Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 124)

124 Rockwell Automation Publication 1769-UM004B-EN-P - March 2010
Appendix C Thermocouple Descriptions
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards [7]
specifies that the initial calibration tolerances for type R commercial
thermocouples be ±1.5 °C (±34.7 °F) or ±0.25% (whichever is greater) between
0 °C (32 °F) and 1450 °C (2642 °F). Type R thermocouples can be supplied to
meet special tolerances of ±0.6 °C (±33.08 °F) or ±0.1% (whichever is greater).
The suggested upper temperature limit, 1480 °C (2696 °F), given in the ASTM
standard [7] for protected type R thermocouples applies to 0.51 mm
2
(24 AWG)
wire. This temperature limit applies to thermocouples used in conventional
closed-end protecting tubes and it is intended only as a rough guide to the user.
It does not apply to thermocouples having compacted mineral oxide insulation.
Type S Thermocouples This section describes platinum-10% rhodium alloy versus platinum
thermocouples, commonly known as type S thermocouples. This type is often
referred to by the nominal chemical composition of its positive (SP)
thermoelement: platinum-10% rhodium. The negative (SN) thermoelement is
commercially available platinum that has a nominal purity of 99.99% [21].
An industrial consensus standard (ASTM E1159-87) specifies that rhodium
having a nominal purity of 99.98% shall be alloyed with platinum of 99.99%
purity to produce the positive thermoelement, which typically contains 10.00
±0.05% rhodium by weight. The consensus standard [21] describes the purity of
commercial type S 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 standard
instruments of the IPTS-68, as transfer standards and reference thermometers in
various laboratory applications, and to develop reference functions and tables
[27,28]. The higher purity alloy material typically contains less than 500 atomic
ppm of impurities and the platinum less than 100 atomic ppm of impurities [27].
Difference between such high purity commercial material and the platinum
thermoelectric reference standard, Pt-67, are described in [27] and [28].
A reference function for the type S thermocouple, based on the ITS-90 and the
SI volt, was determined recently from new data obtained in an international
collaborative effort involving eight national laboratories. The results of this
international collaboration were reported by Burns et al. [28]. The new function
was used to compute the reference table given in this monograph.