C (1000 °F) for 0.8mm
(20 AWG) and 427 °C (800 °F) for 0.5 or 0.3mm (24 or AWG).
S and R Type Thermocouple
S (platinum-10% rhodium
versus platinum)
R (platinum-13% rhodium
versus platinum) 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 changes 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 ±1.4 °C (±34.5 °F) between 0…538 °C
(32…1000 °F) and ±1/4% between 538…1482 °C (1000…2700 °F).
Limits of error are not specified for Type S (or R) thermocouples
below 0 °C (32 °F). The recommended upper temperature limit for
continuous use of protected thermocouples, 1482 °C (2700 °F),
applies to 0.5mm (24 AWG) wire.