Rockwell Automation 1790P-T4T0 CompactBlock LDX I/O Thermocouple Modules - Manual (Page 63)

1 Publication 1790-UM003A-EN-P
Appendix
C
Thermocouple Descriptions
The information in this appendix was extracted from the NIST Monograph
175 issued in January 1990, which supersedes the IPTS-68 Monograph 125
issued in March 1974. NIST Monograph 175 is provided by the United
States Department of Commerce, National Institute of Standards and
Technology.
International
Temperature Scale of
1990 The ITS-90 [1,3] is realized, maintained, and disseminated by NIST to
provide a standard scale of temperature for use in science and industry in
the United States. This scale was adopted by the International Committee
of Weights and Measures (CIPM) at its meeting in September 1989, and it
became the official international temperature scale on January 1, 1990.
The ITS-90 supersedes the IPTS-68(75) [2] and the 1976 Provisional 0.5 K
to 30 K Temperature Scale (EPT-76) [4].
The adoption of the ITS-90 removed several deficiencies and limitations
associated with IPTS-68. Temperatures on the ITS-90 are in closer
agreement with thermodynamic values than were those of the IPTS-68
and EPT-76. Additionally, improvements have been made in the
non-uniqueness and reproducibility of the temperature scale, especially in
the temperature range from t68 = 630.74°C to 1064.43°C, where the type S
thermocouple was the standard interpolating device on the IPTS-68.
For additional technical information regarding ITS-90, refer to the NIST
Monograph 175.
Type B Thermocouples This section discusses Platinum-30 percent Rhodium Alloy Versus
Platinum-6 percent Rhodium Alloy thermocouples, commonly called type
B thermocouples. This type is sometimes referred to by the nominal
chemical composition of its thermoelements: platinum - 30 percent
rhodium versus platinum - 6 percent rhodium or “30-6”. The positive (BP)
thermoelement typically contains 29.60 ± 0.2 percent rhodium and the
negative (BN) thermoelement usually contains 6.12 ± 0.02 percent
rhodium. The effect of differences in rhodium content are described later
in this section. An industrial consensus standard [21] (ASTM E1159-87)
specifies that rhodium having a purity of 99.98 percent shall be alloyed
with platinum of 99.99 percent purity to produce the thermoelements.
This consensus standard [21] describes the purity of commercial type B
materials that are used in many industrial thermometry applications that
meet the calibration tolerances described later in this section. Both
thermoelements will typically have significant impurities of elements such
as palladium, iridium, iron, and silicon [38].