| The national standards have adopted most of the
thermocouples listed in the IEC standard. Table below contains thermocouples
typically used in international trade.
A thermocouple is identified by the Type, for example Type B or
B thermocouple. The positive and negative wires identify the polarity
of the thermocouple. When the hot junction is higher in temperature
than the cold junction, a positive emf appears on the positive wire.
A thermocouple can be used above the recommended temperature range,
up to the upper temperature limit. Above the recommended range the
life of the thermocouple becomes short, due to oxidation, crystallization
of the wires and other reasons. In some applications, the only possible,
economical way of temperature measurement is to use a thermocouple
near the upper temperature limit, replacing it frequently.
There are new thermocouples, like Type N (nicrosil/nisil) thermocouple,
which are expected to have a longer life than Type K thermocouples
in an oxidizing atmosphere. DIN includes Types U and L. In the U.
S., Type W is used in very high temperature (up to 2300°C or
3700°F).
The standards carry the temperature-emf tables of each thermocouple,
referenced to 0°C (32°F). In other words, it is the emf
when the cold junction is at 0°C. |
What
kinds of the thermocouples are used?
The
table below has seven widely used thermocouples. There are about ten thermocouples
listed in the international (IEC) and/or national standards, like ANSI of
the U. S. A., DIN of Germany and JIS of Japan. The listed thermocouples
are recognized as reliable in research and application, among possible combinations
of dissimilar metals. 