Rockwell Automation 1771-IXHR , D17716.5.80 HIGH RESOL.THERMOCOUPLE - Manual (Page 54)

Appendix
C
CĆ1
Thermocouple Restrictions
(Extracted from NBS Monograph 125
(IPTS-68))
Following are some restrictions extracted from NBS Monograph 125 (IPTS–68)
issued March 1974 on thermocouples B, E, J, K, R, S and T:
B (Platinum - 30% Rhodium vs Platinum - 6% Rhodium) Type
Thermocouples
“The ASTM manual STP 470 [1970] indicates the following restrictions on the
use of B type thermocouples at high temperatures: They should not be used in
reducing atmospheres, nor in those containing metallic or nonmetallic vapors,
unless suitably protected wiht nonmetallic protecting tubes. They should never
be inserted directly into a metallic primary tube.”
“At temperatures below 450C the Seebeck coefficient of Type B thermocouples
becomes quite small and is almost negligible in the normal room temperature
range. Consequently, in most applications the reference junction temperature of
the thermocouple does not need to be controlled or even known, as long as it is
between 0 and 50C.”
Studies have shown that “a .1 percent change in the Rhodium content of the
Pt–30% Rh thermoelement produces a corresponding change in the
thermocouple voltage of about 15uV (i.e. 1.3C) at 1500C. In contrast a change
of only .01% in the Rhodium content of Pt–6% Rh thermoelement also
produces a voltage change of about 15uV (1.3C) at this temperature.”
“The thermoelectric voltages of Type B thermocouples is sensitive to their
history of annealing, heat treatment and quenching. Calibration of Type B wires
above 1600C is undesirable in most circumstances.”
“ASTM Standard E230–72 in the Annual Book of ASTM Standards [1972]
specifies that the standard limits of error for Type B commercial thermocouples
be + 1/2 percent between 871 and 1705C. Limits of error are not specified for
Type B thermocouples below 871C. The recommended upper temperature limit
for protected thermocouples, 1705C, applies to AWG 24 (0.5mm) wire.”
E (Nickel-Chromium vs Copper-Nickel <Constantan*>) Type Thermocouple
“Type E thermocouples are recommended by the ASTM Manual [1970] for use
in the temperature range from –250 to 871C in oxidizing or inert atmospheres.
General