Rockwell Automation 1762-IT4 Thermocouple/mV Input Module - Manual (Page 111)

Publication 1762-UM002A-EN-P - July 2002
Thermocouple Descriptions C-11
one-half the standard tolerances given above. Tolerances are not
specified for type N thermocouples below 0°C.
The suggested upper temperature limit of 1260°C given in the ASTM
standard [7] for protected type N thermocouples applies to AWG 8
(3.25 mm) wire. It decreases to 1090°C for AWG 14 (1.63 mm), 980°C
for AWG 20 (0.81 mm), 870°C for AWG 24 or 28 (0.51 mm or 0.33
mm), and 760°C for AWG 30 (0.25 mm). 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 percent 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 percent rhodium. The negative (RN)
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 13.00 ±
0.05 percent 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 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