Thermocouple wire: understanding how different wires affect thermocouple types

Thermocouple wire has a big effect on the design of the different sensors. There are different thermocouples: J, K, T, E and so on. Different thermocouple types are made up of different types of metal in the wire. Thermocouples are the most common sensors used by many industries.

Determining the length of thermocouple wire

The length of thermocouple wire can be determined by several things. The general guideline says that thermocouple wire should be 20 AWG gauge or thicker wire that is under 100 feet long. The wire should be used in an area that has no electromagnetic interference. The two main factors that decide what is considered usable thermocouple wire length are the total loop resistance and the prevention of excessive electrical noise from interfering with the signal.

Since each type of thermocouple is made from different materials, the resistance will vary according to the thermocouple type and the wire diameter and length. Allowable loop resistance is related to the input resistance of the amplifier circuit attached to it. The goal is the keep the total loop resistance under 100 ohms. The loop resistance is obtained by multiplying the length in feet by the resistance in double feet. When calculating this information, make sure to include the thermocouple probe if applicable and the wire length.

Identifying insulated wires

Insulation on a thermocouple wire is identified by the color coding n the wire. For example, the negative lead in insulated thermocouple wire has a red label, and the positive lead wire is the same color as the thermocouple and the color of the insulated extension grade wire. The outer covering of the thermocouple wire is usually brown, where as the high temperature wire will usually have a color coded thread embedded in the white material.

Standard versus special limits of error (SLE) wire

The difference between standard and special limits of error (SLE) wire is related to the accuracy of the wire. SLE wire is said to be the same grade as standard wire with a slightly better accuracy spec. The accuracy of each type of thermocouple wire will be different.

Thermocouple grade versus extension grade wire

Thermocouples used to make the probe part of the thermocouple is known as thermocouple grade wire. Extension wire is used only for extending a thermocouple signal from a probe back up to the device that is reading the signal. The extension grade wire has a lower ambient temperature limit for the wire's use. The extension grade thermocouple wire may pass a signal representing a higher temperature as received from the probe, but this wire cannot be exposed to high temperatures. You can get away with using thermocouple wire as extension wire, but you cannot used extension wire in the probe part of a thermocouple. Look for part numbers on extension wire that start with "EX."

There are many technical aspects that must be followed to the letter when using thermocouple wire to ensure that you get the desired result for the thermocouple type.