Thermocouple wires are used for conveying a voltage differential that has been established at one end - usually by a thermocouple probe - to the other end - usually a display device such as a digital thermometer. Thermocouple probes are made by joining two dissimilar metals, the most common being Chromel and Alumel and the point at which they are joined - usually referred to as the hot junction - will develop a potential which is proportional to the temperature. Now because dissimilar metals when joined together will create a thermocouple junction - then the same type of wire needs to be used to connect the thermocouple to the measuring device. (copper wires wont do - that would only create another junction).. Therefore if the thermocouple probe is a type K, then the wire used must be also a type K wire, also the polarity of the wire must be observed when hooking up. Positive to positive etc., also positive is NOT red, its Yellow (for type K). Thermocouple wires are available in various sizes and just about any size will do, however other factors may determine using large wires -eg. when the measured point is far away from the measuring device then a larger wire is preferable. Thermocouple wires comes in a wide variety however specific characteristics usually determine the type used. The widest range of thermocouple wire is the type K but that does come with some drawbacks such as the resolution which would be lower so if higher accuracy at lower temperatures is a priority then other type such as the type T thermocouple may be desireable. Below is a table listing the temperature limits of various kinds of thermocouple wire - based on the insulation of the wire.:
Type K Thermocouple wire with glass insulation: 482 deg. C.
Type J Thermocouple wire with PVC insulation: 105 deg. C.
Type T Thermocouple wire with Teflon Insulation: 260 deg. C.
Type E Thermocouple wire with Kapton Insulation: 260 deg. C.
In addition to the temperature range of thermocouple wires, there is also the measurement range of the wire. Below is the temperature measurement range of the most common types of thermocouple wire.
Type K Thermocouple wire -200 deg. C to 1,350 deg. C
Type J Thermocouple wire -40 deg. C to 750 deg. C
Type T Thermocouple wire -200 deg. C to 350 deg. C
Type E Thermocouple wire -110 deg. C to 140 deg. C
Thermocouple wires are available in two levels of accuracy: Normal and SLE. For the Normal accuracy, there is no SLE notation in the thermocouple wire model number. For SLE the thermocouple wire part number will have the SLE within it. eg GG-K-24-SLE-500. This is a special limits of error thermocouple wire with glass insulation, thermocouple wire size of AWG24, and thermocouple wire length of 500 ft.
Thermocouple wires are available in two grades, i.e. thermocouple grade and extension grade. Thermocouple grade wire is used for making thermocouple probes whereas Extension grade is used for conveying the signal from the thermocouple probe to the measuring device. Thermocouple grade wires are available in a variety of temperature levels and during usage is usually exposed to the temperatures being measured - sometimes high temperatures-. Extension grade thermocouple wires on the other hand are usually routed in wire trays or in areas that are at ambient temperature. Extension grade thermocouple wires are usually not routed in areas exposed to process temperature and are not usually exposed to high temperature. Thermocouple grade wires may be used in place of Extension grade thermocouple wires but not vice versa. i.e. Extension grade wire cannot be used for thermocouple grade wiring.
Thermocouple wires are available as Bare wires - without insulation, Duplex insulated thermocouple wire - the most common, and , Metal Sheathed thermocouple wire - usually for high temperature applications where the wire may be subject to vibration and harsh environment.