Thermocouple Wire Types

Thermocouple wires are metal junctions made of alloys with high heat and electrical conductivity. They are used wherever ambient temperature-based automatic cutoffs and activations are required, such as in heaters or heat alarms. Thermocouples are used to measure and control over a wide temperature range. Generally, thermocouple wires are used as thermopiles, which are multiple thermocouples connected in series or parallel. Thermocouples vary by the metal alloy used in manufacturing. The alloy determines a thermocouple's physical properties, such as conductivity, magnetism and the temperature range. They are also classified based on the insulation type.

Standard Thermocouple Wire

• Type K is a general purpose thermocouple and is made of chromel-alumel alloy. It is inexpensive and has a wide temperature range from -200 to 1350 degrees Celsius. It has nickel, which makes it magnetic. Type E is also a standard thermocouple wire. It is composed of chromel-constantan. This nonmagnetic thermocouple wire is most suitable for cryogenic use. It can measure over a temperature range of -40 to 900 degrees Celsius. Another standard thermocouple wire, Type J, is made of iron-constantan. Because of the presence of iron, it has a relatively restricted temperature range of -40 to 750 degrees Celsius, but it has a higher sensitivity. Standard type N thermocouple wire is a nicrosil-nisil alloy, and is most effective in measuring temperature ranges over 1200 degrees Celsius. The composition also makes it less susceptible to high-temperature oxidation.

Low-Sensitivity Thermocouple Wire

• Low-sensitivity thermocouple wires are made of platinum-rhodium alloy and are the most stable thermocouple materials available. Presence of platinum makes them expensive and only used in specialized situations, such as measuring very high temperatures. Type B is a low-sensitivity platinum-rhodium thermocouple that measures temperatures from 0 to 1800 degrees Celsius. Type R and S measure temperatures up to 1600 degrees Celsius.

Thermocouple Wires of Other Alloys

• Type T is a nonmagnetic thermocouple made of copper-constantum and can measure temperatures in the range of -200 to 350 degrees Celsius. Type C thermocouple is made of tungsten and rhenium, with a very wide range of 0 to 2300 degrees Celsius. However, do not use this type in the presence of oxygen and at temperatures above 260 degrees Celsius. Type T and C are best suited for use in vacuum furnaces. Nickel is used in Type N thermocouples and can measure temperatures up to 1400 degrees Celsius.

Thermocouple Wires by Insulation Type

• Metal-sheathed thermocouple wires have magnesium oxide insulation and are covered with a metal sheath. Duplex insulated wires, which is the most common type, has a simple insulation that can vary by the application. Thermocouple wire is also available without insulation.