Allen-Bradley MicroLogix 1200 User Manual page 115

Thermocouple Descriptions C-15
family of copper-nickel alloys containing anywhere from 45 to 60
percent 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, etc. The constantan
for type T thermocouples usually contains about 55 percent copper,
45 percent nickel, and small but thermoelectrically significant
amounts, about 0.1 percent 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 4K) but that its Seebeck coefficient
becomes quite small below 20K. Its Seebeck coefficient at 20K is only
about 5.6µV/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 70K 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 20K. Type E thermocouples are recommended as the
most suitable of the letter-designated thermocouple types for general
low-temperature use, since 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°C to 370°C 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 for AWG 14 (1.63 mm) thermoelements
since type TP thermoelements oxidize rapidly above this temperature.
However, the thermoelectric properties of type TP thermoelements
are apparently not grossly affected by oxidation since negligible
AB Parts changes in the thermoelectric voltage were observed at NBS [10] for
AWG 12, 18, and 22 type TP thermoelements during 30 hours of
Publication 1762-UM002A-EN-P - July 2002