Spectrum Controls 1746sc-NI8u - Manual (Page 103)

Thermocouple Descriptions
The following information 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.
Inter Inter Inter Inter Inter na na na na national tional tional tional tional T TT T Temper emper emper emper emperaaaaatur tur tur tur ture Scale of e Scale of e Scale of e Scale of e Scale of 1990 1990 1990 1990 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 has 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.
J Type
Thermocouples Ir Ir Ir Ir Iron on on on on V V V V Ver er er er ersus Copper-Nic sus Copper-Nic sus Copper-Nic sus Copper-Nic sus Copper-Nickkkkkel el el el el Allo Allo Allo Allo Alloy (SAMA) y (SAMA) y (SAMA) y (SAMA) y (SAMA)
T TT T T her her her her her mocouples mocouples mocouples mocouples mocouples
This is one of the most common types of industrial thermocouples,
because of its relatively high Seebeck coefficient and low cost. It has
been reported that more than 200 tons of type J materials are supplied
annually to industry in this country. However, this type is least suitable for
accurate thermometry because there are significant nonlinear deviations in
the thermoelectric output of thermocouples obtained from different
manufacturers. These irregular deviations lead to difficulties in obtaining
accurate calibrations based on a limited number of calibration points. The
positive thermoelement is commercially pure (99.5% Fe) iron, usually
containing significant impurity levels of carbon, chromium, copper,
manganese, nickel, phosphorus, silicon, and sulfur. Thermocouple wire
represents such a small fraction of the total production of commercial iron
wire that the producers do not control the chemical composition to
maintain constant thermoelectric properties. Instead, instrument
companies and thermocouple fabricators select material most suitable for
Appendix B