Rockwell Automation 1762-IT4 Thermocouple/mV Input Module - Manual (Page 101)

1 Publication 1762-UM002A-EN-P - July 2002
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