Dwyer Instruments, Inc. offers many temperature measuring products that use thermocouples as the sensing element, and also manufactures a wide variety of thermocouples.
A thermocouple is made of two dissimilar metal wires, which product a voltage as a function of temperature change. Thermocouples are based on the Seebeck effect, which is a thermoelectric effect where continuous current is created in a circuit where the junctions of two dissimilar conductive materials are kept at different temperatures. The measuring end of the sensor is created by joining the two metal wires together into what is called the hot or measuring junction.
Thermocouples measure the temperature difference between the measuring junction and a reference temperature, known as the cold junction. Typically, a cold junction compensation is performed using an RTD, thermistor, or diode to measure the temperature at the input connections to the measuring instrument. A small voltage known as the Seebeck voltage is created when the hot junction is exposed to the process temperature, from the temperature difference at the two junctions.
Thermocouples use standard sensors that are made of junctions of specific metal alloys which have a predictable and repeatable relationship between temperature and voltage. Different metal combinations are used for different temperature ranges. Certain combinations of alloys have become popular as industry standards, with different types best suited for different applications. Letter designations are given to the type of thermocouple based on voltage-temperature relationship.
The most common thermocouples are Type K, chromel/alumel, with a ranges around -200 to 1350 degrees Celsius. Type J, iron-constantan, has a smaller range than K, around -40 to 750 degrees Celsius, but has a higher temperature sensitivity. Selection of the thermocouple to use depends on the application needs for range, sensitivity, chemical compatibility, and magnetism.
Color codes standards have been established by national, ANSI, and international, IEC, standards for easy identification. One wire in the pair is always negative and the other positive.
Often thermocouples are sheathed in a thin walled stainless steel tube and packed with magnesium oxide to protect the measuring junction, and for easier insertion into thermowells. Thermocouples designed with their measuring junctions in contact with the sheath are termed grounded junctions. Exposed type thermocouples have the measuring junction exposed to the process. Both offer the advantage of fast response time. In ungrounded junction thermocouples, the junction does not make contact with the sheath, offering the advantage of electrical isolation.
Thermocouples can be wired directly to the measuring instrument or extended with thermocouple wire. Extension wires are made of the same metal alloys as the thermocouple type. Standard electrical wire cannot be used. Sensor wires should never be run in conduit with power sources, as there can be noise problems. Commonly, a thermocouple is wired into a temperature transmitter when sending the signal a long distance.