Spectrum Controls 1746sc-NI8u - Manual (Page 108)

96 SLC 500 ™ Universal Analog Input Module
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
5.6uV/°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, 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 (1.63mm) 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 changes in the thermoelectric voltage were observed at
NBS [10] for AWG 12, 18, and 22 type TP thermoelements during 30
hours of heating in air at 500°C. At this temperature the type TN
thermoelements have good resistance to oxidation and exhibit only small
voltage changes heated in air for long periods of time, as shown by the
studies of Dahl [11]. Higher operating temperatures, up to at least 800°C,
are possible in inert atmospheres where the deterioration of the type TP
thermoelement is no longer a problem. The use of type T thermocouples
in hydrogen atmospheres at temperatures above about 370°C is not
recommended since type TP thermoelements may become brittle.
Type T thermocouples are not well suited for use in nuclear environments
since both thermoelements are subject to significant changes in
composition under thermal neutron irradiation. The copper in the
thermoelements is converted to nickel and zinc.
Because of the high thermal conductivity of type TP thermoelements,
special care should be exercised when using the thermocouples to ensure
that the measuring and reference junctions assume the desired
temperatures.
ASTM Standard E230-87 in the 1992 Annual Book of ASTM Standards
[7] specifies that the initial calibration tolerances for type T commercial
thermocouples be +/-1°C or +/-0.75% (whichever is greater) between 0°C
and 350°C, and +/-1°C or +/-1.5% (whichever is greater) between -200°C
and 0°C. Type T thermocouples can also be supplied to meet special
tolerances which are equal to approximately one-half the standard
tolerances given above. Type T thermocouple materials are normally