Allen-Bradley CompactBlock LDX 1790D-4T0, Руководство пользователя

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Publication 1790-UM003A-EN-P
Thermocouple Descriptions C-15
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 5.6
µ
V/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.
AB Drives