Spectrum Controls 1769sc-IF8u - Manual (Page 87)

Appendix C: Thermocouple Descriptions 77
are intended only as a rough guide to the user. They do not apply to
thermocouples having compacted mineral oxide insulation.
Type J Type J Type J Type J Type J
T TT T T h e r h e r 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 m o c o u p l e s m o c o u p l e s m o c o u p l e s This section discusses Iron Versus Copper-Nickel Alloy (SAMA)
thermocouples, called type J thermocouples. A type J thermocouple 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 percent 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 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
gauge 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