Spectrum Controls 1769sc-IF8u User Manual

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Appendix C: Thermocouple Descriptions

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
transfer standards and reference thermometers in various laboratory
applications and to develop reference functions and tables [22,23]. The
higher purity alloy material typically contains less than 500 atomic ppm of
impurities and the platinum less than 100 atomic ppm of impurities [22].
Differences between such high purity commercial material and the
platinum thermoelectric reference standard, Pt-67, are described in [22]
and [23].

A reference function for the type R thermocouple, based on the ITS-90
and the SI volt, was determined recently from new data obtained in a
collaborative effort by NIST and NPL. The results of this international
collaboration were reported by Burns et al [23]. The function was used to
compute the reference table given in this monograph.

Type R thermocouples have about a 12 percent larger Seebeck coefficient
than do Type S thermocouples over much of the range. Type R
thermocouples were not standard interpolating instruments on the IPTS-68
for the 630.74°C to gold freezing-point range. Other than these two points,
and remarks regarding history and composition, all of the precautions and
restrictions on usage given in the section on type S thermocouples also
apply to type R thermocouples. Glawe and Szaniszlo [24], and Walker et al
[25,26] have determined the effects that prolonged exposure at elevated
temperatures (>1200°C) in vacuum, air, and argon atmospheres have on
the thermoelectric voltages of type R thermocouples.

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 or ±0.25 percent (whichever is greater)
between 0°C and 1450°C. Type R thermocouples can be supplied to meet
special tolerances of ±0.6°C or ±0.1 percent (whichever is greater). The
suggested upper temperature limit, 1480°C, given in the ASTM standard
[7] for protected type R thermocouples applies to AWG 24 (0.51 mm)
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

T h e r

h e r

h e r m o c o u p l e s

m o c o u p l e s

This section describes Platinum-10 percent 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 percent rhodium. The negative
(SN) thermoelement is commercially available platinum that has a nominal
purity of 99.99 percent [21]. An industrial consensus standard (ASTM
E1159-87) specifies that rhodium having a nominal purity of 99.98 percent
shall be alloyed with platinum of 99.99 percent purity to produce the