Rockwell Automation 1790P-T4T0 CompactBlock LDX I/O Thermocouple Modules - Manual (Page 73)

Publication 1790-UM003A-EN-P
Thermocouple Descriptions C-11
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards [7]
specifies that the initial calibration tolerances for type N commercial
thermocouples be ±2.2°C or ±0.75 percent (whichever is greater) between
0°C and 1250°C. Type N thermocouples can also be supplied to meet
special tolerances that are equal to approximately 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.