Spectrum Controls 1756sc-IF8u - Manual (Page 88)

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ControlLogix ™ Universal Analog Input Module
the thermocouple usage. The total and specific types of impurities that
occur in commercial iron change with time, location of primary ores, and
methods of smelting. Many unusual lots have been selected in the past,
for example spools of industrial iron wire and even scrapped rails from an
elevated train line. At present, iron wire that most closely fits these tables
has about 0.25 percent manganese and 0.12 percent copper, plus other
minor impurities.
The negative thermoelement for type J thermocouples is a copper-nickel
alloy known ambiguously as constantan. The word constantan has
commonly referred to copper-nickel alloys containing anywhere from 45 to
60 percent copper, plus minor impurities of carbon, cobalt, iron, and
manganese. Constantan for type J thermocouples usually contains about
55 percent copper, 45 percent nickel, and a small but thermoelectrically
significant amount of cobalt, iron, and manganese, about 0.1 percent or
more. It should be emphasized that type JN thermoelements are
NOT generally interchangeable with type TN (or EN)
thermoelements, although they are all referred to as “constantan”.
In order to provide some differentiation in nomenclature, type JN is often
referred to as SAMA constantan.
Type J thermocouples are recommended by the ASTM [5] for use in the
temperature range from 0°C to 760°C in vacuum, oxidizing, reducing, or
inert atmospheres. If used for extended times in air above 500°C, heavy
gage wires are recommended because the oxidation rate is rapid at
elevated temperatures. Oxidation normally causes a gradual decrease in
the thermoelectric voltage of the thermocouple with time. Because iron
rusts in moist atmospheres and may become brittle, type J thermocouples
are not recommended for use below 0°C. In addition, they should not be
used unprotected in sulfurous atmospheres above 500°C.
The positive thermoelement, iron, is relatively insensitive to composition
changes under thermal neutron irradiation, but does exhibit a slight
increase in manganese content. The negative thermoelement, a copper-
nickel alloy, is subject to substantial composition changes under thermal
neutron irradiation since copper is converted to nickel and zinc.
Iron undergoes a magnetic transformation near 769°C and an alpha-
gamma crystal transformation near 910°C [6]. Both of these
transformations, especially the latter, seriously affect the thermoelectric
properties of iron, and therefore of type J thermocouples. This behavior
and the rapid oxidation rate of iron are the main reasons why iron versus
constantan thermocouples are not recommended as a standardized type
above 760°C. If type J thermocouples are taken to high temperatures,
especially above 900°C, they will lose the accuracy of their calibration
when they are recycled to lower temperatures. If type J thermocouples
are used in air at temperatures above 760°C, only the largest wire, AWG 8
(3.3mm) should be used and they should be held at the measured
temperature for 10 to 20 minutes before readings are taken. The
thermoelectric voltage of the type J thermocouples may change by as