Spectrum Controls 1746sc-NI8u - Manual (Page 111)

Appendix B: Thermocouple Descriptions 99
AWG 20 (0.81mm), 430°C for AWG 24 or 28 (0.51mm or 0.33mm), and
370°C for AWG 30 (0.25mm). These temperature limits apply to
thermocouples used in conventional closed-end protecting tubes and they
are intended only as a rough guide to the user. They do not apply to
thermocouples having compacted mineral oxide insulation.
R Type
Thermocouples Pla Pla Pla Pla Platin tin tin tin tinum-13% Rhodium um-13% Rhodium um-13% Rhodium um-13% Rhodium um-13% Rhodium Allo Allo Allo Allo Alloy y y y y V V V V Ver er er er ersus Pla sus Pla sus Pla sus Pla sus Platin tin tin tin tinum um um um um
T TT T T her her her her her mocouples mocouples mocouples mocouples mocouples
This type is often referred to by the nominal chemical composition of its
positive (RP) thermoelement: platinum-13% rhodium. The negative (RN)
thermoelement is commercially-available platinum that has a nominal
purity of 99.99% [21]. An industrial consensus standard (ASTM E1159-
87) specifies that rhodium having a nominal purity of 99.98% shall be
alloyed with platinum of 99.99% purity to produce the positive
thermoelement, which typically contains 13.00 +/-0.05% rhodium by
weight. This consensus standard [21] describes the purity of commercial
type R 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 transfer
standards and reference thermometers in various laboratory applications
and to develop reference functions and tables [22,23]. The higher purity
alloy material typically contains less than 500 atomic ppm of impurities and
the platinum less than 100 atomic ppm of impurities [22]. Differences
between such high purity commercial material and the platinum
thermoelectric reference standard, Pt-67, are described in [22] and [23].
A reference function for the type R thermocouple, based on the ITS-90
and the SI volt, was determined recently from new data obtained in a
collaborative effort by NIST and NPL. The results of this international
collaboration were reported by Burns et al [23]. The function was used to
compute the reference table given in this monograph.
Type R thermocouples have about a 12% 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% (whichever is greater) between