Spectrum Controls 1769sc-IF8u - Manual (Page 97)

Appendix C: Thermocouple Descriptions 87
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 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 percent (whichever is greater) between
0°C and 350°C, and ±1°C or ±1.5 percent (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
supplied to meet the tolerances specified for temperatures above 0°C.
However, the same materials may not satisfy the tolerances specified for
the -200°C to 0°C range. If materials are required to meet the tolerances
below 0°C, this should be specified when they are purchased.
The suggested upper temperature limit of 370°C given in the ASTM
standard [7] for protected type T thermocouples applies to AWG 14 (1.63
mm) wire. It decreases to 260°C for AWG 20 (0.81 mm), 200°C for AWG