Spectrum Controls 1756sc-IF8u - Manual (Page 98)

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ControlLogix ™ Universal Analog Input Module
weight percent increase in rhodium content; the Seebeck coefficient
increases by about 4% per weight percent increase at the same
temperature.
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards
[7] specifies that the initial calibration tolerances for type S commercial
thermocouples be +/-1.5°C or +/-0.25% (whichever is greater) between
0°C and 1450°C. Type S thermocouples can be supplied to meet special
tolerances of +/-0.6°C or +/-0.1% (whichever is greater).
The suggested upper temperature limit, 1480°C, given in the ASTM
standard [7] for protected type S thermocouples applies to AWG 24
(0.51mm) 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.
B Type
Thermocouples Pla Pla Pla Pla Platin tin tin tin tinum-30% Rhodium um-30% Rhodium um-30% Rhodium um-30% Rhodium um-30% Rhodium Allo Allo Allo Allo Alloy y y y y V V V V Ver er er er ersus Pla sus Pla sus Pla sus Pla sus Platin tin tin tin tinum- um- um- um- um-
6% Rhodium 6% Rhodium 6% Rhodium 6% Rhodium 6% Rhodium Allo Allo Allo Allo Alloy y y y y T TT T Ther her her her her mocouples mocouples mocouples mocouples mocouples
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].
Studies by Ehringer [39], Walker et al. [25,26], and Glawe and Szaniszlo
[24] have demonstrated that thermocouples, in which both legs are
platinum-rhodium alloys, are suitable for reliable temperature
measurements at high temperatures. Such thermocouples have been
shown to offer the following distinct advantages over types R and S
thermocouples at high temperatures: (1) improved stability, (2) increased
mechanical strength, and (3) higher operating temperatures.
The research by Burns and Gallagher [38] indicated that the 30-6
thermocouple can be used intermittently (for several hours) up to 1790°C
and continuously (for several hundred hours) at temperatures up to about
1700°C with only small changes in calibration. The maximum temperature
limit for the thermocouple is governed, primarily, by the melting point of the
Pt-6% rhodium thermoelement which is estimated to be about 1820°C by
Acken [40]. The thermocouple is most reliable when used in a clean