Rockwell Automation 1794-IT8,D17946.5.7 Thermocouple/Millivolt Input Module - Manual (Page 65)

Appendix
B
Publication 1794-6.5.7
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 0.1 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.”
General