Spectrum Controls 1746sc-NI8u - Manual (Page 107)

Appendix B: Thermocouple Descriptions 95
(3.25mm) wire. It decreases to 1090°C for AWG 14 (1.63mm), 980°C for
AWG 20 (0.81mm), 870 for AWG 24 or 28 (0.51mm or 0.33mm), and
760°C for AWG 30 (0.25mm). 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.
T Type
Thermocouples Copper Copper Copper Copper Copper V V V V Ver er er er ersus Copper-Nic sus Copper-Nic sus Copper-Nic sus Copper-Nic sus Copper-Nickkkkkel el el el el Allo Allo Allo Allo Alloyyyyy
T TT T T her her her her her mocouples mocouples mocouples mocouples mocouples
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% pure copper with an oxygen content
varying from 0.02 to 0.07% (depending upon sulfur content) and with
other impurities totaling about 0.01%. 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% 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% 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, it 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