Spectrum Controls 1769sc-IF8u - Manual (Page 96)

86 Compact IO ™ Universal Input Module
Type T Type T Type T Type T Type T
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 describes Copper Versus Copper-Nickel Alloy thermocouples,
called type T thermocouples. This type is one of the oldest and most
popular thermocouples for determining temperatures within the range from
about 370°C down to the triple point of neon (-248.5939°C). Its positive
thermoelement, TP, is typically copper of high electrical conductivity and
low oxygen content that conforms to ASTM Specification B3 for soft or
annealed bare copper wire. Such material is about 99.95 percent pure
copper with an oxygen content varying from 0.02 to 0.07 percent
(depending upon sulfur content) and with other impurities totaling about
0.01 percent. Above about -200°C, the thermoelectric properties of type
TP thermoelements, which satisfy the above conditions, are exceptionally
uniform and exhibit little variation between lots. Below about -200°C the
thermoelectric properties are affected more strongly by the presence of
dilute transition metal solutes, particularly iron.
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 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