Rockwell Automation 1790P-T4T0 CompactBlock LDX I/O Thermocouple Modules - Manual (Page 74)

Publication 1790-UM003A-EN-P
C-12 Thermocouple Descriptions
Type R thermocouples have about a 12 percent larger Seebeck coefficient
than do Type S thermocouples over much of the range. Type R
thermocouples were not standard interpolating instruments on the
IPTS-68 for the 630.74°C to gold freezing-point range. Other than these
two points, and remarks regarding history and composition, all of the
precautions and restrictions on usage given in the section on type S
thermocouples also apply to type R thermocouples. Glawe and 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].