Thermocouples are dissimilar wires so joined as to produce a thermally generated mini-voltage when its joint ends are exposed to temperature. Several combinations of dissimilar combinations have become standards and are letter coded. Thermocouples have normally standard lead wires of 12 inches but can also be ordered with longer lead wires. Thermocouples are perishable items and the controller where they connect to may be many feet away. To safe on maintenance costs, use jacks and panels to connect the lead wires from the thermocouples to the extension thermocouple wire (which connect into the control panel). A correct chosen thermocouple extension wire and connetions allows the extension of up to 100ft without affecting the thermoelectric characteristics. If a thermocouple needs to be replaced, it makes sense to replace just the thermocouple with the standard lead wire.
When ordering thermocouple wire, we need at least to know:
wire gauge, solid or stranded, wire insulation + jacketing
Standard thermocouple wire can be ordered with high temperature insulation, while for the thermocouple extension wire there is usually no need for the high temp protection. Thermocouple extension wire should not be used in the sensing point, because of this lower ambient temperature limit. A thermocouple extension wire has often a brown outside insulation. The maximum length of an extension wire should not exceed 100ft with 20 AWG and avoid electromagnetic interferences. Subzero and vacuum application should also be mentioned when ordering thermocouple wire. For high temperature wire applications, click here.
Thermocouple wires are sold in several versions:
Below are some of the most common thermocouple types:
Thermocouple extension wire
Thermocouple extension wire for base metal types (example: type J, K or T) has the same composition as the corresponding thermocouple wire. For noble metal compositions (like type B,R, S), a different composition is used due to high pricing of the noble material. (example: Platinum) Jack's and plugs are also color coded, matching the calibration. For easier installation, wires can be stranded instead of solid. If installing thermocouple wires close to other electrical conduits (which is not recommanded...), use shielded thermocouple wires. The longer the wire, the less accurate temperature readings you'll achieve and you may have to consider thermocouple wire with special limits.