Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 117)

Rockwell Automation Publication 1769-UM004B-EN-P - March 2010 117
Thermocouple Descriptions Appendix C
The suggested upper temperature limit, 870 °C (1598 °F), given in the ASTM
standard [7] for protected type E thermocouples applies to 3.25 mm
2
(8 AWG)
wire. It decreases to 650 °C (1202 °F) for 1.63 mm
2
(14 AWG), 540 °C (1004 °F)
for 0.81 mm
2
(20 AWG), 430 °C (806 °F) for 0.51 or 0.33 mm
2
(24 or 28 AWG),
and 370 °C (698 °F) for 0.25 mm
2
(30 AWG). 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% 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% manganese and
0.12% 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…60% copper, plus minor
impurities of carbon, cobalt, iron, and manganese. Constantan for type J
thermocouples usually contains about 55% copper, 45% nickel, and a small but
thermoelectrically significant amount of cobalt, iron, and manganese, about 0.1%
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’. To provide some differentiation in
nomenclature, type JN is often referred to as SAMA constantan.