Rockwell Automation 1762-IT4 Thermocouple/mV Input Module - Manual (Page 112)

Publication 1762-UM002A-EN-P - July 2002
C-12 Thermocouple Descriptions
Szaniszlo [24], and Walker et al [25,26] have determined the effects that
prolonged exposure at elevated temperatures (>1200°C) in vacuum,
air, and argon atmospheres have on the thermoelectric voltages of
type R thermocouples.
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards
[7] specifies that the initial calibration tolerances for type R commercial
thermocouples be ±1.5°C or ±0.25 percent (whichever is greater)
between 0°C and 1450°C. Type R thermocouples can be supplied to
meet special tolerances of ±0.6°C or ±0.1 percent (whichever is
greater).
The suggested upper temperature limit, 1480°C, given in the ASTM
standard [7] for protected type R thermocouples applies to AWG 24
(0.51 mm) wire. This temperature limit applies to thermocouples used
in conventional closed-end protecting tubes and it is intended only as
a rough guide to the user. It does not apply to thermocouples having
compacted mineral oxide insulation.
Type S Thermocouples This section describes Platinum-10 percent Rhodium Alloy Versus
Platinum thermocouples, commonly known as type S thermocouples.
This type is often referred to by the nominal chemical composition of
its positive (SP) thermoelement: platinum-10 percent rhodium. The
negative (SN) thermoelement is commercially available platinum that
has a nominal purity of 99.99 percent [21]. An industrial consensus
standard (ASTM E1159-87) specifies that rhodium having a nominal
purity of 99.98 percent shall be alloyed with platinum of 99.99 percent
purity to produce the positive thermoelement, which typically
contains 10.00 ± 0.05 percent rhodium by weight. The consensus
standard [21] describes the purity of commercial type S materials that
are used in many industrial thermometry applications and that meet
the calibration tolerances described later in this section. It does not
cover, however, the higher-purity, reference-grade materials that
traditionally were used to construct thermocouples used as standard
instruments of the IPTS-68, as transfer standards and reference
thermometers in various laboratory applications, and to develop
reference functions and tables [27,28]. The higher purity alloy material
typically contains less than 500 atomic ppm of impurities and the
platinum less than 100 atomic ppm of impurities [27]. Difference
between such high purity commercial material and the platinum
thermoelectric reference standard, Pt-67, are described in [27] and [28].
A reference function for the type S thermocouple, based on the ITS-90
and the SI volt, was determined recently from new data obtained in an
international collaborative effort involving eight national laboratories.
The results of this international collaboration were reported by Burns
et al. [28]. The new function was used to compute the reference table
given in this monograph.