LANGE'S HANDBOOK OF CHEMISTRY

PRACTICAL LABORATORY INFORMATION 11.139
The noble metal thermocouples, Types B, R, and S, are all platinum or platinum-rhodium thermocouples
and hence share many of the same characteristics. Metallic vapor diffusion at high temperatures
can readily change the platinum wire calibration, hence platinum wires should only be
used inside a nonmetallic sheath such as high-purity alumina.
Type B thermocouples (Table 11.56) offer distinct advantages of improved stability, increased
mechanical strength, and higher possible operating temperatures. They have the unique advantage
that the reference junction potential is almost immaterial, as long as it is between 0C and 40C.
Type B is virtually useless below 50C because it exhibits a double-value ambiguity from 0C to
42C.
Type E thermoelements (Table 11.57) are very useful down to about liquid hydrogen temperatures
and may even be used down to liquid helium temperatures. They are the most useful of the commercially
standardized thermocouple combinations for subzero temperature measurements because
of their high Seebeck coefficient (58 V/C), low thermal conductivity, and corrosion resistance.
They also have the largest Seebeck coefficient (voltage response per degree Celsius) above 0C of
any of the standardized thermocouples which makes them useful for detecting small temperature
changes. They are recommended for use in the temperature range from 250 to 871C in oxidizing
or inert atmospheres. They should not be used in sulfurous, reducing, or alternately reducing and
oxidizing atmospheres unless suitably protected with tubes. They should not be used in vacuum at
high temperatures for extended periods of time.
Type J thermocouples (Table 11.58) are one of the most common types of industrial thermocouples
because of the relatively high Seebeck coefficient and low cost. They are recommended for use
in the temperature range from 0 to 760C (but never above 760C due to an abrupt magnetic transformation
that can cause decalibration even when returned to lower temperatures). Use is permitted
in vacuum and in oxidizing, reducing, or inert atmospheres, with the exception of sulfurous atmospheres
above 500C. For extended use above 500C, heavy-gauge wires are recommended. They are
not recommended for subzero temperatures. These thermocouples are subject to poor conformance
characteristics because of impurities in the iron.
The Type K thermocouple (Table 11.59) is more resistant to oxidation at elevated temperatures
than the Type E, J, or T thermocouple, and consequently finds wide application at temperatures
above 500C. It is recommended for continuous use at temperatures within the range 250 to 1260C
in inert or oxidizing atmospheres. It should not be used in sulfurous or reducing atmospheres, or in
vacuum at high temperatures for extended times.
The Type N thermocouple (Table 11.60) is similar to Type K but it has been designed to minimize
some of the instabilities in the conventional Chromel-Alumel combination. Changes in the alloy
content have improved the order/disorder transformations occurring at 500C and a higher silicon
content of the positive element improves the oxidation resistance at elevated temperatures.
The Type R thermocouple (Table 11.61) was developed primarily to match a previous platinum–
10% rhodium British wire which was later found to have 0.34% iron impurity in the rhodium.
Comments on Type S also apply to Type R.
The Type S thermocouple (Table 11.62) is so stable that it remains the standard for determining
temperatures between the antimony point (630.74C) and the gold point (1064.43C). The other fixed
point used is that of silver. The Type S thermocouple can be used from 50C continuously up to
about 1400C, and intermittently at temperatures up to the freezing point of platinum (1769C). The
thermocouple is most reliable when used in a clean oxidizing atmosphere, but may also be used in
inert gaseous atmospheres or in a vacuum for short periods of time. It should not be used in reducing
atmospheres, nor in those containing metallic vapor (such as lead or zinc), nonmetallic vapors (such
as arsenic, phosphorus, or sulfur), or easily reduced oxides, unless suitably protected with nonmetallic
protecting tubes.
The Type T thermocouple (Table 11.63) is popular for the temperature region below 0C (but
see under Type E). It can be used in vacuum, or in oxidizing, reducing, or inert atmospheres.