Rockwell Automation 1762-IT4 Thermocouple/mV Input Module - Manual (Page 105)

Publication 1762-UM002A-EN-P - July 2002
Thermocouple Descriptions C-5
may not satisfy the tolerances specified for the -200°C to 0°C range. If
materials are required to meet the tolerances below 0°C, this should
be specified when they are purchased.
The suggested upper temperature limit, 870°C, given in the ASTM
standard [7] for protected type E thermocouples applies to AWG 8
(3.25 mm) wire. It decreases to 650°C for AWG 14 (1.63 mm), 540°C
for AWG 20 (0.81 mm), 430°C for AWG 24 or 28 (0.51 mm or 0.33
mm), and 370°C for AWG 30 (0.25 mm). These temperature limits
apply to thermocouples used in conventional closed-end protecting
tubes and they are intended only as a rough guide to the user. They
do not apply to thermocouples having compacted mineral oxide
insulation.
Type J Thermocouples 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