Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 113)

Rockwell Automation Publication 1769-UM004B-EN-P - March 2010 113
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…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% rhodium alloy versus platinum-6% rhodium
alloy thermocouples, commonly called type B thermocouples. This type is
sometimes referred to by the nominal chemical composition of its
thermoelements: platinum-30% rhodium versus platinum-6% rhodium or ‘30-6’.
The positive (BP) thermoelement typically contains 29.60 ±0.2% rhodium and
the negative (BN) thermoelement usually contains 6.12 ±0.02% 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% shall be alloyed with platinum of 99.99% 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].