Rockwell Automation 1769-IT6 Compact I/O 1769-IT6 Thermocouple/mV Input Module - Manual (Page 127)

Rockwell Automation Publication 1769-UM004B-EN-P - March 2010 127
Thermocouple Descriptions Appendix C
The negative thermoelement, TN or EN, is a copper-nickel alloy known
ambiguously as constantan. The word constantan refers to a family of
copper-nickel alloys containing anywhere from 45…60% copper. These alloys also
typically contain small percentages of cobalt, manganese and iron, as well as trace
impurities of other elements such as carbon, magnesium, silicon, and so forth.
The constantan for type T thermocouples usually contains about 55% copper,
45% nickel, and small but thermoelectrically significant amounts, about 0.1% or
larger, of cobalt, iron, or manganese. It should be emphasized that type TN (or
EN) thermoelements are not generally interchangeable with type JN
thermoelements although they are all referred to as ‘constantan’. In order to
provide some differentiation in nomenclature, type TN (or EN) is often referred
to as Adams’ (or RP1080) constantan and type JN is usually referred to as SAMA
constantan.
The thermoelectric relations for type TN and type EN thermoelements are the
same, that is the voltage versus temperature equations and tables for platinum
versus type TN thermoelements apply to both types of thermoelements over the
temperature range recommended for each thermocouple type. However, if
should not be assumed that type TN and type EN thermoelements may be used
interchangeably or that they have the same commercial initial calibration
tolerances.
The low temperature research [8] by members of the NBS Cryogenics Division
showed that the type T thermocouple may be used down to liquid helium
temperatures (about 4 °K) but that its Seebeck coefficient becomes quite small
below 20 °K. Its Seebeck coefficient at 20 °K is only about 5.6 μV/K, being
roughly two-thirds that of the type E thermocouple. The thermoelectric
homogeneity of most type TP and type TN (or EN) thermoelements is
reasonably good. There is considerable variability, however, in the thermoelectric
properties of type TP thermoelements below about 70 °K caused by variations in
the amounts and types of impurities present in these nearly pure materials. The
high thermal conductivity of the type TP thermoelements can also be
troublesome in precise applications. For these reasons, type T thermocouples are
generally unsuitable for use below about 20 °K. Type E thermocouples are
recommended as the most suitable of the letter-designated thermocouple types
for general low-temperature use, because they offer the best overall combination
of desirable properties.
Type T thermocouples are recommended by the ASTM [5] for use in the
temperature range from -200…370 °C (-328…698 °F) in vacuum or in oxidizing,
reducing, or inert atmospheres. The suggested upper temperature limit for
continuous service of protected type T thermocouples is set at 370 °C (698 °F)
for 1.63 mm
2
(14 AWG) thermoelements because type TP thermoelements
oxidize rapidly above this temperature. However, the thermoelectric properties of
type TP thermoelements are apparently not grossly affected by oxidation because
negligible changes in the thermoelectric voltage were observed at NBS [10] for
12, 18, and 22 AWG type TP thermoelements during 30 hours of heating in air at
500 °C (932 °F).