Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 123)

Rockwell Automation Publication 1769-UM004B-EN-P - March 2010 123
Thermocouple Descriptions Appendix C
The suggested upper temperature limit of 1260 °C (2300 °F) given in the ASTM
standard [7] for protected type N thermocouples applies to 3.25 mm
2
(8 AWG)
wire. It decreases to 1090 °C (1994 °F) for 1.63 mm
2
(14 AWG), 980 °C
(1796 °F) for 0.81 mm
2
(20 AWG), 870 °C (1598 °F) for 0.51 or 33 mm
2
(24 or 28 AWG), and 760 °C (1400 °F) for 0.25 mm
2
(30 AWG).
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.
Type R Thermocouples This section describes platinum-13% rhodium alloy versus platinum
thermocouples, called type R thermocouples. 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
(1167.33 °F) 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 (>2192 °F)) in vacuum, air, and argon atmospheres have on the
thermoelectric voltages of type R thermocouples.