A thermocouple is an
electrical device consisting of two dissimilar electrical conductors forming
electrical junctions at differing temperatures. A thermocouple produces a
temperature-dependent voltage as a result of the thermoelectric effect, and
this voltage can be interpreted to measure temperature. Thermocouples are a
widely used type of temperature sensor.
There are many types
of thermocouples. Type J, K, T, & E are “Base Metal” thermocouples, the
most common types of thermocouples. Type R, S, and B thermocouples are “Noble
Metal” thermocouples, which are used in high temperature applications.
This thermocouple consists of an Iron and a Constantan. It has a more restricted range (−40 °C to +750 °C) but higher sensitivity of about 50 µV/°C. The Curie point of the iron (770 °C) causes a smooth change in the characteristic, which determines the upper temperature limit. Type J thermocouple may be used in vacuum, reducing, oxidizing and inert atmospheres.
This thermocouple consists of a Chromel and an Alumel leg. It is the most common general-purpose thermocouple with a sensitivity of approximately 41 µV/°C. It is inexpensive, and a wide variety of probes are available in its −200 °C to +1350 °C) range. This thermocouple is fairly accurate and stable at high temperatures.
This thermocouple consists of a Copper and a Constantan leg. These thermocouples are suited for measurements in the −200 to +350 °C range. Type-T thermocouples have a sensitivity of about 43 µV/°C. It may be used in vacuum, oxidizing, reducing and inert atmospheres. It maintains good resistance to corrosion in most atmospheres and high stability at sub-zero temperatures.
This thermocouple consists of one Chromel leg and one Constantan leg. This thermocouple is not subject to corrosion in most atmospheres. It has a high output (68 µV/°C), which makes it well suited to cryogenic use. Additionally, it is non-magnetic.
Types B, R, and S thermocouples use platinum or a rhodium alloy for each conductor. These are among the most stable thermocouples, but have lower sensitivity than other types, approximately 10 µV/°C. Types R & S may be used up to 1480° C.
