What is a thermocouple?

A thermocouple is a sensor for measuring temperature. It consists of two dissimilar metal wires that are joined together (normally by welding) at the sensing end. Different thermocouple types (e.g. K, T, N, J, etc) are made from different combinations of metals. For example, a Type K thermocouple has a Nickel-Chromium positive conductor and a Nickel-Aluminium negative conductor.

Why do I need thermocouple cable?

To maintain the accuracy of a thermocouple measurement system, thermocouple cable of the same alloys is required to extend from the thermocouple sensor to the measurement device.

How are insulated thermocouple cables identified?

The insulation on thermocouple wire is colour coded for identification. The BS EN 60584-3 standard specifies that the negative conductor insulation is coloured white for all thermocouple types. The positive conductor insulation and any overall insulation has the colour of the thermocouple type. For example, Type K thermocouple cable would have white coloured insulation on the negative (Nickel-Aluminium) conductor, green coloured insulation on the positive (Nickel-Chromium) conductor and the overall insulating jacket would be coloured green. For information on International (IEC) and American (ANSI) colour codes refer to our Thermocouple Colour Code Guide.

What is the difference between Class 1 and Class 2 accuracy cable?

The BS EN 60584-2 standard defines the accuracy requirements for thermocouple cable. There are 2 main tolerances: Class 1 (most accurate) and Class 2. The actual tolerance varies by thermocouple type and temperature range. Full details of the tolerances for different thermocouple types are listed in our Thermocouple Tolerance Guide.

What is the difference between Thermocouple grade and Extension grade cable?

Thermocouple grade cable is wire that is used to make the sensing point (or probe part) of the thermocouple. Extension grade cable is only used to extend a thermocouple signal from a probe back to the instrument reading the signal. The extension grade cable typically will have a lower ambient temperature limit in which it may be used. Namely, it may pass a signal representing a higher temperature as received from the probe, but the cable physically may not be exposed to higher temperatures. Thermocouple cable may be used as extension cable, but extension grade cable may not be used in the sensing point (or probe part) of the thermocouple. Omega part numbers for thermocouple extension cable typically begin with an "EX" prefix.

What is the maximum length of thermocouple cable?

There are many factors that determine the usable length of a thermocouple cable. As a guideline, under 30 metres with 0.8mm dia or thicker conductors in an area free of electromagnetic interference usually is fine. Two of the main factors in determining useable thermocouple length are total loop resistance and preventing electrical noise getting into the signal. Because different thermocouple cables are made of different alloys, the resistance will vary based on the thermocouple type as well as the wire diameter and length. The allowable loop resistance is affected by the input resistance of the amplifier circuit to which it is attached. But as a guideline, typically the objective is to keep the total loop resistance under 100 ohms. Loop resistance is determined by multiplying the length in metres by the resistance per double metre (remember 1 metre length of cable includes 1 metre from each of the two conductors) as shown in Resistance Vs Conductor Diameter table for thermocouple type and conductor diameter. The second major factor in running a thermocouple cable is to keep it away from any electromagnetic fields. Thermocouples produce a very low voltage signal and should not be run near power cables, motors, etc. To help minimise electrical noise pickup, a metal overbraid or twisted and screened wire is commonly used.